Heap Buffer Overflow in MongoDB C Driver with Cyrus SASL

Publication date: 2026-05-06

Last updated on: 2026-06-18

Assigner: MongoDB, Inc.

Description

The MongoDB C Driver's Cyrus SASL integration performs unsafe string copying during username canonicalization, enabling a heap buffer overflow before any authentication or network traffic. This may be triggered by passing untrusted input in the username of a MongoDB URI with authMechanism=GSSAPI.

CVSS Scores

EPSS Scores

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Meta Information

Published

2026-05-06

Last Modified

2026-06-18

Generated

2026-07-06

AI Q&A

2026-05-06

EPSS Evaluated

2026-07-05

NVD

CVE-2026-6691

EUVD

EUVD-2026-27838

Affected Vendors & Products

Vendor	Product	Version / Range
mongodb	c_driver	From 2.1.0 (inc) to 2.1.2 (exc)

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-120	The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer.
CWE-787	The product writes data past the end, or before the beginning, of the intended buffer.

Attack-Flow Graph

Executive Summary

The vulnerability exists in the MongoDB C Driver's Cyrus SASL integration, where unsafe string copying occurs during the process of username canonicalization. This unsafe operation can lead to a heap buffer overflow before any authentication or network traffic takes place. The issue can be triggered by providing untrusted input in the username field of a MongoDB URI when using the authMechanism=GSSAPI.

Impact Analysis

This vulnerability can lead to a heap buffer overflow, which may allow an attacker to execute arbitrary code or cause a denial of service. Since the overflow occurs before authentication or network traffic, it could be exploited remotely by passing malicious input in the username field, potentially compromising the security and stability of the affected system.

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