MongoDB libmongocrypt EUVD-2026-95162

The target application must use MongoDB libmongocrypt with Client-Side Field Level Encryption or Queryable Encryption enabled - this is an opt-in feature, not default MongoDB usage. The attacker must either (1) be an authenticated actor with permission to modify key vault documents, or (2) control a MongoDB server that returns a malicious key document to the client. The specific trigger is a key document containing duplicate masterKey fields, which causes provider-specific KMS data to be handled as an incompatible type during key-document cleanup. Limiting factors: encryption must be in use, some privilege (key-vault write access, PR:L) or a malicious/compromised server is required, and the confirmed impact is client-side crash or memory corruption rather than confirmed remote code execution.

This is a genuine but conditional memory-safety flaw (CWE-843 type confusion) in MongoDB's libmongocrypt, the library underpinning Client-Side Field Level Encryption (CSFLE) and Queryable Encryption. The source-provided CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:L/UI:N, score 8.7) indicates a network-reachable, low-complexity, low-privilege issue with no user interaction. However, the PR:L metric is important: exploitation is NOT unauthenticated - it requires an actor already able to modify key vault documents, or control of a MongoDB server that returns a malicious key document, so a trust boundary must already be crossed. The source rates all three impact dimensions High (VC:H/VI:H/VA:H), but the description's concrete, confirmed outcomes are application termination (DoS) and process-memory corruption via invalid frees/invalid memory access; full confidentiality compromise or code execution is plausible for invalid-free bugs but is NOT explicitly claimed, so the VC:H/VI:H ratings should be treated as conservative worst-case rather than demonstrated. No EPSS score, no CISA KEV listing, and no public exploit code are present in the supplied data, which argues against imminent widespread exploitation. Net: a real priority for environments actively using CSFLE/Queryable Encryption against untrusted servers or multi-tenant key vaults, but lower practical urgency than the 8.7 suggests for deployments where the key vault and server are fully trusted.

An authenticated user with write access to the encryption key vault (or an attacker controlling/compromising the MongoDB server the client talks to) stores or returns a data-encryption-key document containing duplicate masterKey fields. When the libmongocrypt-based client later processes and cleans up that key document, provider-specific KMS data is treated as the wrong type, causing invalid memory access and invalid frees that crash the client application or corrupt its memory. No public proof-of-concept exploit was identified in the provided data.