Data Resiliency in Microsoft Office 365

Data Resiliency in Microsoft Office 365
Data Resiliency in
Microsoft Office 365
Published: January 9, 2017
This document describes how Microsoft prevents customer data from becoming lost
or corrupt in Office 365, and how Office 365 protects customer data against
malware and ransomware
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Given the complex nature of cloud computing, Microsoft is mindful that it’s not a case of if things will
go wrong, but rather when. We design our cloud services to maximize reliability and minimize the
negative effects on customers when things do go wrong. We have moved beyond the traditional
strategy of relying on complex physical infrastructure, and we have built redundancy directly into our
cloud services. We use a combination of less complex physical infrastructure and more intelligent
software that builds data resiliency into our services and delivers high availability to our customers.
This document describes data resiliency in Microsoft Office 365 from two perspectives:
How Microsoft prevents customer data from becoming lost or corrupt in Exchange Online,
SharePoint Online, and Skype for Business; and
2. How Exchange Online, SharePoint Online, and Skype for Business protect customer data
against malware and ransomware
Resiliency and Recoverability Are Built-in
Building in resiliency and recovery starts with the assumption that the underlying infrastructure and
processes will fail at some point: hardware (infrastructure) will fail, humans will make mistakes, and
software will have bugs. While it would be incorrect to say that software developers were not thinking
about these things before the cloud, how these issues were handled in a typical IT implementation
was very different before the cloud:
First, hardware and infrastructure protections were significant. This meant having datacenters
with 99.99% reliability required significant power and network redundancy, and servers were
implemented with hardware-based clustering, dual power supplies, dual network interfaces,
and the like.
Second, process was paramount. Operations teams maintained rigorous procedures, change
windows were employed, and there was often significant project management overhead.
Third, deployment took place at a glacial pace. Deploying code without owning the source
meant waiting for patch releases, and major version releases involved hardware replacement
and significant capital outlay. Moreover, the only way to correct a problem was to rollback.
Thus, most IT organizations would deploy only major releases to avoid the work to keep upto-date.
Finally, the scale of deployed systems, as well as the level of their interconnectedness was
historically much smaller than it is now.
Today, customers expect continuous innovation from Microsoft without compromising quality, and
this is one of the reasons why Microsoft’s services and software are built with resiliency and
recoverability in mind.
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Office 365 Data Resiliency Principles
Resiliency refers to the ability of a cloud-based service to withstand certain types of failures and yet
remain fully-functional from the customers’ perspective. Data resiliency means that no matter what
failures occur within Office 365, critical customer data remains intact and unaffected. To that end,
Office 365 services have been designed around five specific resiliency principles:
There is critical and non-critical data. Non-critical data (for example, whether a message was
read) can be dropped in rare failure scenarios. Critical data (for example, customer data such
as email messages) should be protected at extreme cost. As a design goal, delivered mail
messages are always critical, and things like whether a message has been read is non-critical.
Copies of customer data must be separated into different fault zones or as many fault
domains as possible (e.g., datacenters, accessible by single credentials (process, server, or
operator)) to provide failure isolation.
Critical customer data must be monitored for failing any part of Atomicity, Consistency,
Isolation, Durability (ACID).
Customer data must be protected from corruption. It must be actively scanned or monitored,
repairable, and recoverable.
Most data loss results from customer actions, so allow customers to recover on their own
using a GUI that enables them to restore accidentally deleted items.
Through the building of our cloud services to these principles, coupled with robust testing and
validation, Office 365 is able meet and exceed the requirements of customers while ensuring a
platform for continuous innovation and improvement.
Dealing with Data Corruption
One of the challenging aspects of running a large-scale cloud service is how to handle data
corruption, given the large volume of data and independent systems. Data corruption can be caused
Application or infrastructure bugs, corrupting some or all of the application state
Hardware issues that result in lost data or an inability to read data
Human operational errors
Malicious hackers and disgruntled employees
Incidents in external services that result in some loss of data
Because greater resiliency in data integrity means fewer data corruption incidents, Microsoft has built
into Office 365 protection mechanisms to prevent corruption from happening, as well as systems and
processes that enable us to recover data if it does. Checks and processes exist within the various
stages of the engineering release process to increase resiliency against data corruption, including:
System Design
Code organization and structure
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Code review
Unit tests, integration tests, and system tests
Trip wires tests/gates
Within Office 365 production environments, peer replication between datacenters ensures that there
are always multiple live copies of any data. Standard images and scripts are used to recover lost
servers, and replicated data is used to restore customer data. Because of the built-in data resiliency
checks and processes, Microsoft maintains backups only of Office 365 information system
documentation (including security-related documentation), using built-in replication in SharePoint
Online and our internal code repository tool, Source Depot. System documentation is stored in
SharePoint Online, and Source Depot contains system and application images. Both SharePoint
Online and Source Depot use versioning and are replicated in near real-time.
