libmongocrypt constructs requests to external key management services by inserting values from key vault documents, provider configuration and KMS responses directly into request lines, headers and bodies without validation or proper encoding. This allows a user with write access to the key vault collection to modify those values, adding arbitrary members or token claims that the library then signs with the application's credentials. The result is that the application may perform KMS operations using parameters it did not intend, potentially exposing sensitive keys or altering cryptographic operations.

MongoDB libmongocrypt, all versions susceptible until patched. The vulnerability is present in releases that construct requests as described; the Advisory indicates that the fix is available in 1.21.1. No specific earlier versions are enumerated in the input, but all affected releases prior to the fix are considered vulnerable.

The CVSS score of 6.0 denotes moderate severity. Exploit probability is not quantified, as EPSS data is not available, and the vulnerability is not listed in CISA’s KEV catalog. The attack requires an attacker to have write permission to the key vault collection, making it a local privilege requirement; however, an insider or compromised credentials could satisfy this. Once the attacker modifies the request, KMS services may accept malformed or malicious requests, leading to unintended key operations.