The US cybersecurity agency CISA on Wednesday issued a warning on threat actors exploiting a critical Microsoft SharePoint Server vulnerability in the wild.
The security defect, tracked as CVE-2023-29357 (CVSS score of 9.8) and patched on June 2023 Patch Tuesday, is described as an elevation of privilege (EoP) flaw that allows unauthenticated attackers to gain administrator privileges.
An attacker can exploit the bug by sending a spoofed JSON Web Token (JWT) authentication token to a vulnerable SharePoint server. According to Microsoft, no user interaction is required for successful exploitation.
“An attacker who has gained access to spoofed JWT authentication tokens can use them to execute a network attack which bypasses authentication and allows them to gain access to the privileges of an authenticated user,” Microsoft explains in a June 2023 advisory.
Microsoft credited Nguyễn Tiến Giang (Jang) of StarLabs SG for identifying the vulnerability. The researcher had demonstrated the issue at ZDI’s Pwn2Own hacking contest in Vancouver, in March 2023, earning a $100,000 prize for his discovery.
In September 2023, the researcher published a technical writeup of the flaw and its use in a two-bug exploit chain to achieve pre-authentication remote code execution (RCE) on the SharePoint server, along with proof-of-concept (PoC) code demonstrating the attack.
The second vulnerability exploited in the attack had been patched in May 2023.
This week, more than three months after the PoC publication, CISA has added CVE-2023-29357 to its Known Exploited Vulnerabilities (KEV) catalog, sending a clear message that threat actors are actively exploiting it in attacks targeting SharePoint servers.
While CISA does not provide specific information on the observed exploitation, the agency told SecurityWeek in the past that it only adds security defects to the KEV list based on solid evidence of active exploitation.
Microsoft has yet to update its advisory to confirm that the vulnerability has been exploited. The flaw does have an ‘exploitation more likely’ classification in the tech giant’s advisory.
Now that CVE-2023-29357 has been added to KEV, federal agencies have 21 days to identify vulnerable SharePoint instances within their environments and apply the available patches, as mandated by the Binding Operational Directive (BOD) 22-01.
BOD 22-01 only applies to federal agencies but CISA advises all organizations to review the entries in the KEV catalog and to apply patches as soon as possible, or discontinue vulnerable products if patches are not available.