An oversized SASL username value can bypass a size check in the C Driver and be copied past the end of a small buffer, causing the application that uses the driver to crash. This vulnerability does not provide remote code execution or privilege escalation, but it can disrupt the service by terminating the application. The weakness is a classic integer overflow or signed/unsigned mismatch, identified as CWE-190.

The MongoDB C Driver is affected. The issue arises only in builds that include the optional external SASL authentication backend and when a connection is configured to use that backend. No specific version numbers are supplied in the advisory.

The severity is a moderate CVSS score of 5.7. No EPSS score is available, and the vulnerability is not listed in the CISA KEV catalog. Exploitation requires an attacker who can influence the driver's connection configuration on a system that hosts the driver; this may imply remote code supplied by an application that initializes the driver, or local compromise. The likelihood of exploitation is low to moderate because the vulnerable code path is conditional on the optional SASL backend being present and enabled, but any application using that feature must consider the risk of denial of service.