Security News > 2021 > August > WARNING: Microsoft Exchange Under Attack With ProxyShell Flaws

The U.S. Cybersecurity and Infrastructure Security Agency is warning of active exploitation attempts that leverage the latest line of "ProxyShell" Microsoft Exchange vulnerabilities that were patched earlier this May, including deploying LockFile ransomware on compromised systems.

Tracked as CVE-2021-34473, CVE-2021-34523, and CVE-2021-31207, the vulnerabilities enable adversaries to bypass ACL controls, elevate privileges on the Exchange PowerShell backend, effectively permitting the attacker to perform unauthenticated, remote code execution.

The development comes a little over a week after cybersecurity researchers sounded the alarm on opportunistic scanning and exploitation of unpatched Exchange servers by taking advantage of the ProxyShell attack chain.

Originally demonstrated at the Pwn2Own hacking contest in April this year, ProxyShell is part of a broader trio of exploit chains discovered by DEVCORE security researcher Orange Tsai that includes ProxyLogon and ProxyOracle, the latter of which concerns two remote code execution flaws that could be employed to recover a user's password in plaintext format.

Now according to researchers from Huntress Labs, at least five distinct styles of web shells have been observed as deployed to vulnerable Microsoft Exchange servers, with over over 100 incidents reported related to the exploit between August 17 and 18.

Web shells grant the attackers remote access to the compromised servers, but it isn't clear exactly what the goals are or the extent to which all the flaws were used.

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