Exchange Online Data Resilience
There are two types of corruption that can affect an Exchange database: physical corruption, which is
typically caused by hardware (in particular, storage hardware) problems, and logical corruption, which
occurs due to other factors. Generally, there are two types of logical corruption that can occur within
an Exchange database:
Database logical corruption The database page checksum matches, but the data on the
page is wrong logically. This can occur when the database engine (the Extensible Storage
Engine (ESE)) attempts to write a database page and even though the operating system
returns a success message, the data is either never written to the disk or it's written to the
wrong place. This is referred to as a lost flush. ESE includes numerous features and safeguards
that are designed to prevent physical corruption of a database and other data loss scenarios.
To prevent lost flushes from losing data, ESE includes a lost flush detection mechanism in the
database along with a feature (single page restore) to correct it.
Store logical corruption Data is added, deleted, or manipulated in a way that the user
doesn't expect. These cases are generally caused by third-party applications. It's generally
only corruption in the sense that the user views it as corruption. The Exchange store considers
the transaction that produced the logical corruption to be a series of valid MAPI operations.
The In-Place Hold features in Exchange Online provides protection from store logical
corruption (because it prevents content from being permanently deleted by a user or an
Exchange Online performs several consistency checks on replicated log files during both log
inspection and log replay. These consistency checks prevent physical corruption from being replicated
by the system. For example, during log inspection, there is a physical integrity check which verifies the
log file and validates that the checksum recorded in the log file matches the checksum generated in
memory. In addition, the log file header is examined to make sure the log file signature recorded in
the log header matches that of the log file. During log replay, the log file undergoes further scrutiny.
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For example, the database header also contains the log signature which is compared with the log
file’s signature to ensure they match.
Protection against corruption of mailbox data in Exchange Online is achieved by using Exchange
Native Data Protection, a resiliency strategy that leverages application-level replication across multiple
servers and multiple datacenters along with other features that help protect data from being lost due
to corruption or other reasons. These features include native features that are managed by Microsoft
or the Exchange Online application itself, such as:
Database availability groups and multiple database copies
Lagged database copies
Transport resiliency features, such as Safety Net and Shadow Redundancy
Single Bit Correction
Online Database Scanning
Lost Flush Detection
Single Page Restore
Mailbox Replication Service
Log File Checks
Deployment on Resilient File System
For more information on the native features listed above, click on the above hyperlinks, and see
below for additional information and for details on items without hyperlinks. In addition to these
native features, Exchange Online also includes data resiliency features that customers can manage,
such as:
Single Item Recovery (enabled by default)
In-Place Hold and Litigation Hold
Deleted Item Retention and Soft-Deleted Mailboxes (both enabled by default)
Database Availability Groups
Every mailbox database in Office 365 is hosted in a database availability group (DAG) and replicated
to geographically-separate datacenters within the same region. The most common configuration is
four database copies in four datacenters; however, some regions have fewer datacenters (databases
are replicated to three datacenters in India, and two datacenters in Australia and Japan). But in all
cases, every mailbox database has four copies that are distributed across multiple datacenters,
thereby ensuring that mailbox data is protected from software, hardware, and even datacenter
Out of these four copies, three of them are configured as highly available. The fourth copy is
configured as a lagged database copy. The lagged database copy is not intended for individual
mailbox recovery or mailbox item recovery. Its purpose is to provide a recovery mechanism for the
rare event of system-wide, catastrophic logical corruption.
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Lagged database copies in Exchange Online are configured with a seven-day log file replay lag time.
In addition, the Exchange Replay Lag Manager is enabled to provide dynamic log file play down for
lagged copies to allow lagged database copies to self-repair and manage log file growth. Although
lagged database copies are used in Exchange Online, it is important to understand that they are not a
guaranteed point-in-time backup. Lagged database copies in Exchange Online have an availability
threshold, typically around 90%, due to periods where the disk containing a lagged copy is lost due to
disk failure, the lagged copy becoming a highly-available copy (due to automatic play down), as well
as the periods where the lagged database copy is re-building the log replay queue.
Transport Resilience
Exchange Online includes two primary transport resilience features: Shadow Redundancy and Safety
Net. Shadow Redundancy keeps a redundant copy of a message while it is in transit. Safety Net keeps
a redundant copy of a message after the message is successfully delivered.
With Shadow Redundancy, each Exchange Online transport server makes a copy of each messages it
receives before it acknowledges successfully receiving the message to the sending server. This makes
all messages in the transport pipeline redundant while in transit. If Exchange Online determines the
original message was lost in transit, a redundant copy of the message is redelivered.
Safety Net is a transport queue that is associated with the Transport service on a Mailbox server. This
queue stores copies of messages that were successfully processed by the server. When a mailbox
database or server failure requires activating an out-of-date copy of the mailbox database, messages
in the Safety Net queue are automatically resubmitted to the new active copy of the mailbox
database. Safety Net is also redundant, thereby eliminating transport as a single point of failure. It
uses the concept of a Primary Safety Net and a Shadow Safety Net wherein if the Primary Safety Net
is unavailable for more than 12 hours, resubmit requests become shadow resubmit requests, and
messages are re-delivered from the Shadow Safety Net.
