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On Wednesday, the Cybersecurity and Infrastructure Security Agency (CISA) added a privilege escalation vulnerability affecting Microsoft SharePoint servers to its list of Known Exploited Vulnerabilities (KEV).

SharePoint is a popular, cloud-based document management and storage system, which is also variously used by companies to implement internal applications and business processes, and share resources via an intranet. As recently as 2020, it enjoyed more than 200 million active monthly users.

The latest addition to KEV, CVE-2023-29357, is a "critical" 9.8 out of 10 vulnerability on the CVSS scale, affecting SharePoint Server 2016 and 2019. With no user engagement required, it allows an attacker to bypass authentication checks and gain administrative access to a server using spoofed JSON Web Token (JWT) authentication tokens.

Researchers first demonstrated the utility of CVE-2023-29357 at March 2023's Pwn2Own event, combining it with a second SharePoint vulnerability to create a successful exploit chain — and winning $100,000 in the process. Another independent researcher developed a proof-of-concept (PoC) exploit in September.

Microsoft issued a patch back in June. However, it's still being actively exploited, according to CISA's new alert. In a Mastodon post on Thursday, security researcher Kevin Beaumont provided a bit of extra context, writing that "I am aware of one ransomware group that finally has a working exploit for this."

For organizations still in the firing line, the June patch can be found here.