The aggressive Zloader banking malware campaign is exploiting Microsoft’s digital signature verification method to inject code into a signed system DLL, according to researchers at Check Point.
The campaign has been ongoing since November 2021, leveraging legitimate enterprise remote monitoring and management application Atera for initial access to the target machines.
The attacks abuse Atera’s ability to install an agent on the endpoint and assign it to a specific account by including the owner’s email address in a unique .msi file. As part of the campaign, Zloader’s operators created an installer with a temporary email address inside, Check Point said in a note documenting the malicious activity.
As in previous Zloader campaigns, the file imitates a Java installation, but Check Point security researchers say they haven’t been able determine how the file is being distributed.
The agent provides the attacker with full access to the system, including the ability to download and upload files or run scripts. The malicious actors were seen executing two .bat files on the target machines, to change Windows Defender settings and load the rest of the malware from an external server.
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The malware installation chain relies on the execution of several scripts to avoid detection, elevate privileges, disable security features, achieve persistence, and inject the main payload into running processes.
The appContast.dll – a file signed by Microsoft, to which the attackers appended a script – is called using mshta.exe, which results in the execution of the main Zloader payload, using regsvr32.exe.
Check Point discovered that the malware made modifications to the legitimate appContast.dll and reboot.dll files to ensure that malicious code could be executed without issues.
With only “File checksum and two places that match the signature size” modified, the signature’s validity was maintained, but the attackers were able to append data to the signature section.
“As we can’t run compiled code from the signature section of a file, placing a script written in VBscript or JavaScript and running the file using mshta.exe is an easy solution that could evade some EDRs,” the company said.
Although Microsoft released a fix for the issue almost a decade ago, the strict file verification was later pulled and an opt-in update was instead pushed. This means the fix is disabled and the malware authors can launch attacks similar to the latest Zloader campaign.
Check Point said it discovered an open folder on the attacker’s server and noticed that the adversary changes the DLL files on a regular basis. As of January 2022, the malicious DLL file appears to have been downloaded to 2170 unique victim IPs, with most of the victim systems located in the United States (864).
“Zloader campaigns have been previously spotted in the wild in multiple forms. In this particular case, we see that the authors put a lot of effort into the evasion methods. Two noteworthy ways seen here are using legitimate RMM software as an initial access to a target machine, and appending code to a file’s signature while still maintaining the signature’s validity and running it using mshta.exe,” Check Point added.
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