Microsoft’s Patch Tuesday (Valentine’s Edition) released information on four remote code execution vulnerabilities in Microsoft Exchange, impacting the following versions:

  • Exchange Server 2019
  • Exchange Server 2016 
  • Exchange Server 2013

Attackers must have functional authentication to attempt exploitation. If they are successful, they may be able to execute code on the Exchange server as SYSTEM, a mighty Windows account.

Exchange remote code execution vulnerabilities have a bit of a pattern in their history. This history is notable due to authentication being a requirement for exploitation of these newly announced vulnerabilities.

CVE-2023-21529, CVE-2023-21706, and CVE-2023-21707 have similarities to CVE-2022-41082 due to them all requiring authentication to achieve remote code execution, which GreyNoise covered back in September 2022. Readers may know those previous September 2022 vulnerabilities under the “ProxyNotShell” moniker, since an accompanying Server-Side Request Forgery (SSRF) vulnerability was leveraged to bypass the authentication constraint. “As per our last email” we noted this historical pattern of Exchange exploitation in prior blogs as well as tracked recent related activity under the Exchange ProxyNotShell Vuln Check tag which sees regular activity.

Shadowserver, a nonprofit organization which proactively scans the internet and notifies organizations and regional emergency response centers of outstanding exposed vulnerabilities, noted that there were over 87,000 Exchange instances vulnerable to CVE-2023-21529 (the most likely vulnerability entry point of the four new weaknesses). 

As of the publishing date of this post, there are no known, public proof-of-concept exploits for these new Exchange vulnerabilities. Unless attackers are attempting to bypass web application firewall signatures that protect against the previous server-side request forgery (SSRF) weakness, it is unlikely we will see any attempts to mass exploit these new weaknesses any time soon. Furthermore, determined attackers have been more stealthy when it comes to attacking self-hosted Exchange servers, amassing solid IP address and domain inventories of these systems, and retargeting them directly for new campaigns.

GreyNoise does not have a tag for any of the four, new Exchange vulnerabilities but is continuing to watch for emergent proof-of-concept code and monitoring activity across the multi-thousand node sensor network for anomalous Exchange exploitation. Specifically, we are keeping a keen eye on any activity related to a SSRF bypass or Exchange credential brute-force meant to meet the authentication constraints needed by an attacker to leverage these vulnerabilities.

GreyNoise researchers will update this post if and when new information becomes available.

Given the likely targeted nature of new, malicious Exchange exploit campaigns, you may be interested in how GreyNoise can help you identify targeted attacks, so you can focus on what matters to your organization.

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This article is a summary of the full, in-depth version on the GreyNoise Labs blog.
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