Message resubmissions from Safety Net are automatically initiated by the Active Manager component
of the Microsoft Exchange Replication service that manages DAGs and mailbox database copies. No
manual actions are required to resubmit messages from Safety Net.
Single Bit Correction
ESE includes a mechanism to detect and resolve single-bit CRC errors (aka single-bit flips) that are the
result of hardware errors (and as such they represent physical corruption). When these errors occur,
ESE automatically corrects them and logs an event in the event log.
Online Database Scanning
Online database scanning (also known as database checksumming) is the process where an ESE uses
a database consistency checker to read each page and check for page corruption. The primary
purpose is to detect physical corruption and lost flushes that may not be getting detected by
transactional operations. Database scanning also performs post-store crash operations. Space can be
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leaked due to crashes, and online database scanning finds and recovers lost space. The system is
designed with the expectation that every database is fully scanned once every seven days.
Lost Flush Detection
A lost flush occurs when a database write operation that the disk subsystem/operating system
returned as completed did not actually get written to disk, or was written in the wrong location. Lost
flush incidents can result in database logical corruption, so to prevent lost flushes from resulting in lost
data, ESE includes a lost flush detection mechanism. As database pages are written to passive copies,
a check is performed for lost flushes on the active copy. If a lost flush is detected, ESE can repair the
process using a page patching process.
Single Page Restore
Single page restore, aka page patching, is an automatic process where corrupt database pages are
replaced by healthy copies from a healthy replica. The repair process for a corrupt page depends on
whether the database copy is active or passive. When an active database copy encounters a
corrupted page, it can copy a page from one of its replicas, provided the page it copies is completely
up-to-date. This is accomplished by putting a request for the page into the log stream, which is the
basis of mailbox database replication. As soon as a replica encounters the page request it responds
by sending a copy of the page to the requesting database copy. Single page restore also provides an
asynchronous communication mechanism for the active to request a page from replicas, even if the
replicas are currently offline.
In case of corruption in a passive database copy, including a lagged database copy, because these
copies are always behind their active copy, it is always safe to copy any page from the active copy to
a passive copy. A passive database copy is by nature highly available, so during the page patching
process, log replaying is suspended, but log copying continues. The passive database copy retrieves a
copy of the corrupted page from the active copy, waits until the log file which meets the maximum
required log generation requirement is copied and inspected, and then patches the corrupt page.
Once the page has been patched, log replay resumes. The process is the same for the lagged
database copy, except that the lagged database first replays all log files that are necessary to achieve
a patchable state.
Mailbox Replication Service
Moving mailboxes is a key part of managing a large-scale email service. There are always updated
technologies and hardware and version upgrades to deal with, so having a robust, throttled system
that enables our engineers to accomplish this work while keeping the mailbox moves transparent to
users (by making sure they stay online throughout the process) is key and making sure that the
process scales up gracefully as mailboxes get larger and larger.
The Exchange Mailbox Replication Service (MRS) is responsible for moving mailboxes between
databases. During the move, MRS performs a consistency check on all items within the mailbox. If a
consistency issue is found, MRS will either correct the problem, or skip the corrupted items, thereby
removing the corruption from the mailbox.
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Because MRS is a component of Exchange Online, we can make changes in its code to address new
forms of corruption that are detected in the future. For example, if we detect a consistency issue that
MRS is not able to fix, we can analyze the corruption, change the MRS code and correct the
inconsistency (if we understand how to).
Log File Checks
All transaction log files generated by an Exchange database undergo several forms of consistency
checks. When a log file is created, the first thing done is a bit pattern is written and then a series of
log writes is performed. This enables Exchange Online to execute a series of checks (lost flush, CRC
and other checks) to validate each log file as it is written, and again as it is replicated.
Deployment on Resilient File System
To help prevent corruption from occurring at the file system level, Exchange Online is being deployed
on Resilient File System (ReFS) partitions to provide improved recovery capabilities. ReFS is a file
system in Windows Server 2012 and later that is designed to be more resilient against data corruption
thereby maximizing data availability and integrity. Specifically, ReFS brings improvements in the way
that metadata is updated which offers better protection for data and reduces data corruption cases. It
also uses checksums to verify the integrity of file data and metadata ensuring that data corruption is
easily found and repaired.
Exchange Online takes advantage of several ReFS benefits:
More resiliency in data integrity means fewer data corruption incidents. Reducing the number
of corruption incidents means fewer unnecessary database reseeds.
Checksum running on metadata enabling detections of corruption cases sooner and more
deterministically, allowing us to fix customer data corruption before grey failures occur on
data volumes.
Designed to work well with extremely large data sets—petabytes and larger—without
performance impact
Support for other features used by Exchange Online, such as BitLocker encryption.
