A threat actor associated with the FIN8 hacking group has exploited the CVE-2023-3519 remote code execution vulnerability to target unpatched Citrix NetScaler systems in widespread attacks.
Sophos, an established cybersecurity firm, has closely monitored this campaign, revealing that the actor employs payload injections, BlueVPS for malware distribution, obfuscated PowerShell scripts, and deploys PHP webshells on compromised machines. The campaign bears resemblances to a previous attack observed by Sophos analysts, suggesting a connection between the two activities and highlighting the threat actor’s specialization in ransomware attacks.
The focus of these attacks lies on Citrix NetScaler systems, with the vulnerability CVE-2023-3519, a critical-severity code injection flaw in Citrix NetScaler ADC and NetScaler Gateway, identified as an actively exploited zero-day vulnerability in mid-July 2023. Although security updates were released on July 18th, evidence emerged that cybercriminals were selling an exploit for this flaw since at least July 6th, 2023.
Shadowserver’s findings by August 2nd revealed the discovery of 640 webshells in compromised Citrix servers, with Fox-IT subsequently identifying 1,952 compromised servers two weeks later. Worryingly, by mid-August, over 31,000 Citrix NetScaler instances remained susceptible to CVE-2023-3519, despite the availability of security updates.
Sophos X-Ops has identified the threat actor behind the exploitation as ‘STAC4663’ and reports that this campaign is likely connected to the same campaign reported by Fox-IT. The payload delivered in these recent attacks, injected into “wuauclt.exe” or “wmiprvse.exe,” is being analyzed, but Sophos suggests it is part of a ransomware attack chain based on the attacker’s profile. The campaign’s association with the FIN8 hacking group is assessed with moderate confidence, with the analysis founded on factors such as domain discovery, BlueVPS hosting, and unusual PowerShell scripting.
Sophos has shared a list of IoCs (indicators of compromise) on GitHub to aid defenders in detecting and mitigating the threat posed by these attacks. If security updates have not been applied to Citrix ADC and Gateway appliances, the vendor’s security bulletin provides recommended actions to safeguard against these exploits.
