CVE-2026-114654 in C# Driver
Summary
by MITRE • 10/22/2026
A numeric-type conversion flaw in the BSON handling library of the MongoDB C# Driver allows an out-of-range binary subtype value supplied through untrusted textual input to bypass the library's binary-element integrity checks. When such a value is later encoded, it is narrowed to a smaller wire representation, producing binary elements that misdeclare their type or length. An unauthenticated party able to supply text parsed by an affected application can cause it to emit and persist malformed binary data, which subsequent readers either reject with an error or silently interpret as shorter than the data that was validated.
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Analysis
by VulDB Data Team • 10/22/2026
The vulnerability identified in the MongoDB C# Driver stems from a critical numeric-type conversion flaw within its BSON handling library. This issue specifically affects how binary subtype values are processed when they originate from untrusted textual input sources. The core technical defect lies in the failure of the library to properly validate these values against expected ranges before processing them for encoding. When an attacker supplies a binary subtype value that falls outside the acceptable numerical bounds, the driver fails to reject it during the initial parsing phase. Instead, the system proceeds with the conversion process, which leads to a subsequent narrowing of the data type when it is encoded into its wire representation format. This narrowing results in binary elements that incorrectly declare their type or length, effectively bypassing the library's built-in integrity checks designed to ensure data consistency and correctness.
The operational impact of this flaw is significant for applications relying on the MongoDB C# Driver for persistent storage and data exchange. An unauthenticated attacker who can supply text parsed by an affected application has a direct path to inject malformed binary data into the database. This misrepresentation occurs because the encoded elements do not accurately reflect the original input, leading to discrepancies between what was validated during ingestion and what is actually stored on disk or transmitted over the network. Subsequent readers of this data face two primary failure modes: they may reject the corrupted entries with explicit errors, causing application instability or denial of service for legitimate users, or more dangerously, they may silently interpret the malformed binary as being shorter than it was originally validated to be. This silent misinterpretation can lead to subtle logic errors where applications process truncated data without realizing that integrity has been compromised.
From a security architecture perspective, this vulnerability aligns with CWE-190 Integer Overflow or Wraparound and CWE-20 Improper Input Validation. The failure occurs because the application does not adequately verify user-supplied input before using it to control program flow or data structure creation. In terms of attack vectors, this falls under ATT&CK technique T1190 Exploit Public-Facing Application, as it leverages a flaw in software that is accessible to external actors. The ability to persist malformed data creates opportunities for further exploitation depending on how downstream systems handle the corrupted binary blobs. For instance if those binaries are interpreted by other services or scripts, they could trigger buffer overflows, deserialization attacks, or logic bypasses based on the incorrect length fields.
Mitigation strategies must focus on strict input validation and defensive coding practices within the application layer as well as ensuring the use of patched driver versions provided by MongoDB. Developers should implement explicit range checks for all numeric values derived from untrusted sources before they are passed to BSON encoding functions. It is crucial to validate that binary subtype identifiers fall strictly within the defined standard ranges established by the BSON specification. Additionally, applications should employ robust error handling mechanisms that detect and reject malformed data at ingestion points rather than allowing it to propagate into persistent storage. Regular security audits of code paths involving database interactions can help identify similar weaknesses in type conversion logic across other components of the system. Keeping dependencies updated is also vital as vendors frequently release patches for such low-level library flaws to prevent exploitation through standard attack vectors.