CVE-2026-120813: Plaintext storage of encrypted fields via duplicate keys in client-side encryption schema parsing

An issue in MongoDB Server's client-side field-level encryption (CSFLE) schema processing component allows encryption metadata to be silently dropped during schema parsing under certain conditions. A party who can deliver a specially crafted schema to a user's CSFLE client may cause sensitive data intended for encryption to be stored in plaintext, bypassing confidentiality protections. Exploitation requires the ability to deliver the malicious schema to the user's client, such as through a man-in-the-middle position or an untrusted server.