Exchange Online also benefits from other ReFS features:
Integrity (Integrity Streams). ReFS stores data in a way that protects it from many of the
common errors that can normally cause data loss. Office 365 Search uses Integrity Streams to
help with early disk corruption detection and checksums of file content. The feature also
reduces corruption incidents caused by “Torn Writes” (when a write operation does not
complete due to power outages, etc.).
Availability (Salvage). ReFS prioritizes the availability of data. Historically, file systems were
often susceptible to data corruption that would require the system to be taken offline for
repair. Although rare, if corruption does occur, ReFS implements salvage, a feature that
removes the corrupt data from the namespace on a live volume and ensures that good data
is not adversely affected by non-repairable corrupt data. Applying the Salvage feature and
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isolating data corruption to Exchange Online database volumes means that we can keep nonaffected databases on a corrupted volume healthy between the time of corruption and repair
action. This increases the availability of databases that would normally be affected by such
disk corruption issues.
SharePoint Online Data Resilience
A key principle for SharePoint Online is to never have a single copy of any piece of data. SharePoint
Online uses SQL Server replication, which is a set of technologies for copying and distributing data
and database objects from one database to another, and then synchronizing between databases to
maintain consistency.
For example, when a user saves a file in SharePoint Online, the file is chunked, encrypted, and stored
within Azure Blob storage. Azure Blob service provides mechanisms to ensure data integrity both at
the application and transport layers. This post will detail these mechanisms from the service and client
perspective. MD5 checking is optional on both PUT and GET operations; however, it does provide a
convenience facility to ensure data integrity across the network when using HTTP. Additionally, since
HTTPS provides transport layer security additional MD5 checking is not needed while connecting over
HTTPS as it would be redundant. Azure Blob service provides a durable storage medium, and uses its
own integrity checking for stored data. The MD5's that are used when interacting with an application
are provided for checking the integrity of the data when transferring that data between the
application and service via HTTP.
To ensure data integrity the Azure Blob service uses MD5 hashes of the data in a couple different
manners. It is important to understand how these values are calculated, transmitted, stored, and
eventually enforced to appropriately design your application to utilize them to provide data integrity.
For more information, see Windows Azure Blob MD5 Overview.
Metadata and pointers to the file are stored in a SQL Server database (the content database). All the
chunks – files, pieces of files, and update deltas – are stored as blobs in Azure storage that are
randomly distributed across multiple Azure storage accounts. The SQL database is hosted on a RAID
10 storage array which is synchronously mirrored to another RAID 10 storage array in a separate rack
within the same datacenter. Asynchronous log shipping is then used to replicate the data to another
RAID 10 storage array in a second datacenter. In addition to protecting data with RAID 10 and
synchronous and asynchronous replication, scheduled data backups are taken which are also
asynchronously replicated to the second datacenter.
In SharePoint Online, data backups are performed every 12 hours and retained for 14 days.
SharePoint Online also uses a hot standby system that includes paired geographically-separate
datacenters within the same customer data location region (for example, Chicago and San Antonio
for customers who have provisioned their tenant in the United States) configured as active/active. For
example, there are live users that have Chicago as their primary datacenter and San Antonio as a
failover datacenter, and live users that have San Antonio as their primary datacenter and Chicago as
their failover datacenter.
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Skype for Business Data Resilience
The key principles for Skype for Business are to:
1. Never have a single copy of any piece of customer data; and
2. Keep the system loosely-coupled
Skype for Business uses a blob store, which contains persistent data that is used to provision new
front-end servers, as well as user data. An internal Microsoft technology is then deployed as part of
the Skype for Business service and used for creating highly reliable, distributable, and scalable
applications. This technology defines votes, determines where users are homed, and replicates data.
Persistent user data is synchronously replicated to two or more front-end servers and Skype for
Business clients only see success when the data is written to all replicas correctly.
A backup service replicates data between blob stores, and an internal monitoring service
automatically probes the system for latencies or malfunctions at all the times. Instances that fail can be
replaced seamlessly while other instances continue to operate and run independent stacks in more
than one pool, either in the same region or in another region. In case of an event affecting availability,
the system performs failover from the primary to one of the backups while recovery steps (automatic
and/or initiated by the engineering team monitoring the service 24/7) are performed in the
background, usually without the users being aware of the incident.
Customer data created in Skype for Business resides in the user’s Exchange Online mailbox, and that
data is protected by the resiliency features described above in Exchange Online Data Resilience.
Protecting Customer Data from Malware
Malware consists of viruses, spyware and other malicious software. Office 365 includes protection
mechanisms to prevent malware from being introduced into Office 365 by a client or by an Office 365
server. The use of anti-malware software is a principal mechanism for protection of Office 365 assets
from malicious software. The anti-malware software detects and prevents computer viruses, malware,
rootkits, worms, and other malicious software from being introduced into any service systems. Antimalware software provides both preventive and detective control over malicious software.
