High-severity WinRAR 0-day exploited for weeks by 2 groups

BI.ZONE said the Paper Werewolf delivered the exploits in July and August through archives attached to emails impersonating employees of the All-Russian Research Institute. The ultimate goal was to install malware that gave Paper Werewolf access to infected systems.

While the discoveries by ESET and BI.ZONE were independent of each other, it’s unknown if the groups exploiting the vulnerabilities are connected or acquired the knowledge from the same source. BI.ZONE speculated that Paper Werewolf may have procured the vulnerabilities in a dark market crime forum.

ESET said the attacks it observed followed three execution chains. One chain, used in attacks targeting a specific organization, executed a malicious DLL file hidden in an archive using a method known as COM hijacking that caused it to be executed by certain apps such as Microsoft Edge. It looked like this:

The DLL file in the archive decrypted embedded shellcode, which went on to retrieve the domain name for the current machine and compare it with a hardcoded value. When the two matched, the shellcode installed a custom instance of the Mythic Agent exploitation framework.

A second chain ran a malicious Windows executable to deliver a final payload installing SnipBot, a known piece of RomCom malware. It blocked some attempts at being forensically analyzed by terminating when opened in an empty virtual machine or sandbox, a practice common among researchers. A third chain made use of two other known pieces of RomCom malware, one known as RustyClaw and the other as Melting Claw.

WinRAR vulnerabilities have previously been exploited to install malware. One code-execution vulnerability from 2019 came under wide exploitation in 2019 shortly after being patched. In 2023, a WinRAR zero-day was exploited for more than four months before the attacks were detected.

Besides its massive user base, WinRAR makes a perfect vehicle for spreading malware because the utility has no automated mechanism for installing new updates. That means users must actively download and install patches on their own. What’s more, ESET said Windows versions of the command-line utilities UnRAR.dll and the portable UnRAR source code are also vulnerable. People should steer clear of all WinRAR versions prior to 7.13, which, at the time this post went live, was the most current. It has fixes for all known vulnerabilities, although given the seemingly unending stream of WinRAR zero-days, it isn’t much of an assurance.