CISA Warns Organizations of ProxyShell Attacks on Exchange Servers

The U.S. Cybersecurity and Infrastructure Security Agency (CISA) over the weekend issued an alert to warn of malicious actors actively exploiting the recently disclosed Microsoft Exchange vulnerabilities named ProxyShell.

Tracked as CVE-2021-34473, CVE-2021-34523, and CVE-2021-31207, the ProxyShell bugs can allow an attacker to execute arbitrary code on a victim system. By chaining the flaws, attackers are able to run code remotely without authentication.

Microsoft patched CVE-2021-34473 and CVE-2021-34523 in April 2021, but released information on them only in July. Fixes for CVE-2021-31207 were released in May 2021. Microsoft Exchange Server 2013, 2016, and 2019 were found vulnerable.

“CISA strongly urges organizations to identify vulnerable systems on their networks and immediately apply Microsoft’s Security Update from May 2021—which remediates all three ProxyShell vulnerabilities—to protect against these attacks,” CISA says in its alert.

The vulnerabilities were first exploited at the 2021 Pwn2Own hacking contest by researchers at security consulting firm DEVCORE, who earned a $200,000 bug bounty for their findings.

Shortly after the details of the security holes were made public at the Black Hat and DEF CON cybersecurity conferences in early August, attackers started scanning for vulnerable servers, and at least 30,000 affected systems are believed to be accessible from the Internet, most of which are located in the United States and Germany.

Actual attacks were observed a few days later. Security firm Huntress Labs reported last week that attackers deployed at least five distinct types of webshells onto the compromised servers. Overall, the researchers found more than 140 webshells deployed on roughly 1,900 unpatched systems.

The ProxyShell vulnerabilities, security researcher Kevin Beaumont warns, are being exploited by multiple threat actors, including in attacks aimed at ransomware deployment. What makes these issues extremely serious, he notes, is the fact that no authentication is required pre-exploitation.

“I have been watching multiple threat actors, including groups operating from US internet service providers again and deploying in methods similar to Hafnium back in January-March,” Beaumont says, underlining that “mass and increasing exploitation” has been observed for weeks.

The researcher also points out that hundreds of US government systems that are vulnerable to ProxyShell remain unpatched, directly exploitable, and exposed to the Internet. These systems have “*.gov SSL certificate hostnames within the US,” he notes.

To stay protected, organizations are advised to ensure their Exchange servers are kept up to date, or that they at least run a patch level that addresses these particular security holes.

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