Each anti-malware solution in place tracks the version of the software and what signatures are
running. The automatic download and application of signature updates at least daily from the
vendor's virus definition site is centrally managed by the appropriate anti-malware tool for each
service team.
The following functions are centrally managed by the appropriate anti-malware tool on each
endpoint for each service team:
Automatic scans of the environment
Periodic scans of the file system (at least weekly)
Real-time scans of files as they are downloaded, opened, or executed
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Automatic download and application of signature updates at least daily from the vendor's
virus definition site
Alerting, cleaning, and mitigation of detected malware
When anti-malware tools detect malware, they block the malware and generate an alert to Office 365
service team personnel, Office 365 Security, and/or the security and compliance team of the Microsoft
organization that operates our datacenters. The receiving personnel initiate the incident response
process. Incidents are tracked and resolved, and post-mortem analysis is performed.
SharePoint Online and OneDrive for Business Protection Against Malware
To further protect the service against malicious files, SharePoint Online (which includes OneDrive for
Business) prohibits certain file types from being uploaded and prevents content from being executed
directly in the service. This prohibits the potential spread of malware from within the service. Antimalware software is installed both as part of the initial build on all systems, and on all SharePoint
Online servers, enabling further protection by actively scanning document repositories and code
within SharePoint Online sites and libraries.
Exchange Online Protection Against Malware
All email messages for Exchange Online travel through Exchange Online Protection (EOP), which
quarantines and scans in real time all email and email attachments both entering and leaving the
system for viruses and other malware. Administrators do not need to set up or maintain the filtering
technologies; they are enabled by default. However, administrators can make company-specific
filtering customizations using the Exchange Admin Center.
Using multiple anti-malware engines, EOP offers multilayered protection that’s designed to catch all
known malware. Messages transported through the service are scanned for malware (including
viruses and spyware). If malware is detected, the message is deleted. Notifications may also be sent to
senders or administrators when an infected message is deleted and not delivered. You can also
choose to replace infected attachments with either default or custom messages that notify the
recipients of the malware detection.
The following helps provide anti-malware protection:
Layered Defenses Against Malware Multiple anti-malware scan engines used in EOP help
protect against both known and unknown threats. These engines include powerful heuristic
detection to provide protection even during the early stages of a malware outbreak. This
multi-engine approach has been shown to provide significantly more protection than using
just one anti-malware engine.
Real-time Threat Response During some outbreaks, the anti-malware team may have
enough information about a virus or other form of malware to write sophisticated policy rules
that detect the threat even before a definition is available from any of the engines used by the
service. These rules are published to the global network every 2 hours to provide your
organization with an extra layer of protection against attacks.
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Fast Anti-Malware Definition Deployment The anti-malware team maintains close
relationships with partners who develop anti-malware engines. As a result, the service can
receive and integrate malware definitions and patches before they are publicly released. Our
connection with these partners often allows us to develop our own remedies as well. The
service checks for updated definitions for all anti-malware engines every hour.
Advanced Threat Protection
Advanced Threat Protection (ATP) is an email filtering service that provides additional protection
against specific types of advanced threats, including malware and viruses. Exchange Online Protection
currently uses a robust and layered anti-virus protection powered by multiple engines against known
malware and viruses. ATP extends this protection through a feature called Safe Attachments, which
protects against unknown malware and viruses, and provides better zero-day protection to safeguard
your messaging system. All messages and attachments that don’t have a known virus/malware
signature are routed to a special hypervisor environment, where a behavior analysis is performed
using a variety of machine learning and analysis techniques to detect malicious intent. If no suspicious
activity is detected, the message is released for delivery to the mailbox.
Exchange Online Protection also scans each message in transit in Office 365 and provides time of
delivery protection, blocking any malicious hyperlinks in a message. Attackers sometimes try to hide
malicious URLs with seemingly safe links that are redirected to unsafe sites by a forwarding service
after the message has been received. Safe Links proactively protects your users if they click such a
link. That protection remains every time they click the link, and malicious links are dynamically blocked
while good links are accessible.
ATP also offers rich reporting and tracking capabilities, so you can gain critical insights into who is
getting targeted in your organization and the category of attacks you are facing. Reporting and
message tracing allows you to investigate messages that have been blocked due to an unknown virus
or malware, while the URL trace capability allows you to track individual malicious links in the
messages that have been clicked.
For more information about ATP, see Introducing Exchange Online Advanced Threat Protection and
Office 365 Advanced Threat Protection.
SharePoint Online and OneDrive for Business Protection Against Ransomware
There are many forms of ransomware attacks, but one of the most common forms is where a
malicious individual encrypts a user’s important files and then demands something from the user,
such as money or information, in exchange for the key to decrypt them. Ransomware attacks are on
the rise, particularly those that encrypt files that are stored in the user’s cloud storage. For example,
Crowti (also known as Cryptowall) and Tescrypt (also known as Teslacrypt) are two ransomware
families that have infected over half a million PCs in the first half of 2015. For more information about
ransomware, see the Microsoft Malware Protection Center.
