CVE-2026-84966 in C++ Driver
Summary
by MITRE • 09/03/2026
An incorrect numeric type conversion in the BSON document building component of the MongoDB C++ Driver may cause a length value to be interpreted incorrectly. When an application supplies an extremely large, non-terminated field name to the builder, the library may read memory outside the intended buffer and terminate the calling process. No authentication is required, but the calling application must pass the oversized name in a specific form.
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Analysis
by VulDB Data Team • 09/03/2026
The MongoDB C++ Driver contains a critical vulnerability within its BSON document building component that stems from an incorrect numeric type conversion during length calculation. This flaw specifically manifests when applications attempt to construct BSON documents using field names of extreme size without proper termination or boundary checks. The underlying issue involves the driver failing to correctly validate and convert the integer representation of the string length, leading to a miscalculation of buffer boundaries. When an oversized, non-terminated field name is passed to the builder, the internal logic interprets this incorrect numeric value as valid memory access parameters rather than rejecting it as invalid input. Consequently, the library proceeds to read from memory locations outside the intended buffer allocation, resulting in an out-of-bounds read condition that corrupts process state and typically causes the calling application to terminate abruptly due to a segmentation fault or similar fatal error.
From a technical perspective, this vulnerability aligns with CWE-190 Integer Overflow or Wraparound, as the root cause lies in the mishandling of numeric data types during size calculations. The failure to properly validate input length before memory access operations allows an attacker who controls the field name payload to trigger undefined behavior within the application process. Although no authentication is required to exploit this specific code path, the attack vector is constrained by the requirement that the calling application must explicitly pass the oversized name in a specific form compatible with the BSON builder interface. This implies that while remote exploitation may be possible if the driver processes untrusted input directly from network requests without intermediate sanitization, local or authenticated scenarios are also viable depending on how the application integrates the C++ Driver into its request handling pipeline.
The operational impact of this vulnerability is primarily denial of service against applications utilizing the affected version of the MongoDB C++ Driver. By supplying a crafted field name that triggers the out-of-bounds read, an attacker can cause persistent crashes in services relying on dynamic BSON construction for database operations such as inserts or updates. This disrupts availability and can lead to data loss if transactions are interrupted mid-execution. Furthermore, depending on the specific memory layout and compiler optimizations, there may be secondary risks related to information disclosure if the out-of-bounds read exposes sensitive process memory contents before termination occurs. However, given that the primary symptom is process termination, the most immediate threat remains service disruption rather than arbitrary code execution or data exfiltration in typical deployment scenarios.
Mitigation strategies should focus on both software updates and defensive coding practices. The most effective remediation is to upgrade the MongoDB C++ Driver to a version where this numeric type conversion flaw has been patched with proper bounds checking and input validation for field name lengths. Applications integrating the driver must implement strict input sanitization at the application layer, ensuring that any user-supplied data used as BSON keys undergoes length verification before being passed to the builder functions. Additionally, developers should consider using safer string handling libraries or wrappers that enforce maximum key size limits consistent with MongoDB specifications. Monitoring for abnormal process crashes in environments running vulnerable driver versions can also serve as an early detection mechanism for potential exploitation attempts targeting this weakness.