Update [03/16/2021]: Microsoft released updated tools and investigation guidance to help IT Pros and incident response teams identify, remediate, defend against associated attacks: Guidance for responders: Investigating and remediating on-premises Exchange Server vulnerabilities.
Update [03/15/2021]: Microsoft released a new one-click mitigation tool, the Microsoft Exchange On-Premises Mitigation Tool, to help customers who do not have dedicated security or IT teams to apply security updates for Microsoft Exchange Server.
Update [03/08/2021]: Microsoft continues to see multiple actors taking advantage of unpatched systems to attack organizations with on-premises Exchange Server. To aid defenders in investigating these attacks where Microsoft security products and tooling may not be deployed, we are releasing a feed of observed indicators of compromise (IOCs). The feed of malware hashes and known malicious file paths observed in related attacks is available in both JSON and CSV formats at the below GitHub links. This information is being shared as TLP:WHITE: CSV format | JSON format
Update [03/05/2021]: Microsoft sees increased use of these vulnerabilities in attacks targeting unpatched systems by multiple malicious actors beyond HAFNIUM. To aid customers in investigating these attacks, Microsoft Security Response Center (MSRC) has provided additional resources, including new mitigation guidance: Microsoft Exchange Server Vulnerabilities Mitigations – March 2021
Update [03/04/2021]: The Exchange Server team released a script for checking HAFNIUM indicators of compromise (IOCs). See Scan Exchange log files for indicators of compromise.
Microsoft has detected multiple 0-day exploits being used to attack on-premises versions of Microsoft Exchange Server in limited and targeted attacks. In the attacks observed, the threat actor used these vulnerabilities to access on-premises Exchange servers which enabled access to email accounts, and allowed installation of additional malware to facilitate long-term access to victim environments. Microsoft Threat Intelligence Center (MSTIC) attributes this campaign with high confidence to HAFNIUM, a group assessed to be state-sponsored and operating out of China, based on observed victimology, tactics and procedures.
The vulnerabilities recently being exploited were CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065, all of which were addressed in today’s Microsoft Security Response Center (MSRC) release – Multiple Security Updates Released for Exchange Server. We strongly urge customers to update on-premises systems immediately. Exchange Online is not affected. We have established a resource center that is constantly updated as more information becomes available at https://aka.ms/ExchangeVulns.
We are sharing this information with our customers and the security community to emphasize the critical nature of these vulnerabilities and the importance of patching all affected systems immediately to protect against these exploits and prevent future abuse across the ecosystem. This blog also continues our mission to shine a light on malicious actors and elevate awareness of the sophisticated tactics and techniques used to target our customers. The related IOCs, Azure Sentinel advanced hunting queries, and Microsoft Defender for Endpoint product detections and queries shared in this blog will help SOCs proactively hunt for related activity in their environments and elevate any alerts for remediation.
Microsoft would like to thank our industry colleagues at Volexity and Dubex for reporting different parts of the attack chain and their collaboration in the investigation. Volexity has also published a blog post with their analysis. It is this level of proactive communication and intelligence sharing that allows the community to come together to get ahead of attacks before they spread and improve security for all.
Who is HAFNIUM?
HAFNIUM primarily targets entities in the United States across a number of industry sectors, including infectious disease researchers, law firms, higher education institutions, defense contractors, policy think tanks, and NGOs.
HAFNIUM has previously compromised victims by exploiting vulnerabilities in internet-facing servers, and has used legitimate open-source frameworks, like Covenant, for command and control. Once they’ve gained access to a victim network, HAFNIUM typically exfiltrates data to file sharing sites like MEGA.
In campaigns unrelated to these vulnerabilities, Microsoft has observed HAFNIUM interacting with victim Office 365 tenants. While they are often unsuccessful in compromising customer accounts, this reconnaissance activity helps the adversary identify more details about their targets’ environments.
HAFNIUM operates primarily from leased virtual private servers (VPS) in the United States.
Technical details
Microsoft is providing the following details to help our customers understand the techniques used by HAFNIUM to exploit these vulnerabilities and enable more effective defense against any future attacks against unpatched systems.
CVE-2021-26855 is a server-side request forgery (SSRF) vulnerability in Exchange which allowed the attacker to send arbitrary HTTP requests and authenticate as the Exchange server.
CVE-2021-26857 is an insecure deserialization vulnerability in the Unified Messaging service. Insecure deserialization is where untrusted user-controllable data is deserialized by a program. Exploiting this vulnerability gave HAFNIUM the ability to run code as SYSTEM on the Exchange server. This requires administrator permission or another vulnerability to exploit.
