CVE-2026-96746: MongoDB C Driver Buffer Overflow Vulnerability

CVE-2026-96746 Overview

CVE-2026-96746 is an out-of-bounds write vulnerability [CWE-787] in the connection-monitoring logic of the MongoDB C Driver. An unauthenticated attacker who controls name resolution and the responses of hosts named in a client's connection string can write beyond the end of a heap buffer. The resulting memory corruption can terminate the application using the driver, producing a denial-of-service condition. The flaw affects applications that embed the MongoDB C Driver and resolve hostnames through attacker-influenced infrastructure such as a hostile DNS resolver or compromised upstream service.

Critical Impact
An attacker-controlled DNS response can trigger a heap out-of-bounds write in the driver, crashing the host application without authentication.

Affected Products

MongoDB C Driver versions prior to 1.30.12 (1.x branch)
MongoDB C Driver versions prior to 2.5.5 (2.x branch)
Applications and language bindings that embed the MongoDB C Driver connection-monitoring logic

Discovery Timeline

2026-09-24 - CVE CVE-2026-96746 published to NVD
2026-09-24 - Last updated in NVD database

Technical Details for CVE-2026-96746

Vulnerability Analysis

The defect lives inside the connection-monitoring code path of the MongoDB C Driver. When the driver resolves hosts specified in a client's connection string, it processes responses from name resolution and connection setup. Under attacker-controlled conditions, the monitoring logic writes past the end of a heap-allocated buffer, corrupting adjacent memory.

The practical outcome described by the vendor is unexpected termination of the application linked against the driver. Because the write occurs on the heap, the condition may also carry integrity implications where attacker-shaped bytes land in neighboring allocations before the crash occurs.

Exploitation does not require credentials to the MongoDB cluster itself. Instead, the attacker must control what the client sees during hostname resolution and the subsequent handshake with the resolved endpoint.

Root Cause

The root cause is insufficient bounds enforcement in the connection-monitoring logic when handling data derived from host resolution and server responses. The driver sizes or writes to a heap buffer using values that are influenced by untrusted responses, producing a classic out-of-bounds write condition classified as [CWE-787]. Detailed root-cause commentary is provided in the MongoDB C Driver security advisory GHSA-frjf-h5jg-4v46.

Attack Vector

The attack is remote and unauthenticated. An attacker who controls name resolution for the hosts in a client's connection string, or who controls the responses of those hosts, can craft replies that drive the driver into the vulnerable write path. Positions enabling this include operating a hostile DNS resolver, poisoning a resolver the client trusts, standing up a rogue host at a resolved address, or compromising an intermediate network device. The MongoDB server itself is not the attack surface.

No public proof-of-concept code is available. For technical fix details, see the GitHub release notes for v1.30.12 and the GitHub release notes for v2.5.5.

Detection Methods for CVE-2026-96746

Indicators of Compromise

Unexpected crashes, SIGABRT, or SIGSEGV signals from processes linked against libmongoc or libbson shortly after establishing new MongoDB client connections.
Heap corruption diagnostics from AddressSanitizer, glibc malloc detection (free(): invalid pointer, malloc_consolidate), or Windows heap verifier events in affected applications.
DNS query logs showing resolution of MongoDB connection string hostnames to unexpected or externally controlled IP addresses.
Core dumps whose stack frames point into MongoDB C Driver connection-monitoring routines.

Detection Strategies

Inventory applications and container images that bundle libmongoc and compare the embedded version against 1.30.12 and 2.5.5.
Enable crash reporting and heap hardening (MALLOC_CHECK_, glibc tcache integrity checks, Windows Fault Tolerant Heap) on hosts running MongoDB C Driver workloads.
Correlate application crash events with recent DNS resolver changes, new upstream resolvers, or anomalous replies for MongoDB endpoints.

Monitoring Recommendations

Monitor DNS resolution integrity for MongoDB connection string hostnames, including unexpected TTL changes and answers from non-authoritative resolvers.
Alert on repeated abnormal termination of services that use the MongoDB C Driver, especially when crashes cluster around connection setup.
Track outbound connections from application hosts to confirm MongoDB endpoints resolve to approved cluster addresses.

How to Mitigate CVE-2026-96746

Immediate Actions Required

Upgrade the MongoDB C Driver to 1.30.12 on the 1.x branch or 2.5.5 on the 2.x branch and redeploy all applications linked against the driver.
Rebuild and republish container images, language bindings, and statically linked binaries that embed the vulnerable driver versions.
Restrict client hosts to trusted internal DNS resolvers and block egress to untrusted recursive resolvers.

Patch Information

MongoDB has released fixed versions in the v1.30.12 release and the v2.5.5 release. The full advisory, including affected version ranges, is documented in GitHub Security Advisory GHSA-frjf-h5jg-4v46. Operators should also validate downstream language bindings such as PyMongo C extensions and PHP MongoDB extensions, which may embed the vulnerable driver.

Workarounds

Pin MongoDB client connection strings to IP addresses or validated internal hostnames resolved by trusted authoritative DNS infrastructure.
Enforce DNSSEC validation or DNS-over-TLS between application hosts and recursive resolvers to reduce the risk of response tampering.
Place MongoDB client traffic on segmented networks that prevent attacker-in-the-middle positioning between the application and cluster nodes.
Enable application-level heap hardening and crash isolation so that a triggered write terminates only the affected worker rather than a shared service.

bash

# Verify installed MongoDB C Driver version on Linux
pkg-config --modversion libmongoc-1.0
pkg-config --modversion libbson-1.0

# Example upgrade on Debian/Ubuntu after enabling the fixed package repository
sudo apt-get update
sudo apt-get install --only-upgrade libmongoc-1.0-0 libbson-1.0-0

# Rebuild and restart services linked against the driver
sudo systemctl restart <your-application-service>