Clop gang exploiting CVE-2021-35211 RCE in SolarWinds Serv-U in recent attack

Pierluigi Paganini November 09, 2021

The Clop ransomware gang is exploiting CVE-2021-35211 vulnerability in SolarWinds Serv-U to compromise corporate networks.

Threat actors always look for new ways to compromise target networks, Clop ransomware gang (aka TA505, FIN11) is exploiting CVE-2021-35211 SolarWinds Serv-U vulnerability to breach businesses’ infrastructures and deploy its ransomware.

The flaw is a remote code execution vulnerability that allows threat actors to execute arbitrary commands on a vulnerable server with elevated privileges.

In September Microsoft revealed that the flaw was exploited in attacks against a limited, targeted set of customers by a single threat actor. The issue resides in Serv-U version 15.2.3 HF1 and all prior versions, the vendor released Serv-U version 15.2.3 hotfix (HF) 2 to fix the issue. All other SolarWinds and N-able (formerly SolarWinds MSP) are not affected by this issue, including the Orion Platform, and all Orion Platform modules. 

The experts pointed out that this issue is not linked to the SolarWinds supply chain attack.

Microsoft tracked the attackers as DEV-0322, they exploited the flaw in attacks aimed at the U.S. Defense Industrial Base Sector and software companies.

SolarWinds addressed the vulnerability in July 2021 after it became aware of a threat actor exploiting the issue in attacks.

Now researchers from NCC Group reported a surge in Clop ransomware infections over the past weeks and most of the attacks leveraged the exploitation of the CVE-2021-35211 vulnerability.

“The surge can be traced back to a vulnerability in SolarWinds Serv-U that is being abused by the TA505 threat actor. ” reads the analysis published by NCC Group. “During multiple incident response investigations, NCC Group found that a vulnerable version of SolarWinds Serv-U server appeared to be the initial access used by TA505 to breach its victims’ IT infrastructure. The vulnerability being exploited is known as CVE-2021-35211.”

While the Clop gang is known to use vulnerabilities in their attacks, such as the Accellion zero-day attacks, the researchers state that TA505 more commonly uses phishing emails with malicious attachments to breach networks.

Upon exploiting the issue, Serv-U spawns a subprocess controlled by the attackers that can allow to run commands and deploy tools for further penetration into the target network.

The exploitation of the flaw triggers an exception in the Serv-U that could be easily spotted in the log to check for indicators of compromise.

“NCC Group recommends looking for potentially vulnerable Serv-U FTP-servers in your network and checking these logs for traces of similar exceptions as suggested by the SolarWinds security advisory.” continues the report. “This log file is usually located in the Serv-U installation folder. Looking at this log file it contains exceptions at the time of exploitation of CVE-2021-35211. NCC Group’s analysts encountered the following exceptions during their investigations:

EXCEPTION: C0000005; CSUSSHSocket::ProcessReceive();

Anyway, Microsoft’s post mortem analysis of the attacks states that the above exception is not by definition an indicator of successful exploitation, NCC experts suggest checking for suspicious PowerShell commands.

The execution of PowerShell scripts is used to

Another sign of exploitation is traces of PowerShell command execution, which is used to deploy Cobalt Strike Beacon on the system running the vulnerable Serv-U software.

The attackers achieve persistence hijacking a legitimate scheduled task (RegIdleBackup) and abusing the COM handler associated with it to execute a loader used to drop the FlawedGrace RAT.

Below is the list of summary of checks to determine exploitation of the CVE-2021-35211 vulnerability observed in recent attacks.

  • Check if your Serv-U version is vulnerable
  • Locate the Serv-U’s DebugSocketlog.txt
  • Search for entries such as ‘EXCEPTION: C0000005; CSUSSHSocket::ProcessReceive();’ in this log file
  • Check for Event ID 4104 in the Windows Event logs surrounding the date/time of the exception and look for suspicious PowerShell commands
  • Check for the presence of a hijacked Scheduled Task named RegIdleBackup using the provided PowerShell command
    • In case of abuse: the CLSID in the COM handler should NOT be set to {CA767AA8-9157-4604-B64B-40747123D5F2}
  • If the task includes a different CLSID: check the content of the CLSID objects in the registry using the provided PowerShell command, returned Base64 encoded strings can be an indicator of compromise.

Experts also warn of the large presence of vulnerable Serv-U servers exposed online, most of them located in China, located in China, followed by the United States. Most of the vulnerable installs run SSH-2.0-Serv-U_15.1.6.25 version.

549United States
88Hong Kong
28South Korea
27United Kingdom

“There are currently still many vulnerable internet-accessible Serv-U servers online around the world.” concludes the report. “In July 2021 after Microsoft published about the exploitation of Serv-U FTP servers by DEV-0322, NCC Group mapped the internet for vulnerable servers to gauge the potential impact of this vulnerability. In July, 5945 (~94%) of all Serv-U (S)FTP services identified on port 22 were potentially vulnerable. In October, three months after SolarWinds released their patch, the number of potentially vulnerable servers is still significant at 2784 (66.5%).”

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Pierluigi Paganini

(SecurityAffairs – hacking, SolarWinds)

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