CVE-2021-26858 is a post-authentication arbitrary file write vulnerability in Exchange. If HAFNIUM could authenticate with the Exchange server then they could use this vulnerability to write a file to any path on the server. They could authenticate by exploiting the CVE-2021-26855 SSRF vulnerability or by compromising a legitimate admin’s credentials.
CVE-2021-27065 is a post-authentication arbitrary file write vulnerability in Exchange. If HAFNIUM could authenticate with the Exchange server then they could use this vulnerability to write a file to any path on the server. They could authenticate by exploiting the CVE-2021-26855 SSRF vulnerability or by compromising a legitimate admin’s credentials.
Attack details
After exploiting these vulnerabilities to gain initial access, HAFNIUM operators deployed web shells on the compromised server. Web shells potentially allow attackers to steal data and perform additional malicious actions that lead to further compromise. One example of a web shell deployed by HAFNIUM, written in ASP, is below:
Following web shell deployment, HAFNIUM operators performed the following post-exploitation activity:
Using Procdump to dump the LSASS process memory:
Using 7-Zip to compress stolen data into ZIP files for exfiltration:
Adding and using Exchange PowerShell snap-ins to export mailbox data:
Using the Nishang Invoke-PowerShellTcpOneLine reverse shell:
Downloading PowerCat from GitHub, then using it to open a connection to a remote server:
HAFNIUM operators were also able to download the Exchange offline address book from compromised systems, which contains information about an organization and its users.
Our blog, Defending Exchange servers under attack, offers advice for improving defenses against Exchange server compromise. Customers can also find additional guidance about web shell attacks in our blog Web shell attacks continue to rise.
Can I determine if I have been compromised by this activity?
The below sections provide indicators of compromise (IOCs), detection guidance, and advanced hunting queries to help customers investigate this activity using Exchange server logs, Azure Sentinel, Microsoft Defender for Endpoint, and Microsoft 365 Defender. We encourage our customers to conduct investigations and implement proactive detections to identify possible prior campaigns and prevent future campaigns that may target their systems.
Check patch levels of Exchange Server
The Microsoft Exchange Server team has published a blog post on these new Security Updates providing a script to get a quick inventory of the patch-level status of on-premises Exchange servers and answer some basic questions around installation of these patches.
Scan Exchange log files for indicators of compromise
The Exchange Server team has created a script to run a check for HAFNIUM IOCs to address performance and memory concerns. That script is available here: https://github.com/microsoft/CSS-Exchange/tree/main/Security.
CVE-2021-26855 exploitation can be detected via the following Exchange HttpProxy logs:
These logs are located in the following directory: %PROGRAMFILES%\Microsoft\Exchange Server\V15\Logging\HttpProxy
Exploitation can be identified by searching for log entries where the AuthenticatedUser is empty and the AnchorMailbox contains the pattern of ServerInfo~*/*
Here is an example PowerShell command to find these log entries:
Import-Csv -Path (Get-ChildItem -Recurse -Path "$env:PROGRAMFILES\Microsoft\Exchange Server\V15\Logging\HttpProxy" -Filter '*.log').FullName | Where-Object { $_.AnchorMailbox -like 'ServerInfo~*/*' -or $_.BackEndCookie -like 'Server~*/*~*'} | select DateTime, AnchorMailbox, UrlStem, RoutingHint, ErrorCode, TargetServerVersion, BackEndCookie, GenericInfo, GenericErrors, UrlHost, Protocol, Method, RoutingType, AuthenticationType, ServerHostName, HttpStatus, BackEndStatus, UserAgent
If activity is detected, the logs specific to the application specified in the AnchorMailbox path can be used to help determine what actions were taken.
These logs are located in the %PROGRAMFILES%\Microsoft\Exchange Server\V15\Logging directory.
