CVE-2026-84964 in C Driver
Summary
by MITRE • 09/03/2026
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.
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Analysis
by VulDB Data Team • 09/03/2026
The vulnerability identified in the MongoDB C Driver involves a critical memory management error within its TLS implementation, specifically affecting the certificate revocation checking mechanism. This flaw is classified as a double free condition, which occurs when an application attempts to release the same block of heap-allocated memory more than once. In this specific context, the issue resides in the code path responsible for validating whether a server's SSL/TLS certificate has been revoked by its issuing authority. The vulnerability can be triggered during the TLS handshake process if the client is configured to trust the endpoint or if the connection relies on existing trusted certificates that are subsequently manipulated. An attacker who controls or acts as this trusted TLS endpoint can craft specially formed certificate data designed to exploit the logic error in the revocation checking routine, leading directly to the double free condition.
From a technical perspective, the root cause lies in improper handling of memory pointers during the cleanup phase of the certificate validation process. When the driver processes the revocation status, it allocates resources for parsing and verifying the certificate chain. Due to the flawed logic, under certain conditions involving specific malformed or crafted certificate structures, the application fails to nullify pointers after freeing them or incorrectly tracks reference counts. Consequently, when subsequent cleanup routines execute, they attempt to deallocate memory that has already been returned to the system heap. This violates fundamental principles of safe memory management and is categorized under CWE-415: Double Free, which describes situations where a program frees an object twice without ensuring it remains valid between the two operations. Such errors often lead to undefined behavior within the application runtime environment.
The operational impact of this vulnerability is primarily centered on service availability rather than data confidentiality or integrity in most scenarios. Because the double free occurs during the connection establishment phase, exploiting this flaw allows an unauthenticated attacker positioned as a malicious but seemingly trusted TLS endpoint to cause the connecting client application to crash unexpectedly. This results in a denial-of-service condition for any MongoDB clients attempting to establish connections through such endpoints. While modern memory allocators and operating system protections may sometimes mitigate the severity by detecting invalid free operations, relying on these safeguards is not sufficient given the deterministic nature of this logic error. The attack does not require prior authentication because it exploits the trust relationship established during the TLS handshake, making it accessible to any network adversary capable of intercepting or spoofing traffic directed at MongoDB clients configured with specific trust settings.
This vulnerability aligns with ATT&CK technique T1499: Endpoint Denial of Service, as it involves manipulating system resources to disrupt normal operations. Furthermore, it relates to CWE-20: Improper Input Validation, as the driver fails to adequately sanitize or validate the structure and state of certificate data before processing revocation checks. The presence of this flaw highlights risks associated with complex cryptographic libraries where edge cases in protocol handling can lead to severe stability issues. Organizations utilizing the MongoDB C Driver must recognize that even trusted endpoints are not immune from causing client-side failures if they present malformed certificates during handshake negotiations, particularly when strict certificate validation policies are enforced but implemented with flawed memory management logic.
To mitigate this risk, immediate patching of the MongoDB C Driver to a version where this double free issue has been resolved is essential. Developers should ensure that their build environments include updated dependencies and verify that no custom forks retain vulnerable code paths. Additionally, implementing network-level monitoring can help detect anomalous TLS handshakes or repeated connection failures originating from specific endpoints, which may indicate exploitation attempts. While the vulnerability requires a trusted endpoint context, applying strict certificate pinning or using mutual TLS with rigorous client-side validation policies can reduce the attack surface by limiting which certificates are accepted as valid during handshake negotiations. Regular security audits of cryptographic integration code and adherence to secure coding standards that emphasize proper resource lifecycle management will further prevent similar memory corruption vulnerabilities in future implementations.