CVE-2026-84969 in C Driver
Summary
by MITRE • 09/03/2026
A memory-handling error in the BSON-to-JSON conversion helpers of the MongoDB C Driver can write a small number of bytes past the end of a heap buffer when a binary field is encoded and the output is cut short at a caller-configured length limit. A party who supplies the document content, with no privileges on the application that links the driver, may cause a small amount of data outside the intended buffer to be altered.
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Analysis
by VulDB Data Team • 09/03/2026
The vulnerability identified in the MongoDB C Driver represents a critical memory safety flaw within its BSON-to-JSON conversion utilities. Specifically, this issue resides in the helper functions responsible for serializing Binary Extended JSON (BSON) fields into standard JSON format. The core technical defect is an out-of-bounds write error that occurs during the encoding process when specific conditions are met regarding buffer limits and data length. When a binary field within a BSON document is being converted, the driver allocates a heap buffer to hold the resulting JSON string. If the caller configures a maximum output length limit that truncates the expected full conversion of the binary data, the internal logic fails to correctly account for this boundary constraint during the finalization or padding steps of the encoding routine. Consequently, the function attempts to write metadata or termination characters beyond the allocated heap buffer size, resulting in a small number of bytes being written past the end of the intended memory region.
This type of vulnerability is classified under CWE-787: Out-of-bounds Write, which describes writing data outside the bounds of a designated buffer. In the context of modern software security and attack surface analysis, this flaw aligns with MITRE ATT&CK technique T1059.006: Command and Scripting Interpreter via System Utilities or potentially T1203: Exploitation for Defense Evasion if leveraged to bypass controls, though primarily it falls under the broader category of Memory Corruption (T1204). The significance of this flaw lies in its potential impact on application stability and security. While the description indicates that only a small amount of data outside the buffer is altered, heap overwrites are notoriously dangerous because they can corrupt adjacent memory structures such as allocation metadata or function pointers. An attacker who controls the input document content could potentially trigger this condition to cause a denial of service through application crashes due to segmentation faults or corrupted state detection by runtime guards like ASan or MSVC CRT checks.
The operational impact is particularly severe given that the vulnerability requires no privileges on the target application linking the driver. This means that any client connecting to a MongoDB instance using the C Driver, provided they can supply arbitrary document content for conversion via these specific helper functions, can exploit this flaw. In environments where untrusted data ingestion is common, such as public-facing APIs or services processing user-generated documents, an attacker could craft malicious BSON payloads designed to trigger the length limit truncation during JSON serialization. The resulting memory corruption may lead to unpredictable behavior, including crashes that disrupt service availability, or in more complex exploitation scenarios involving heap grooming and specific allocator behaviors, potentially leading to arbitrary code execution if adjacent memory structures can be manipulated effectively.
Mitigation strategies must focus on both immediate patching and long-term defensive coding practices. The primary remediation is to update the MongoDB C Driver to a version that includes patches for this out-of-bounds write vulnerability, ensuring that buffer boundary checks are strictly enforced during BSON-to-JSON conversion operations. Developers should also review their usage of these helper functions to ensure that length limits are validated against expected output sizes before invocation. Furthermore, integrating static analysis tools and dynamic memory checkers like AddressSanitizer into the CI/CD pipeline can help detect similar off-by-one or buffer overflow errors early in development. For applications relying on this driver, implementing strict input validation for BSON documents and limiting the complexity of binary fields processed through these conversion helpers can reduce the attack surface until patches are deployed.