CVE-2021-26858 exploitation can be detected via the Exchange log files:
C:\Program Files\Microsoft\Exchange Server\V15\Logging\OABGeneratorLog
Files should only be downloaded to the %PROGRAMFILES%\Microsoft\Exchange Server\V15\ClientAccess\OAB\Temp directory
In case of exploitation, files are downloaded to other directories (UNC or local paths)
Windows command to search for potential exploitation:
findstr /snip /c:"Download failed and temporary file" "%PROGRAMFILES%\Microsoft\Exchange Server\V15\Logging\OABGeneratorLog\*.log"
CVE-2021-26857 exploitation can be detected via the Windows Application event logs
Exploitation of this deserialization bug will create Application events with the following properties:
Source: MSExchange Unified Messaging
EntryType: Error
Event Message Contains: System.InvalidCastException
Following is PowerShell command to query the Application Event Log for these log entries:
Get-EventLog -LogName Application -Source "MSExchange Unified Messaging" -EntryType Error | Where-Object { $_.Message -like "*System.InvalidCastException*" }
CVE-2021-27065 exploitation can be detected via the following Exchange log files:
C:\Program Files\Microsoft\Exchange Server\V15\Logging\ECP\Server
All Set-<AppName>VirtualDirectory properties should never contain script. InternalUrl and ExternalUrl should only be valid Uris.
Following is a PowerShell command to search for potential exploitation:
Select-String -Path "$env:PROGRAMFILES\Microsoft\Exchange Server\V15\Logging\ECP\Server\*.log" -Pattern 'Set-.+VirtualDirectory'
Host IOCs
Microsoft is releasing a feed of observed indicators of compromise (IOCs) in related attacks. This feed is available in both CSV and JSON formats. This information is being shared as TLP:WHITE.
Hashes
Web shell hashes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We observed web shells in the following paths:
C:\inetpub\wwwroot\aspnet_client\
C:\inetpub\wwwroot\aspnet_client\system_web\
In Microsoft Exchange Server installation paths such as:
%PROGRAMFILES%\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\owa\auth\
C:\Exchange\FrontEnd\HttpProxy\owa\auth\
The web shells we detected had the following file names:
web.aspx
help.aspx
document.aspx
errorEE.aspx
errorEEE.aspx
errorEW.aspx
errorFF.aspx
healthcheck.aspx
aspnet_www.aspx
aspnet_client.aspx
xx.aspx
shell.aspx
aspnet_iisstart.aspx
one.aspx
Check for suspicious .zip, .rar, and .7z files in C:\ProgramData\, which may indicate possible data exfiltration.
Customers should monitor these paths for LSASS dumps:
C:\windows\temp\
C:\root\
Tools
Many of the following detections are for post-breach techniques used by HAFNIUM. So while these help detect some of the specific current attacks that Microsoft has observed it remains very important to apply the recently released updates for CVE-2021-26855, CVE-2021-26857, CVE-2021-27065 and CVE-2021-26858.
Microsoft Defender Antivirus detections
Please note that some of these detections are generic detections and not unique to this campaign or these exploits.
Exploit:Script/Exmann.A!dha
Behavior:Win32/Exmann.A
Backdoor:ASP/SecChecker.A
Backdoor:JS/Webshell (not unique)
Trojan:JS/Chopper!dha (not unique)
Behavior:Win32/DumpLsass.A!attk (not unique)
Backdoor:HTML/TwoFaceVar.B (not unique)
Microsoft Defender for Endpoint detections
Suspicious Exchange UM process creation
Suspicious Exchange UM file creation
Possible web shell installation (not unique)
Process memory dump (not unique)
Azure Sentinel detections
Advanced hunting queries
To locate possible exploitation activity related to the contents of this blog, you can run the following advanced hunting queries via Microsoft Defender for Endpoint and Azure Sentinel:
Microsoft Defender for Endpoint advanced hunting queries
Microsoft 365 Defender customers can find related hunting queries below or at this GitHub location: https://github.com/microsoft/Microsoft-365-Defender-Hunting-Queries/
Additional queries and information are available via Threat Analytics portal for Microsoft Defender customers.
UMWorkerProcess.exe in Exchange creating abnormal content
Look for Microsoft Exchange Server’s Unified Messaging service creating non-standard content on disk, which could indicate web shells or other malicious content, suggesting exploitation of CVE-2021-26858 vulnerability:
DeviceFileEvents | where InitiatingProcessFileName == "UMWorkerProcess.exe" | where FileName != "CacheCleanup.bin" | where FileName !endswith ".txt" | where FileName !endswith ".LOG" | where FileName !endswith ".cfg" | where FileName != "cleanup.bin"
UMWorkerProcess.exe spawning
Look for Microsoft Exchange Server’s Unified Messaging service spawning abnormal subprocesses, suggesting exploitation of CVE-2021-26857 vulnerability:
DeviceProcessEvents | where InitiatingProcessFileName == "UMWorkerProcess.exe" | where FileName != "wermgr.exe" | where FileName != "WerFault.exe"
Please note excessive spawning of wermgr.exe and WerFault.exe could be an indicator of compromise due to the service crashing during deserialization.
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