CVE-2026-84971 in libmongocrypt
Summary
by MITRE • 09/03/2026
Improper handling of an unexpected value size in the decryption path of a client-side encryption library can cause a failed internal check that terminates the process using the library. A party able to place a suitably formed encrypted value where an application will decrypt it, or able to control the responses the application receives, may cause that application to stop running.
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Analysis
by VulDB Data Team • 09/03/2026
The vulnerability described constitutes a denial of service condition arising from improper input validation within the decryption routine of a client-side encryption library. This flaw is rooted in a failure to adequately verify the size and structure of data before processing it through cryptographic operations, which aligns with CWE-20 Improper Input Validation. When an attacker provides a maliciously crafted encrypted value that exceeds expected bounds or contains malformed parameters, the internal integrity checks within the decryption logic fail. Rather than handling this error gracefully by returning an appropriate exception code to the calling application, the library triggers a fatal error condition that terminates the entire process hosting it. This behavior indicates a lack of robust error handling and resource management in the face of unexpected input conditions.
From an operational perspective, the impact of this vulnerability is significant for any application relying on this specific client-side encryption component. Since the failure results in the termination of the host process, an attacker who can influence the data being decrypted or control the responses received by the application can effectively crash the service. This capability allows for a remote denial of service attack if the vulnerable code path is exposed to user-controlled input over a network interface. The severity of this impact depends on the criticality of the affected application; however, in most contexts, an unexpected process termination leads to data loss, interrupted transactions, and degraded availability for end-users. This scenario maps directly to ATT&CK technique T1499 Endpoint Denial of Service, where adversaries aim to disrupt access to or availability of computing resources by exhausting them through malicious inputs that cause system crashes or service interruptions.
The root cause lies in the assumption that incoming encrypted payloads will always conform to strict structural and size constraints defined during development. In real-world scenarios, network transmission errors, malformed client requests, or deliberate attacks can result in data that violates these assumptions. The library's design fails to implement defensive programming practices such as boundary checks on buffer sizes, validation of cipher block lengths, or safe error propagation mechanisms. Instead of catching the exception and logging it for debugging purposes while maintaining application stability, the unhandled exception propagates up the call stack until it causes a process abort. This highlights a critical gap in security engineering where availability is sacrificed due to insufficient fault tolerance in cryptographic modules.
To mitigate this vulnerability, developers must implement rigorous input validation before passing data to the decryption functions. Specifically, checks should be added to verify that the size of the encrypted payload does not exceed predefined maximum limits and conforms to expected formats for the specific encryption algorithm used. Additionally, the library's error handling logic needs to be refactored to catch internal exceptions during the decryption process. Instead of allowing these errors to terminate the host application, they should be caught, logged securely without exposing sensitive details, and handled in a way that allows the application to continue running or shut down gracefully if necessary. Updating to a patched version of the library is the primary remediation strategy, as vendors typically address such flaws by hardening the input validation routines and improving exception handling mechanisms within the cryptographic core.