CVE-2026-106428: MongoDB C Driver Buffer Overflow Vulnerability

CVE-2026-106428 Overview

CVE-2026-106428 is an out-of-bounds read vulnerability in the MongoDB C Driver. The flaw resides in the Salted Challenge Response Authentication Mechanism (SCRAM) response parsing logic. When the driver processes a malformed server-final message, it reads one byte beyond a fixed-size buffer. A malicious server or network intermediary positioned before server-signature verification can trigger application termination in any process using the driver. The extra byte is not returned through the protocol, limiting the issue to availability impact rather than information disclosure.

Critical Impact
A malicious MongoDB server or in-path network attacker can crash client applications linked against the MongoDB C Driver by sending a crafted SCRAM server-final authentication message.

Affected Products

MongoDB C Driver (libmongoc)
Applications linking the MongoDB C Driver for SCRAM-based authentication
Downstream language bindings depending on libmongoc

Discovery Timeline

2026-10-08 - CVE-2026-106428 published to the National Vulnerability Database (NVD)
2026-10-08 - Last updated in NVD database

Technical Details for CVE-2026-106428

Vulnerability Analysis

The vulnerability is classified as an out-of-bounds read [CWE-125] in the MongoDB C Driver's SCRAM authentication response handler. SCRAM is a challenge-response protocol used by MongoDB clients to authenticate to a server without sending plaintext credentials. During the final step of the exchange, the client parses a server-final message that normally contains the server signature or an error field.

When a malformed message arrives, the parser advances one byte past the end of a fixed-size stack or heap buffer. The resulting read can access unmapped memory and cause the client process to terminate. The disclosed byte is consumed internally and never transmitted back on the wire, so the issue does not leak buffer contents to the attacker.

Root Cause

The root cause is missing or incorrect bounds validation during parsing of the SCRAM server-final payload. The parser trusts length or delimiter cues from the untrusted server response without confirming that the index remains within the allocated buffer. Because the check occurs before server-signature verification completes, authentication state does not protect the parsing path.

Attack Vector

Exploitation requires an attacker to control either the MongoDB server or an unauthenticated intermediary capable of injecting bytes into the authentication stream. The attacker sends a crafted server-final SCRAM message to a connecting client. On parsing, the client reads one byte out of bounds and the host application terminates. No client credentials, user interaction, or authenticated session are required on the attacker side. The impact is limited to denial of service of the client process.

No public proof-of-concept exploit is referenced in the advisory. Technical details are tracked in the MongoDB C Driver Issue CDRIVER-6370.

Detection Methods for CVE-2026-106428

Indicators of Compromise

Repeated or unexpected crashes of processes linked against libmongoc shortly after establishing a MongoDB connection
Core dumps or AddressSanitizer reports pointing to SCRAM response parsing functions in the driver
Client connection failures correlated with MongoDB endpoints hosted outside trusted infrastructure

Detection Strategies

Enable AddressSanitizer or Valgrind in test and staging environments to flag the one-byte out-of-bounds read during integration testing
Monitor application logs for abnormal termination during the SCRAM authentication phase of MongoDB sessions
Compare installed libmongoc versions against the patched release referenced in CDRIVER-6370 using software bill of materials (SBOM) tooling

Monitoring Recommendations

Alert on MongoDB client processes exiting with signals such as SIGSEGV or SIGBUS immediately after TCP connection to a MongoDB port
Capture network metadata for MongoDB sessions traversing untrusted segments to identify unexpected servers or man-in-the-middle paths
Track crash telemetry from production clients and correlate with driver version metadata to identify unpatched hosts

How to Mitigate CVE-2026-106428

Immediate Actions Required

Upgrade the MongoDB C Driver to the fixed release noted in CDRIVER-6370 as soon as it is available for your platform
Rebuild and redeploy any applications or language bindings that statically link libmongoc
Restrict MongoDB client traffic to authenticated, trusted servers reachable only over controlled network paths

Patch Information

MongoDB tracks the fix under issue CDRIVER-6370. Consult the MongoDB C Driver Issue CDRIVER-6370 tracker for the resolved version and release notes. Package maintainers for Linux distributions should pull the patched upstream release into downstream libmongoc packages.

Workarounds

Terminate MongoDB client connections through TLS with strict certificate validation to prevent network intermediaries from injecting SCRAM responses
Limit client applications to connecting only to MongoDB deployments under direct administrative control until patching completes
Isolate client workloads using process supervisors that restart on crash to reduce the operational impact of denial-of-service attempts

bash

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

# Enforce TLS with certificate validation in connection strings
export MONGODB_URI="mongodb://user:pass@mongo.example.com:27017/?tls=true&tlsAllowInvalidCertificates=false"