Cybersecurity (opens in new tab) researchers have discovered a new malicious campaign that attempts to exploit the already-fixed ProxyShell vulnerability (opens in new tab) in Microsoft Exchange email (opens in new tab) servers together with the Windows PetitPotam vulnerability (opens in new tab), once again highlighting the importance of patching vulnerabilities in critical components.
The new campaign that hopes to find unpatched vulnerable hosts in order to deploy a variant of the Babuk ransomware (opens in new tab) was unearthed by researchers at the Cisco Talos threat intelligence group with the help of Cisco Secure product telemetry.
“We assess with moderate confidence that the initial infection vector is exploitation of ProxyShell vulnerabilities in Microsoft Exchange Server through the deployment of China Chopper (opens in new tab) web shell,” share (opens in new tab) the researchers.
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According to the researchers, the campaign is predominantly finding vulnerable servers in the US, with a smaller number of infections also cropping up in the UK, Germany, Ukraine, Finland, Brazil, Honduras, and Thailand.
Unusual infection chain
The researchers note that the threat actor behind this campaign, sometimes referred to as Tortilla, is using a somewhat unusual infection chain.
It first uses an intermediate unpacking module that is hosted on a pastebin.com (opens in new tab) clone called pastebin.pl. This intermediate unpacking stage is first downloaded in memory before the final payload is executed.
Analyzing the attack, the researchers note that the downloader runs an obfuscated PowerShell (opens in new tab) command to connect and fetch another module from the actor's infrastructure, which appears to be hosted in Russia.
The PowerShell command also executes an Antimalware Scan Interface (AMSI) bypass (opens in new tab) to circumvent endpoint protection (opens in new tab), before it finally deploys the Babuk ransomware.
“The leak of the Babuk builder and its source code in July have contributed to its wide availability, even for the less experienced ransomware operators, such as Tortilla,” conclude the researchers, asking users to implement a layered defense security in order to catch such attacks in its infancy.
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