CVE-2026-3109: Mattermost Plugins CSRF Vulnerability

CVE-2026-3109 Overview

CVE-2026-3109 is an improper validation vulnerability affecting Mattermost Plugins versions <=11.4 and 10.11.11.0. The vulnerability stems from a failure to validate webhook request timestamps, which allows an attacker to corrupt Zoom meeting state in Mattermost through replayed webhook requests. This vulnerability is tracked under Mattermost Advisory ID: MMSA-2026-00584.

Critical Impact

Attackers can replay webhook requests to corrupt Zoom meeting state, potentially disrupting collaboration workflows and meeting integrity within Mattermost deployments.

Affected Products

• Mattermost Plugins versions <=11.4
• Mattermost Plugins version 10.11.11.0

Discovery Timeline

• 2026-03-26 - CVE CVE-2026-3109 published to NVD
• 2026-03-26 - Last updated in NVD database

Technical Details for CVE-2026-3109

Vulnerability Analysis

This vulnerability is classified under CWE-754 (Improper Check for Unusual or Exceptional Conditions). The core issue lies in the webhook processing logic within Mattermost Plugins, which fails to properly validate the timestamps included in incoming webhook requests from Zoom.

When webhooks are received, the application does not verify whether the timestamp falls within an acceptable time window. This absence of temporal validation creates an opportunity for replay attacks, where previously captured legitimate webhook requests can be re-sent at a later time to manipulate the application state.

The attack requires network access and elevated privileges to execute, with high complexity due to the need to capture valid webhook traffic first. While the confidentiality and integrity impacts are minimal, successful exploitation can cause availability disruption to Zoom meeting state management.

Root Cause

The root cause is improper validation of exceptional conditions (CWE-754) in the webhook request handling code. The application accepts webhook requests without verifying that the request timestamp is recent and within an acceptable drift threshold. Standard webhook security practices typically require timestamp validation with a small tolerance window (usually 5-15 minutes) to prevent replay attacks, but this validation is missing in affected versions.

Attack Vector

The attack vector is network-based and requires the attacker to first capture legitimate webhook traffic between Zoom and Mattermost. The attacker would need to:

1. Intercept or obtain valid Zoom webhook requests sent to the Mattermost instance
2. Store these captured requests for later replay
3. Re-send the webhook requests at a time of their choosing to corrupt meeting state

The high complexity (AC:H) reflects the prerequisite of capturing valid traffic, and the high privileges required (PR:H) indicate that some level of access to the communication channel or administrative capabilities may be necessary to execute this attack successfully.

Detection Methods for CVE-2026-3109

Indicators of Compromise

• Duplicate webhook request IDs appearing in Mattermost server logs within short time periods
• Zoom meeting state inconsistencies that don't match actual meeting events
• Webhook requests with timestamps significantly older than the current server time
• Unusual patterns of meeting state changes not correlating with user activity

Detection Strategies

• Implement logging and alerting for webhook requests with timestamps outside acceptable ranges
• Monitor for duplicate webhook request identifiers that may indicate replay attempts
• Review Mattermost plugin logs for anomalous Zoom integration behavior
• Compare Zoom API activity logs against webhook processing events for discrepancies

Monitoring Recommendations

• Enable verbose logging on the Mattermost Zoom plugin to capture all webhook processing events
• Set up alerts for meeting state corruption or inconsistent meeting data
• Monitor network traffic for repeated webhook payloads targeting the Mattermost webhook endpoint
• Implement rate limiting alerts on webhook endpoints to detect potential replay attack patterns

How to Mitigate CVE-2026-3109

Immediate Actions Required

• Update Mattermost Plugins to the latest patched version as referenced in the security advisory
• Review Zoom meeting state data for any signs of corruption or manipulation
• Audit webhook endpoint access logs for suspicious activity
• Consider temporarily disabling the Zoom plugin if an immediate update is not possible

Patch Information

Mattermost has released security updates addressing this vulnerability. Administrators should consult the Mattermost Security Updates page for detailed patch information and upgrade instructions. The advisory ID MMSA-2026-00584 provides specific guidance for this vulnerability.

Workarounds

• Implement network-level controls to restrict webhook source IPs to known Zoom infrastructure ranges
• Deploy a reverse proxy with custom timestamp validation logic in front of the webhook endpoint
• Enable additional authentication mechanisms on webhook endpoints where supported
• Monitor and rate-limit incoming webhook requests to reduce the effectiveness of replay attacks

Organizations should prioritize applying the official patch when available, as workarounds may not fully address the underlying vulnerability.