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You can use versioning to protect SharePoint Online lists and SharePoint Online and OneDrive for
Business libraries from some, but not all, of these types of ransomware attacks. Versioning is enabled
by default in OneDrive for Business and disabled by default in SharePoint Online.1 When versioning is
enabled in SharePoint Online site lists, you can look at earlier versions and recover them, if necessary.
That enables you to recover versions of items that pre-date their encryption by the ransomware.
Some organizations also retain multiple versions of items in their lists for legal reasons or audit
By default, versioning in SharePoint is turned off. To turn it on and implement your versioning
decisions, you must either have Full Control or Design permissions. For detailed steps to enable
versioning for SharePoint libraries and lists, see Enable and configure versioning for a list or library.
For detailed steps to do this, see Restore a previous version of a document in OneDrive for Business.
SharePoint Online and OneDrive for Business Recycle Bins
SharePoint Online administrators can restore a deleted site collection by using the SharePoint Online
admin center. SharePoint Online users have a Recycle Bin where deleted content is stored. They can
access the Recycle Bin to recover deleted documents and lists, if they need to. Items in the Recycle
Bin are retained for 90 days. The following data types are captured by the Recycle Bin:
Site collections
List items
Web Part pages
Site customizations made through SharePoint Designer are not captured by the Recycle Bin. For more
information, see Manage the Recycle Bin of a SharePoint site collection. See also, Restore a deleted
site collection.
As illustrated in the example below, versioning does not protect against ransomware attacks that copy
files, encrypt them, and then delete the original files. However, end-users can leverage the Recycle Bin
to recover OneDrive for Business files after a ransomware attack occurs.
Scenario: Recovering SharePoint Online and OneDrive for Business files after a ransomware
Andrew is the head of Human Resources at Blue Yonder Airlines. One day, Andrew receives an email
from Tom, an employee that was recently terminated. Tom says he is applying for a new job
elsewhere and would like Andrew to review and approve an attachment that contains Tom’s
If you don't see the Version history command, version history may be turned off. Depending on how your organization has set up
personal sites, you may be able to turn on document versioning.
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statements to the prospective employer about his termination from Blue Yonder Airlines. Andrew
opens the attachment which, unbeknownst to him, begins a drive-by download process that silently
installs crypto-based ransomware onto Andrew’s computer. After the ransomware is running on
Andrew’s computer:
It connects to a remote server, where it uploads connection information, such as the public IP
address, location, and system information for Andrew’s computer, including what operating
system is running.
The remote server then generates a random 2048-bit RSA key pair that's associated with
Andrew’s computer.
The ransomware copies the public key to Andrew’s computer and starts copying each file
using a pre-determined list of file extensions. As each copy is created, it is also encrypted
using the public key, and the original file is then deleted from Andrew’s hard drive. This
typically continues until all files with the specified file extensions have been copied and
encrypted, including files that are synchronized to the cloud.
Once the encryption process has completed, the ransomware typically executes some local
commands to stop the Volume Shadow Copy (VSS) service that runs on all modern versions
of Windows and controls versioning (history), backup, and restoration of data on a host
computer. The command run by the ransomware stops the VSS service and deletes the VSS
Every time Andrew tries to open a file with the specified file extensions, he receives a message
similar to the following:
Figure 1 - Example ransomware message
Andrew isn’t worried, though, because his files are located on SharePoint Online and OneDrive for
Business. The files on Andrew’s hard drive are simply copies of files that are also stored in the cloud.
To remove the ransomware and recover his files (without paying the ransom), Andrew follows this
Because he doesn’t need any local data, Andrew can reformat his disk, reinstall Windows and
reinstall his applications.
2. After his computer has been reinstalled, Andrew uses the Web interface for OneDrive for
Business to access the Recycle Bin.
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3. Once in the Recycle Bin, Andrew recovers all his files. The version he recovers is the version
that existed right before the ransomware attack.
4. After the original files have been restored, Andrew can delete the encrypted files from
OneDrive and synchronize the recovered files to his computer.
Monitoring, Alerting and Self-Healing
Given the scale of Office 365, it would be impossible to keep customer data resilient and safe from
malware without built-in monitoring that is comprehensive, alerting that is intelligent, and self-healing
that is fast and reliable. Monitoring a set of services at the scale of Office 365 is very challenging.
New mindsets and methodologies needed to be introduced, and whole new sets of technology
needed to be created to operate and manage the service in a connected global environment. We
have moved away from the traditional monitoring approach of data collection and filtering to create
alerts to an approach that is based on data analysis; taking signals and building confidence in that
data and then using automation to recover or resolve the issue. This approach helps take humans out
of the recovery equation, which in turn makes operations less expensive, faster, and less error prone.
