CVE-2026-96749 Overview
CVE-2026-96749 is an integer overflow vulnerability in the BSON (Binary JSON) document encoding component of the MongoDB Python Driver's bundled native C extension. The flaw occurs when a single BSON document is built from an unusually large amount of caller-supplied data. Size arithmetic is performed in a signed 32-bit type, and the overflow guard relies on behavior that is undefined by the C language standard. An unprivileged party who can place a very large value into data that an application encodes may trigger a write outside the bounds of an allocated buffer within the application's own process.
Critical Impact
Out-of-bounds write within the Python application process, potentially enabling memory corruption, denial of service, or code execution depending on the native extension build configuration.
Affected Products
- MongoDB Python Driver (pymongo) versions prior to 4.18.2
- Applications using the bundled C extension for BSON encoding
- Python services that encode caller-controlled data into BSON documents
Discovery Timeline
- 2026-09-24 - CVE-2026-96749 published to the National Vulnerability Database (NVD)
- 2026-09-26 - Last updated in NVD database
Technical Details for CVE-2026-96749
Vulnerability Analysis
The vulnerability resides in the native C extension that pymongo ships to accelerate BSON serialization. During document encoding, the driver computes cumulative buffer sizes using a signed 32-bit integer. When a caller-supplied value is large enough, that arithmetic wraps around and produces a smaller-than-expected size. The code includes a guard intended to detect this overflow, but the check itself relies on signed integer overflow, which the C standard classifies as undefined behavior. Modern optimizing compilers may remove the check entirely, leaving the oversized write unchecked.
Once the undersized length is accepted, the encoder writes caller-controlled bytes past the end of the allocated buffer. The result is a classic heap out-of-bounds write inside the Python interpreter's address space, with impact bounded only by what the attacker can place into the encoded document.
Root Cause
The root cause is an integer overflow categorized as [CWE-190] combined with reliance on undefined behavior in the overflow check. Signed 32-bit size arithmetic in the BSON encoder cannot represent sizes exceeding INT32_MAX, and the compiler is free to optimize away the post-arithmetic validation. See the GitHub Security Advisory GHSA-v4x9-3549-crwv for the maintainer's analysis.
Attack Vector
Exploitation requires local access to the application process and the ability to inject an unusually large value into data that the application subsequently encodes into a BSON document. No authentication to the application is required if the input path is reachable by an unprivileged party. The exact exploitability profile depends on how the native extension was compiled, including compiler, optimization level, and allocator behavior.
No verified public proof-of-concept is available. Technical details describing the overflow mechanism are documented in the MongoDB Driver Change Log and the MongoDB Driver Release 4.18.2 notes.
Detection Methods for CVE-2026-96749
Indicators of Compromise
- Unexpected crashes or segmentation faults in Python processes using pymongo while encoding documents
- Memory corruption signatures in application logs that correlate with BSON serialization code paths
- Anomalously large field values being submitted to application endpoints that persist data to MongoDB
Detection Strategies
- Inventory Python applications and dependencies to identify installations of pymongo at versions below 4.18.2
- Audit application input paths that accept user-supplied strings, binary blobs, or arrays and later encode them with bson.encode() or driver insert/update operations
- Monitor for abnormally large request bodies or field sizes targeting services that interact with MongoDB
Monitoring Recommendations
- Enable crash reporting and core dump collection on Python workloads to capture out-of-bounds write artifacts
- Instrument application-layer input size limits and log entries exceeding thresholds for review
- Correlate dependency inventory output with vulnerability feeds to flag unpatched pymongo installations
How to Mitigate CVE-2026-96749
Immediate Actions Required
- Upgrade pymongo to version 4.18.2 or later across all Python applications and container images
- Rebuild and redeploy any application artifacts that bundle the vulnerable native extension
- Identify and restrict input paths that allow unauthenticated parties to submit large field values destined for BSON encoding
Patch Information
The fix is included in pymongo 4.18.2. The maintainers corrected the size arithmetic and replaced the undefined-behavior-dependent guard with a well-defined overflow check. Release notes are available in the MongoDB Driver Release 4.18.2 announcement.
Workarounds
- Enforce strict server-side input size limits on fields that are serialized into BSON documents
- Disable the bundled C extension and fall back to the pure-Python BSON encoder where performance permits, until patching is complete
- Place a reverse proxy or API gateway in front of exposed services to cap request body size
# Upgrade pymongo to the fixed release
pip install --upgrade "pymongo>=4.18.2"
# Verify the installed version
python -c "import pymongo; print(pymongo.__version__)"
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