One of the Chinese threat actors targeting Pulse Secure VPN appliances via a recently disclosed vulnerability has been attempting to cover its tracks by removing its webshells from victim networks, FireEye reports.
Tracked as CVE-2021-22893, the vulnerability was made public in late April, after security researchers discovered that threat actors had already been exploiting it in attacks targeting organizations in the defense, financial, government, high tech, and transportation sectors in the U.S. and Europe.
At the time, FireEye revealed that at least two Chinese threat actors believed to be state-sponsored — UNC2630 and UNC2717 — had been exploiting the vulnerability for initial compromise. The company identified 12 malware families used in attacks associated with the exploitation of CVE-2021-22893 and three other bugs in Pulse Secure VPN appliances.
A patch was shipped on May 3, two weeks after the security hole was publicly disclosed, but the activity surrounding it and the other flaws did not stop. In fact, FireEye says it has since identified four other malware families used in these attacks, namely BLOODMINE, BLOODBANK, CLEANPULSE, and RAPIDPULSE.
On the other hand, just days before FireEye made its findings public in April, UNC2630 was spotted removing its webshells from dozens of devices.
However, the security firm reveals that, in at least one instance, UNC2630 deleted the webshells deployed in their attack but did not remove the persistence patcher, thus ensuring they could return to the compromised environment when needed.
“It is unusual for Chinese espionage actors to remove a large number of backdoors across several victim environments on or around the time of public disclosure. This action displays an interesting concern for operational security and a sensitivity to publicity,” FireEye notes.
To avoid detection, both UNC2630 and UNC2717 modify file timestamps, and tamper with forensic evidence, including logs, web server core dumps, and data harvested for exfiltration. Additionally, the adversaries show advanced knowledge of victim networks, making it difficult for defenders to identify all of the tools employed, stolen data, or intrusion artefacts.
In a Thursday update to its alert on the targeting of Pulse Secure VPN appliances, the U.S. Cybersecurity and Infrastructure Security Agency (CISA) also noted that one of the identified threat actors has started cleanup operations, in an attempt to hide their tracks.
CISA also mentions a new tool from IT management and security firm Ivanti, which is designed to help administrators ensure the integrity of Pulse Connect Secure software.