Fundamental to Office 365 monitoring is a collection of technologies that comprise our Data Insights
Engine, which is built on Azure, SQL Azure, and open-source streaming database technology. It is
designed to collect and aggregate data and reach conclusions. Currently, it processes more than 500
million events per hour from more than 100,000 servers (~15 TB per day) scattered across dozens of
datacenters in many regions, and these numbers are growing.
Office 365 uses outside-in monitoring, which involves creating synthetic transactions to test everything
that is important. For example, in Exchange Online each scenario is testing every database worldwide
every five minutes in a scattered fashion, providing near continuous coverage of everything that lives
in the system. From multiple locations, 250 million test transactions per day are performed to create a
robust baseline or heartbeat for the service.
Office 365 also uses the concept of Red Alert, which shrinks down all the monitoring signals from all of
the machines in our datacenters to something manageable by a human being. The concept is quite
simple: If something is happening across multiple signals, there must be something going on. It is not
about building confidence in one signal, it is about having reasonable fidelity for each signal so that
you get greater accuracy. This monitoring system is so powerful that we do not have 24x7 staff
watching our monitors; all we have is the machinery that wakes up if it detects a problem, in which
case it will page the appropriate on-call personnel, or more often as is the case, it will just go ahead
and solve the problem. Once we start collecting signals and building red alerts off them, we can start
triangulating across all our service partitions. Below is an example of an Office Service Alert which
illustrates this.
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Figure 2 - Example Office Service Alert
Based on the combination of the failure alert and the Red Alerts, this alert indicates exactly which
components could be having a problem, and that the system is going to try to correct the problem
by itself by restarting a mailbox server.
In addition to self-healing capabilities such as single page restore, Exchange Online includes several
features that take an approach to monitoring and self-healing which focuses on preserving the enduser experience. These features include Managed Availability, which provides built-in monitoring and
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recovery actions, and AutoReseed, which automatically restores database redundancy after a disk
Managed Availability
Managed availability provides a native health checking and recovery solution that monitors and
protects the end user's experience through recovery-oriented actions. Managed availability is the
integration of built-in monitoring and recovery actions with the Exchange high availability platform.
It's designed to detect and recover from problems as soon as they occur and are discovered by the
system. Unlike previous external monitoring solutions and techniques for Exchange, managed
availability doesn't try to identify or communicate the root cause of an issue. Instead, it’s focused on
recovery aspects that address three key areas of the end-user experience:
Availability Can users access the service?
Latency How is the experience for users?
Errors Are users able to accomplish what they want?
Managed availability is an internal feature that runs on every Office 365 server running Exchange
Online. It polls and analyzes hundreds of health metrics every second. If something is found to be
wrong, most of the time it is fixed automatically. But there will always be issues that managed
availability will not be able to fix on its own. In those cases, managed availability will escalate the issue
to an Office 365 support team by means of event logging.
Exchange Online servers are deployed in a configuration that stores multiple databases and their log
streams on the same non-RAID disk. This configuration is often referred to as just a bunch of disks
(JBOD) because no storage redundancy mechanisms, such as RAID, are being used to duplicate the
data on the disk. When a disk fails in a JBOD environment, the data on that disk is lost.
Given the size of Exchange Online and the fact that deployed within it are millions of disk drives, disk
drive failures are a regular occurrence in Exchange Online. In fact, more than 100 fail every day. When
a disk fails in an on-premises enterprise deployment, an administrator must manually replace the
failed disk and restore the affected data. In a cloud deployment the size of Office 365, having
operators (cloud administrators) manually replacing disks is neither practical nor economically
Automatic Reseed, or AutoReseed, is a feature that is the replacement for what is normally operatordriven action in response to a disk failure, database corruption event, or other issue that necessitates
a reseeding of a database copy. AutoReseed is designed to automatically restore database
redundancy after a disk failure by using spare disks that have been provisioned on the system. If a
disk fails, the database copies stored on that disk are automatically reseeded to a preconfigured spare
disk on the server, thereby restoring redundancy.
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Office 365 services have been designed around specific resiliency principles that are designed to
protect data from corruption, to separate data into different fault zones, to monitor data for failing
any part of the ACID test, and to allow customers to recover on their own. Office 365 is designed to
maximize reliability and minimize the negative affects to customers when things do go wrong. We
have moved beyond the traditional strategy of relying on complex physical infrastructure and we have
built redundancy into our cloud services. We use a combination of less complex physical infrastructure
and more intelligent software that builds customer data resiliency into our services and delivers high
availability to our customers.
