Attackers Target ProxyLogon Exploit to Install Cryptojacker

Threat actors targeted compromised Exchange servers to host malicious Monero cryptominer in an “unusual attack,” Sophos researchers discovered.

Cryptojacking can be added to the list of threats that face any unpatched Exchange servers that remain vulnerable to the now-infamous ProxyLogon exploit, new research has found.

Researchers discovered the threat actors using Exchange servers compromised using the highly publicized exploit chain—which suffered a barrage of attacks from advanced persistent threat (APT) groups to infect systems with everything from ransomware to webshells—to host Monero cryptomining malware, according to a report posted online this week by SophosLabs.

“An unknown attacker has been attempting to leverage what’s now known as the ProxyLogon exploit to foist a malicious Monero cryptominer onto Exchange servers, with the payload being hosted on a compromised Exchange server,” Sophos principal researcher Andrew Brandt wrote in the report.

Researchers were inspecting telemetry when they discovered what they deemed an “unusual attack” targeting the customer’s Exchange server. Sophos researchers Fraser Howard and Simon Porter were instrumental in the discovery and analysis of the novel threat, Brandt acknowledged.

Researchers said they detected the executables associated with this attack as Mal/Inject-GV and XMR-Stak Miner (PUA), according to the report. Researchers published a list of indicators of compromise on the SophosLabs GitHub page to help organizations recognize if they’ve been attacked in this way.

How It Works

The attack as observed by researchers began with a PowerShell command to retrieve a file named win_r.zip from another compromised server’s Outlook Web Access logon path (/owa/auth), according to the report. Under closer inspection, the .zip file was not a compressed archive at all but a batch script that then invoked the built-into-Windows certutil.exe program to download two additional files, win_s.zip and win_d.zip, which also were not compressed.

The first file is written out to the filesystem as QuickCPU.b64, an executable payload in base64 that can be decoded by the certutil application, which by design can decode base64-encoded security certificates, researchers observed.

The batch script then runs another command that outputs the decoded executable into the same directory. Once decoded, the batch script runs the executable, which extracts the miner and configuration data from the QuickCPU.dat file, injects it into a system process, and then deletes any evidence that it was there, according to the report.

The executable in the attack appears to contain a modified version of a tool publicly available on Github called PEx64-Injector, which is described on its Github page as having the ability to “migrate any x64 exe to any x64 process” with “no administrator privileges required,” according to the report.

Once the file runs on an infected system, it extracts the contents of the QuickCPU.dat file, which includes an installer for the cryptominer and its configuration temporarily to the filesystem. It then configures the miner, injects it into a running process, then quits, according to the report. “The batch file then deletes the evidence and the miner remains running in memory, injected into a process already running on the system,” Brandt wrote.

Researchers observed the cryptominer receiving funds on March 9, which is when Microsoft also released updates to Exchange to patch the flaws. Though the attacker lost several servers after this date and the output from the miner decreased, other servers that were gained thereafter more than made up for the early losses, according to the report.

Exploit-Chain History

The ProxyLogon problem started for Microsoft in early March when the company said it had spotted multiple zero-day exploits in the wild being used to attack on-premises versions of Microsoft Exchange Server. The exploit chain is comprised of four flaws (CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, CVE-2021-27065).

Together the flaws created a pre-authentication remote code execution (RCE) exploit, meaning attackers can take over servers without knowing any valid account credentials. This gave them access to email communications and the opportunity to install a web shell for further exploitation within the environment.

As previously mentioned, Microsoft released an out-of-band update soon after in its scramble to patch the flaws in the ProxyLogon chain; however, while the company boasted later that month that 92 percent of affected machines already had been patched, much damage had already been done, and unpatched systems likely exist that remain vulnerable.

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