CVE-2026-6850 in Mattermost
Summary
by MITRE • 07/13/2026
Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to validate the length and content of message attachment field values, which allows an authenticated attacker to cause a denial of service for all users in a channel via a post containing a specially crafted payload that triggers catastrophic backtracking in the client-side markdown parser.. Mattermost Advisory ID: MMSA-2026-00658
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Analysis
by VulDB Data Team • 07/13/2026
This vulnerability affects Mattermost server versions 11.7.2 and earlier, 11.6.4 and earlier, and 10.11.19 and earlier, representing a critical denial of service condition that can impact all channel users. The flaw stems from insufficient validation of message attachment field values during message processing, allowing authenticated attackers to craft malicious payloads that exploit weaknesses in the client-side markdown parser.
The technical implementation involves catastrophic backtracking within the markdown parsing engine when processing specially constructed attachment field data. This occurs because the parser fails to properly validate input length and content constraints before attempting to parse potentially maliciously formatted data. The vulnerability specifically targets the handling of message attachments where field values are processed through a markdown parser that lacks adequate protection against exponential backtracking scenarios.
From an operational perspective, this vulnerability enables authenticated attackers with access to any channel to trigger denial of service conditions affecting all users within that channel. The impact extends beyond individual messages to potentially disrupt entire communication workflows as the markdown parser becomes unresponsive during processing of the crafted payloads. This creates a cascading effect where legitimate users experience interruptions in their messaging capabilities.
The vulnerability maps to CWE-400, which specifically addresses Uncontrolled Resource Consumption, and aligns with ATT&CK technique T1499.004 for Network Denial of Service. The exploitation requires only authenticated access to the system, making it particularly dangerous as it can be leveraged by insiders or compromised accounts. The attack vector involves sending a specially crafted message containing attachment fields with maliciously constructed content that triggers the parser's backtracking behavior.
Mitigation strategies should focus on implementing strict input validation for all message attachment field values including length restrictions and content sanitization before markdown processing occurs. Organizations should consider upgrading to patched versions of Mattermost where this vulnerability has been addressed through improved parsing logic and input validation mechanisms. Additionally, monitoring for unusual message patterns and implementing rate limiting on attachment processing can help detect and prevent exploitation attempts.
Security teams should also implement network-level controls to monitor for abnormal parsing behavior and establish incident response procedures for handling denial of service events. The fix typically involves strengthening the markdown parser to handle malformed inputs gracefully without triggering catastrophic backtracking while maintaining compatibility with legitimate use cases. Organizations must prioritize patch management to ensure all Mattermost installations are updated to versions that contain the necessary security fixes.
This vulnerability demonstrates the importance of proper input validation in client-side processing components and highlights how seemingly minor parsing flaws can have significant operational impacts. The attack requires minimal privileges but can cause substantial disruption to communication channels, making it a critical concern for organizations relying on Mattermost for their messaging infrastructure. Regular security assessments of parsing and validation logic should be conducted to identify similar vulnerabilities in other applications and systems.
The affected versions represent multiple release streams that require coordinated patching efforts across different deployment environments. Administrators should verify their current installation versions against the patched releases and ensure comprehensive testing is performed before applying updates to production systems. The vulnerability's impact on all users within a channel emphasizes the need for immediate remediation when identified in operational environments.
Organizations should also consider implementing additional security controls such as automated scanning for malformed message content and establishing baseline metrics for normal parsing behavior to help identify potential exploitation attempts. This vulnerability serves as a reminder of the critical importance of validating all user-supplied data before processing and demonstrates how parser-based vulnerabilities can create widespread service disruption even when requiring only basic authentication access.