CVE-2026-106429 in libmongocrypt
Summary
by MITRE • 10/08/2026
An integer underflow in the KMS endpoint-parsing logic of MongoDB libmongocrypt can cause an allocation failure that terminates the application process. This can occur when an authenticated user modifies a key document in the key vault collection, or when an application accepts a KMS endpoint containing a colon after its path or query during key creation. The issue does not access memory outside its allocated bounds.
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Analysis
by VulDB Data Team • 10/08/2026
The vulnerability identified involves an integer underflow within the Key Management Service (KMS) endpoint parsing logic of MongoDB libmongocrypt, a critical component responsible for handling encryption operations and secure communication with external KMS providers. This flaw arises specifically during the processing of endpoint strings that contain specific formatting anomalies, such as colons appearing after the path or query parameters in URLs provided during key creation processes. When an authenticated user modifies a key document within the protected key vault collection, they can trigger this parsing routine under conditions where the input string structure leads to incorrect arithmetic calculations regarding buffer sizes or index offsets. The core technical failure is rooted in how the library calculates memory allocation requirements based on parsed endpoint components; specifically, if the logic fails to properly validate that a subtraction operation will not result in a negative value before casting it to an unsigned integer type, an underflow occurs. This mathematical error results in the calculation of a drastically smaller or zero-sized buffer than intended by the application's security model.
The operational impact of this vulnerability is primarily characterized by denial of service rather than data exfiltration or remote code execution. Because the miscalculated allocation size leads to a failure in memory reservation, the application process handling the encryption request terminates abruptly. This behavior effectively allows an authenticated attacker who has access to modify key documents or supply malicious KMS endpoint configurations during key creation to crash the MongoDB server or associated client applications relying on libmongocrypt for transparent field-level encryption. It is important to note that this specific flaw does not involve out-of-bounds memory reads or writes, which mitigates the risk of arbitrary code execution but significantly degrades system availability and reliability. The attack vector requires authentication, meaning it cannot be exploited by unauthenticated external actors over the network without first gaining valid credentials, yet its impact on service continuity remains severe for environments relying heavily on automated key management workflows.
From a classification perspective, this vulnerability aligns with CWE-191, which defines integer underflow errors where an arithmetic operation results in a value smaller than expected, often leading to buffer overflows or allocation failures depending on subsequent usage of the erroneous value. In terms of offensive security frameworks, this scenario maps closely to MITRE ATT&CK technique T1485, specifically related to data destruction via denial-of-service actions, although it is more accurately described as a resource exhaustion vector triggered by logic errors rather than brute-force volume attacks. The vulnerability highlights the importance of rigorous input validation and bounds checking in cryptographic libraries that process external configurations or user-modifiable metadata structures like key vaults.
Mitigation strategies should focus on immediate patching to versions of libmongocrypt where this parsing logic has been hardened with explicit checks for negative results before unsigned conversion. Administrators must ensure that all MongoDB instances utilizing transparent field-level encryption are updated to secure releases. Additionally, defensive coding practices within application layers interacting with the key vault should include strict validation of endpoint formats prior to passing them to the cryptographic library. Implementing input sanitization rules that reject URLs containing unexpected delimiters in non-standard positions can serve as a secondary defense layer. Regular auditing of key document modifications and monitoring for abnormal process terminations related to encryption services will aid in detecting potential exploitation attempts or misconfigurations before they lead to sustained service outages.