CVE-2026-106433 in libmongocrypt
Summary
by MITRE • 10/08/2026
Improper state management in MongoDB libmongocrypt can cause provider-specific data to be treated as an incompatible type when cleaning up a key document containing duplicate masterKey fields. An authenticated actor who can modify key vault documents, or a server that returns such a key document, can cause invalid memory access and invalid frees in the client process. This can terminate the application or corrupt process memory.
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Analysis
by VulDB Data Team • 10/08/2026
The vulnerability identified within MongoDB's libmongocrypt library represents a critical flaw rooted in improper state management during the cleanup phase of cryptographic operations. Specifically, this issue arises when processing key documents that contain duplicate masterKey fields. In such scenarios, the internal logic fails to correctly distinguish between provider-specific data structures and incompatible types, leading to erroneous type handling. This mismanagement occurs not during the initial encryption or decryption process but rather during the resource deallocation phase, where the library attempts to clean up memory associated with the key document. The root cause lies in a lack of robust validation for duplicate fields within the structured data format used by libmongocrypt, allowing malformed input to bypass standard type checks and trigger undefined behavior in the C-based implementation.
From an operational perspective, this vulnerability poses severe risks to application stability and integrity. An authenticated actor who possesses the ability to modify key vault documents can deliberately inject a key document containing duplicate masterKey fields into the system. Alternatively, a compromised or misconfigured server returning such malformed data can also trigger the flaw. When libmongocrypt processes these invalid structures during cleanup, it results in an invalid memory access followed by an incorrect free operation. This sequence of events typically leads to immediate application termination due to segmentation faults or heap corruption. In more complex scenarios involving long-running services, this could lead to silent data corruption within the process memory space, potentially compromising subsequent cryptographic operations or exposing sensitive information through memory leaks and unpredictable state changes.
The security implications extend beyond simple denial of service. While the primary impact is availability disruption via application crash, the potential for heap corruption introduces risks related to code execution if an attacker can precisely control the corrupted memory structures. Although exploitation requires authentication in most configurations, this requirement does not mitigate the severity given that internal services or compromised accounts often have access to key management systems. The vulnerability aligns with CWE-120 Buffer Overflow and CWE-416 Use After Free categories from the Common Weakness Enumeration standards, as it involves improper handling of memory resources leading to invalid accesses. Furthermore, within the MITRE ATT&CK framework, this behavior can be associated with T1537 Transfer Data to Cloud Account or techniques involving resource manipulation that disrupt service availability, particularly if leveraged in conjunction with other vulnerabilities for privilege escalation or data exfiltration attempts.
Mitigation strategies must focus on both immediate patching and long-term defensive coding practices. The primary remediation is the application of vendor-provided patches that update libmongocrypt to versions where input validation has been strengthened to reject documents with duplicate masterKey fields before they reach the cleanup logic. Organizations should prioritize updating their MongoDB client libraries across all services interacting with encrypted data stores. Additionally, implementing strict schema validation at the database level can prevent malformed key documents from being stored or retrieved in the first place. Security teams should also monitor application logs for unexpected terminations or memory-related errors that may indicate attempted exploitation of this flaw. Regular audits of cryptographic implementations and adherence to secure coding guidelines regarding resource management are essential to preventing similar state management vulnerabilities in future development cycles.