libmongocrypt EUVD-2026-108330

Exploitation requires three concrete conditions: (1) the application must use libmongocrypt (version 1.0.0 up to but excluding 1.21.1) with an external cloud KMS provider - AWS, Azure or GCP; (2) the attacker must have authenticated WRITE access to the key vault collection, since the malicious input is delivered by altering key vault documents (and, per the description, KMS provider configuration or KMS responses); and (3) the application must subsequently perform a key-management operation that reads the tampered document, causing the library to build and sign the request. The requirement for prior write access to the key vault - a high-trust resource - is the principal factor limiting real-world exploitation; there is no path for a fully unauthenticated external attacker.

This is a medium-severity issue (CVSS 6.0, vendor 4.0 vector CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:L/VA:N) whose real-world urgency is tempered by a significant trust prerequisite. Although AV:N marks it network-reachable, the decisive gating metrics are PR:L (the attacker must already hold write access to the key vault collection) and AT:P (a specific attack requirement - the application must subsequently perform a KMS operation that consumes the tampered document). The vuln.today Decision label is 'this_month', not act_now or emergency, consistent with that limited exposure. Impact is weighted toward confidentiality (VC:H) with only limited integrity (VI:L) and no availability impact. No EPSS score, CISA KEV listing, or PoC reference was supplied, so there is no signal of active or probable exploitation - the two NVD references are only the fixed release tag and the MongoDB JIRA ticket. Net: a legitimate defect worth patching on a normal cycle, but not a high-CVSS-masquerading-as-emergency item; its risk is bounded because exploitation presupposes an already-privileged insider or a compromised account with key-vault write rights.

An attacker holding write access to the key vault collection (for example a low-privileged database account or a compromised credential) edits a key vault document to inject extra members into the KMS request body or additional token claims. The next time the application performs a key management operation, libmongocrypt interpolates those unvalidated values into the request and signs it with the application's own AWS/Azure/GCP credentials, causing the KMS to carry out operations with parameters the application never intended. No public exploit code was identified at the time of analysis.