Attacks Targeting Recent Microsoft Exchange Flaw Ramping Up

Multiple threat actors are already targeting Microsoft Exchange servers in an attempt to exploit a vulnerability fixed by Microsoft with its February 2020 Patch Tuesday updates.

Tracked as CVE-2020-0688 and found in Microsoft Exchange 2010, 2013, 2016, and 2019, the issue exists because the server doesn’t create unique cryptographic keys at the time of installation, which allows an authenticated attacker to trick the server into deserializing malicious ViewState data.

A couple of weeks after Microsoft released patches to address the bug, Zero Day Initiative (ZDI) published additional information on the vulnerability and how it can be exploited, and attackers immediately started scanning for vulnerable servers.

Only unpatched systems are exposed to attackers, but exploitation requires other conditions to be met as well. Specifically, the attacker needs access to the Exchange Control Panel (ECP) interface and to have working credentials at hand to log in to the ECP.

Over the weekend, the National Security Agency (NSA) warned in a tweet of the existence of this vulnerability and the attackers’ ability to run commands on unpatched servers.

Last week, Rapid7 released a module to incorporate the exploit into the Metasploit penetration testing framework, and attacks targeting vulnerable Exchange installations are beginning to ramp up, Volexity says.

Multiple advanced persistent threat (APT) actors are either exploiting or attempting to exploit Exchange servers, sometimes using long-compromised credentials that proved of no use so far. IP addresses and the targeting of credentials supposedly known from previous breaches suggest APTs are behind the new incidents, Volexity explains.

“This vulnerability gives attackers the ability to gain access to a significant asset within an organization with a simple user credential or old service account. This issue further underscores why changing passwords periodically is a good best practice, regardless of security measures like 2FA,” the security firm says.

The attackers attempted to exploit CVE-2020-0688 to run system commands in an effort to conduct reconnaissance, deploy webshell backdoors, and run in-memory post-exploitation tools.

Additionally, the security firm says multiple adversaries are attempting to brute-force credentials by leveraging Exchange Web Services (EWS), in preparation for attacks targeting this vulnerability. According to Volexity, brute-force attacks at certain organizations have increased in both frequency and intensity following the vulnerability disclosure.

Organizations can detect potential compromises by examining specific directories and resources, including Exchange Server Exception logs, Application Event logs, and IIS logs, as well as Outlook Web Anywhere (OWA) or the default IIS web directory.

“Fortunately, this vulnerability does require a compromised credential to exploit and, as a result, will stave off widespread automated exploitation such as those that often deploy cryptocurrency miners or ransomware. However, more motivated attackers now have a way to compromise a critical piece of the IT infrastructure if it is not updated,” Volexity concludes.

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