Because more resiliency in customer data integrity translates to less data corruption incidents,
Microsoft has built into Office 365 several protection mechanisms to prevent corruption from
happening, as well as systems and processes that enable us to recover when it happens. Protection
against corruption of mailbox data in Exchange Online is achieved by using Exchange Native Data
Protection, a resiliency strategy that leverages application-level replication across multiple servers and
multiple datacenters along with other features that help protect data from being lost due to
corruption or other reasons. SharePoint Online and OneDrive for Business files are stored within a
SQL Server database that is hosted on a RAID 10 storage array and synchronously mirrored to
another RAID 10 array in a separate rack within the same datacenter. Asynchronous log shipping is
then used to replicate the data to another RAID 10 storage array in a second datacenter. In addition
to protecting data with RAID 10 and synchronous and asynchronous replication, scheduled data
backups are taken every few minutes which are also asynchronously replicated to the second
datacenter. Skype for Business uses a blob store, which contains persistent data that is used to
provision new front-end servers, as well as user data. Persistent user data is synchronously replicated
to two or more front-end servers and Skype for Business clients only see success when the data is
written to all replicas correctly.
Office 365 also includes protection mechanisms to prevent malware from being introduced into
Office 365 by a client or by an Office 365 server. The use of anti-malware software is a principal
mechanism for protection of Office 365 assets from malicious software. You can also use versioning
and recycle bins to protect SharePoint Online lists and SharePoint Online and OneDrive for Business
libraries from certain types of ransomware attacks.
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Materials in this Library
Microsoft publishes a variety of content for customers, partners, auditors, and regulators around security, compliance,
privacy, and related areas. Below are links to other content in our Risk Assurance Documentation library.
Auditing and Reporting in
Office 365
Describes the auditing and reporting features in Office 365 and Azure Active Directory available to customers. Also
details the various audit data that is available to customers via the Office 365 Security & Compliance Center, remote
PowerShell, and the Management Activity API. Also describes the internal logging data that is available to Microsoft
Office 365 engineers for detection, analysis, and troubleshooting.
Controlling Access to Office 365
and Protecting Content on
Describes the Conditional Access (CA) features in Microsoft Office 365 and Microsoft Enterprise Mobility + Security,
and how they are designed with built-in data security and protection to keep company data safe, while empowering
users to be productive on the devices they love. It also provides guidance on how to address common concerns
around data access and data protection using Office 365 features.
Data Encryption Technologies
in Office 365
Provides an overview of the various encryption technologies that are used throughout Office 365, including features
deployed and managed by Microsoft and features managed by customers.
Data Resiliency in Office 365
Describes how Microsoft prevents customer data from becoming lost or corrupt in Exchange Online, SharePoint
Online, and Skype for Business, and how Office 365 protects customer data from malware and ransomware.
Defending Office 365 Against
Denial of Service Attacks
Discusses different types of Denial of Service attacks and how Microsoft defends Office 365, Azure, and their networks
against attacks.
Financial Services Compliance in
Microsoft's Cloud Services
Describes how the core contract amendments and the Microsoft Regulatory Compliance Program work together to
support financial services customers in meeting their regulatory obligations as they relate to the use of cloud services.
Microsoft Response to New
FISC Guidelines in Japan
(English) (Japanese)
Explains how Microsoft addresses the risks and requirements described in the FISC Revised Guidelines, and it describes
features, controls, and contractual commitments that customers can use to meet the requirements in the Revised
Microsoft Threat, Vulnerability,
and Risk Assessment of
Datacenter Physical Security
Provides an overview regarding the risk assessment of Microsoft datacenters, including potential threats, controls and
processes to mitigate threats, and indicated residual risks.
Office 365 Administrative
Access Controls
Provides details on Microsoft’s approach to administrative access and the controls that are in place to safeguard the
services and processes in Office 365. For purposes of this document, Office 365 services include Exchange Online,
Exchange Online Protection, SharePoint Online, and Skype for Business. Additional information about some Yammer
Enterprise access controls is also included in this document.
Office 365 Customer Security
Provides organizations with quick access to the security and compliance features in Office 365 and considerations for
using them.
Office 365 End of Year Security
Report 2014
Covers security and legal enhancements made to Office 365 in calendar year 2014 than enables customers and
partners to meet legal requirements surrounding independent verification and audits of Office 365.
Office 365 End of Year Security
Report and Pen Test Summary
Office 365 End of Year Security Report and Pen Test Summary for CY 2015.
Office 365 Mapping of CSA
Cloud Control Matrix 3.0.1
Provides a detailed overview of how Office 365 maps to the security, privacy, compliance, and risk management
controls defined in version 3.0.1-11-24-2015 of the Cloud Security Alliance's Cloud Control Matrix.
Office 365 Risk Management
Provides an overview of how Office 365 identifies, evaluates, and manages identified risks.
Office 365 Security Incident
Describes how Microsoft handles security incidents in Microsoft Office 365.
Privacy in Office 365
Describes Microsoft’s privacy principles and internal privacy standards that guide the collection and use of customer
and partner information at Microsoft and give employees a clear framework to help ensure that we manage data
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Self-Service Handling of Data
Spills in Office 365 (restricted to
Federal customers)
Reviews the spillage support provided by Office 365, the tools available to customers, and the configuration settings
that should be reviewed in environments that are prone to data spills.
Tenant Isolation in Office 365
Describes how Microsoft implements logical isolation of tenant data within Office 365 environment.
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