CVE-2026-81523 in libmongocrypt
Summary
by MITRE • 08/27/2026
A missing input-validation issue in MongoDB libmongocrypt's automatic-encryption context setup allows a caller-supplied database identifier to be accepted without sanitization. The resulting impact is limited to incorrect schema selection, which may lead to limited disclosure or modification of information handled by the application.
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Analysis
by VulDB Data Team • 08/27/2026
The vulnerability identified within the MongoDB client-side encryption library libmongocrypt stems from an insufficient input validation mechanism during the initialization phase of automatic-encryption context setup. Specifically, when a caller supplies a database identifier intended for schema resolution, the system fails to adequately sanitize or validate this parameter before processing it. This oversight allows malformed or maliciously crafted identifiers to be accepted by the underlying cryptographic engine without triggering appropriate error handling or rejection protocols. The root cause lies in the lack of rigorous boundary checks and type enforcement on user-supplied data that dictates how encrypted fields are mapped to their respective schemas within a specific database context.
From a technical perspective, this flaw represents a classic case of improper input validation where trusted code performs untrusted actions without sufficient verification. In the context of MongoDB's client-side encryption architecture, schema selection is critical for determining which field mappings and key references apply to encrypted data. When an invalid or unexpected database identifier is passed, the library may default to incorrect behavior rather than raising a fatal error. This can result in the system attempting to resolve schemas against non-existent or misconfigured databases, leading to logical errors in how encryption keys are retrieved or applied. The absence of strict validation allows attackers to manipulate this flow by supplying identifiers that do not conform to expected naming conventions or access controls.
The operational impact of this vulnerability is primarily centered on information disclosure and potential data modification due to incorrect schema selection. If an attacker can influence the database identifier used during context setup, they may cause the application to load schemas from unintended databases. This misalignment can lead to the decryption of data using keys associated with a different logical tenant or environment, effectively bypassing isolation boundaries enforced at the application level. Consequently, sensitive information protected by client-side encryption could be exposed to unauthorized parties who have control over the input parameters. Furthermore, if the incorrect schema allows for write operations on fields that were not intended to be modified under those conditions, data integrity may also be compromised.
This issue aligns with CWE-20 Improper Input Validation and falls within the broader category of logic flaws in cryptographic implementations. In terms of attack vectors, it relates to ATT&CK techniques involving input manipulation during application initialization phases. The vulnerability does not typically allow for remote code execution or direct system compromise but serves as a vector for lateral movement or privilege escalation within multi-tenant environments where schema isolation is critical. Attackers with the ability to inject parameters into the encryption context setup process can exploit this weakness to bypass logical access controls that rely on correct database routing and schema resolution.
Mitigation strategies should focus on implementing strict input validation at the earliest possible point in the request lifecycle. Developers must ensure that all caller-supplied identifiers, particularly those influencing cryptographic behavior such as database names or collection paths, are validated against a whitelist of allowed values or subjected to rigorous pattern matching before being passed to libmongocrypt. Additionally, error handling should be configured to fail securely if any validation step fails, preventing the system from proceeding with potentially unsafe assumptions about schema location. Regular security audits and static code analysis focused on cryptographic API usage can help identify similar gaps in input sanitization across other components of the application stack.