CVE-2026-106437 in C Driver
Summary
by MITRE • 10/08/2026
The BSON buffer-reservation API in the MongoDB C Driver can record a length smaller than the five-byte BSON minimum. Later append or comparison operations can underflow unsigned length calculations and read or write outside the document buffer. An actor who can influence the length supplied by an embedding application can cause the application to terminate or read or corrupt adjacent process memory. Reaching this issue requires the application to pass an undersized value to bson_reserve_buffer and then perform an affected operation.
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Analysis
by VulDB Data Team • 10/08/2026
The MongoDB C Driver contains a critical vulnerability within its BSON buffer-reservation API, specifically involving the handling of length parameters during document construction. The core technical flaw lies in the insufficient validation of input values passed to the bson_reserve_buffer function. This API is designed to allocate space for BSON elements, which inherently require a minimum size due to their binary structure format. However, the implementation fails to enforce this structural constraint before proceeding with memory operations. When an embedding application supplies a length value that is smaller than the five-byte minimum required by the BSON specification, the driver records this undersized value without raising an immediate error or rejecting the input. This lack of boundary checking creates a latent defect within the allocated buffer metadata, setting the stage for subsequent arithmetic errors during document assembly.
The operational impact of this vulnerability manifests when subsequent operations attempt to append data to the reserved buffer or perform comparisons based on the recorded length. Because the initial reservation was too small, these later calculations involving unsigned integers can result in an underflow condition. In C programming, subtracting a larger number from a smaller unsigned integer wraps around to a very large positive value rather than becoming negative. This arithmetic error causes the application to miscalculate buffer boundaries, leading it to read or write data outside the intended document buffer limits. Such out-of-bounds memory access is particularly dangerous as it can lead to information disclosure if sensitive data from adjacent process memory is leaked, or code execution if an attacker can craft a payload that overwrites critical control structures like return addresses or function pointers within the application's memory space.
From a threat modeling perspective, this vulnerability aligns with CWE-190 Integer Overflow or Wraparound and CWE-787 Out-of-bounds Write, as the root cause is the failure to validate input ranges leading to unsafe memory operations. In terms of attack vectors, it falls under MITRE ATT&CK techniques related to Memory Corruption, specifically Tactic TA0005 Defense Evasion where attackers might use buffer overflows to bypass security mechanisms or Tactic TA0004 Privilege Escalation if the application runs with elevated privileges. The severity is further compounded by the fact that reaching this issue requires an actor who can influence the length supplied by the embedding application, suggesting a potential for remote code execution in network-facing services that process untrusted BSON data without proper sanitization layers.
Mitigation strategies must focus on both immediate patching and long-term defensive coding practices. The primary remediation is to apply vendor-provided updates that enforce strict validation of buffer reservation lengths against the BSON minimum size requirement before any memory allocation occurs. Developers integrating the MongoDB C Driver should also implement additional input validation at the application layer, ensuring that all length parameters are checked for validity prior to being passed to driver functions. Furthermore, enabling compiler-based security features such as stack canaries and Address Sanitizer during development can help detect these types of buffer overflows early in the software lifecycle. Regular code audits focusing on integer arithmetic and memory management boundaries are essential to prevent similar vulnerabilities from arising in complex data serialization libraries.