Description

A double free in the OpenSSL-based TLS certificate revocation checking path of the MongoDB C Driver can be reached by a TLS endpoint that the client already trusts. During the handshake, specially formed certificate data can cause the same heap object to be released twice. An unauthenticated party acting as the trusted endpoint may cause the connecting client application to terminate unexpectedly.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.9",
  "pubDate": "2026-09-03T16:18:25.270Z",
  "pubdate": "2026-09-03T16:18:25.270Z",
  "executiveSummary": "A critical memory corruption vulnerability exists within the MongoDB C Driver, specifically located in the TLS certificate revocation checking logic that utilizes OpenSSL.\nThe vulnerability is classified as a double free, which occurs when an application attempts to deallocate the same heap memory address twice.\nThis flaw allows a malicious or compromised TLS endpoint to trigger an unexpected termination of the client application during the TLS handshake process.\nThe attack vector is network-based and requires the client to initiate a connection to an endpoint it already trusts, or one that it is configured to trust.\nSuccessful exploitation results in a denial-of-service (DoS) condition, potentially disrupting database connectivity and application availability.\nBecause the vulnerability is triggered during the handshake phase using specially formed certificate data, it can be executed by an unauthenticated party positioned as a trusted TLS endpoint.\nThere are no requirements for high-level privileges; the primary prerequisite is the establishment of a TLS-encrypted communication channel with the affected client.",
  "technicalDetails": "The vulnerability originates in the certificate revocation checking path of the MongoDB C Driver, which relies on OpenSSL's certificate validation routines.\nThe root cause is a flaw in memory management during the processing of X.509 certificate data. When the driver processes specific, maliciously crafted certificate fields during the TLS handshake, the internal logic fails to correctly track the reference count or ownership of a heap-allocated object.\nThe attack flow commences when a client initiates a connection to a malicious TLS server. Upon establishing the TLS handshake, the server presents a crafted certificate. As the MongoDB C Driver's revocation checking mechanism parses the certificate's extensions or revocation information, an error-handling path or a parsing loop triggers an premature release of the underlying memory object.\nSubsequently, the same logical execution path—or a secondary verification routine—attempts to free that same memory pointer again. This double free operation causes heap corruption within the process memory space.\nIn most environments, this corruption triggers an immediate process crash, as the memory allocator detects an inconsistency in the heap metadata, leading to the SIGABRT or equivalent fault signal. This results in the abrupt termination of the MongoDB C Driver application instance.\nThe vulnerability specifically leverages the trust relationship established between the client and the certificate authority or the endpoint itself. By acting as a trusted endpoint, the attacker bypasses initial connection restrictions, making the application susceptible during the certificate validation phase, which occurs before application-level authentication can be finalized.\nWhile this is primarily a denial-of-service vulnerability, double free flaws can sometimes be weaponized for arbitrary code execution if the heap state can be precisely controlled, though the immediate impact here is restricted to application termination.\nAffected versions include all configurations of the MongoDB C Driver that employ the susceptible OpenSSL-based certificate revocation logic, as the fault resides in the driver's integration layer with the OpenSSL library."
}