CVE-2026-106431 in C Driver
Summary
by MITRE • 10/08/2026
An off-by-one error in the BSON bulk document writer in the MongoDB C Driver can write one zero byte immediately past a heap allocation when a document ends at a specific buffer boundary. An actor who can influence the size of documents serialized by an embedding application can corrupt adjacent process memory or terminate the process. Reaching this issue requires the application to use the BSON bulk-writer API and produce a precise cumulative document size.
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Analysis
by VulDB Data Team • 10/08/2026
The vulnerability identified in the MongoDB C Driver represents a critical heap-based buffer overflow stemming from an off-by-one error within the BSON bulk document writer component. This specific flaw occurs during the serialization process when handling documents that align precisely with internal buffer boundaries. The root cause lies in how memory is allocated and managed for outgoing data packets; specifically, the driver fails to account for a null terminator or padding byte required at the end of certain serialized structures. Consequently, when an application utilizes the BSON bulk-writer API to serialize multiple documents, there are scenarios where the cumulative size calculation leads the writer to allocate exactly enough memory for the document content but neglects the additional space needed for termination bytes. This miscalculation results in a write operation that extends one byte beyond the allocated heap region, directly impacting adjacent process memory.
From an operational perspective, this vulnerability poses significant risks depending on the context of the embedding application. An attacker who can influence or control the size and content of documents being serialized by the vulnerable application may exploit this out-of-bounds write to corrupt adjacent heap metadata or data structures. Such corruption can lead to arbitrary code execution if the attacker can carefully craft payloads that overwrite function pointers, object headers, or other critical memory locations with controlled values. Alternatively, in less sophisticated attack scenarios, the memory corruption is sufficient to cause immediate process termination through a segmentation fault or access violation, resulting in a denial of service condition for any services relying on the MongoDB connection established via this driver.
The exploitation requirements are somewhat specific but not insurmountable for determined adversaries. The attacker must have some level of influence over the data being processed by the application, specifically the ability to dictate document sizes that trigger the precise buffer boundary conditions necessary to activate the bug. This often implies a need for interaction with an API endpoint or service layer where user-supplied input is directly serialized into BSON format using the bulk-writer functionality without adequate size validation or padding adjustments prior to memory allocation. The precision required suggests that while this may not be trivially exploitable in all configurations, it remains a high-severity issue due to the potential for remote code execution if the application processes untrusted data through the affected API path.
To mitigate this vulnerability, immediate updates to the MongoDB C Driver are essential as patches have been released by the vendor to correct the memory allocation logic and ensure proper boundary checks during BSON serialization. Applications utilizing the bulk-writer API should be reviewed for any custom handling of document sizes that might bypass internal safeguards. Furthermore, developers should implement strict input validation on all data entering the serialization pipeline to prevent maliciously crafted payloads from reaching the vulnerable code paths. Enabling Address Sanitizer (ASan) or similar memory error detection tools during development and testing phases can help identify such off-by-one errors before they reach production environments. Adhering to secure coding standards that mandate explicit bounds checking for all buffer operations is critical in preventing recurrence of this class of defects, which aligns with CWE-193 (Off-by-One Error) and maps to ATT&CK techniques involving memory corruption exploitation for initial access or privilege escalation depending on the target environment.