CVE-2026-20796: Mattermost Information Disclosure Flaw

CVE-2026-20796 Overview

CVE-2026-20796 is a race condition vulnerability in Mattermost that allows deactivated users to improperly access team name information. The flaw exists in the /common_teams API endpoint, where channel membership validation is not properly enforced at the time of data retrieval. This Time-of-Check Time-of-Use (TOCTOU) vulnerability enables unauthorized information disclosure through a timing-based attack.

Critical Impact

Deactivated users can exploit a race condition to learn team names they should no longer have access to, potentially exposing organizational structure and team naming conventions.

Affected Products

• Mattermost versions 10.11.x <= 10.11.9

Discovery Timeline

• 2026-02-13 - CVE CVE-2026-20796 published to NVD
• 2026-02-13 - Last updated in NVD database

Technical Details for CVE-2026-20796

Vulnerability Analysis

This vulnerability is classified as CWE-367 (Time-of-Check Time-of-Use Race Condition). The core issue stems from Mattermost's failure to properly validate channel membership at the exact moment when team data is retrieved. When a user is deactivated, there exists a window of opportunity where the authorization check and the actual data retrieval are not atomic operations.

The attack requires network access and low privileges (a previously authenticated, but now deactivated user account). The exploitation complexity is high due to the timing-dependent nature of race conditions, making successful exploitation less predictable. The impact is limited to confidentiality, specifically the disclosure of team names that the deactivated user should no longer be authorized to view.

Root Cause

The vulnerability originates from non-atomic authorization and data retrieval operations in the /common_teams API endpoint. When processing requests, the system performs membership validation as a separate step from the actual data fetch. This separation creates a race window where a deactivated user's request can slip through between the authorization check and the enforcement of account deactivation status, allowing the retrieval of team information.

Attack Vector

The attack is network-based and requires an attacker to have previously held a valid authenticated session. The attacker must time their requests to the /common_teams API endpoint precisely during the race window when their account deactivation is being processed but not yet fully enforced. By exploiting this timing vulnerability, the deactivated user can extract team names from the response before the system correctly blocks their access.

The attack scenario involves a user who has been deactivated from a Mattermost instance but maintains an active session or quickly initiates requests during the deactivation process. The /common_teams endpoint returns team information without re-validating the user's current membership status at the moment of data retrieval.

Detection Methods for CVE-2026-20796

Indicators of Compromise

• Unusual API requests to /common_teams endpoint from recently deactivated user accounts
• Multiple rapid successive requests to team-related endpoints during user deactivation events
• Authentication logs showing session activity from accounts flagged for deactivation
• Anomalous patterns of team information queries correlating with user account status changes

Detection Strategies

• Implement logging correlation between user deactivation events and subsequent API activity from those accounts
• Monitor for /common_teams API calls that occur within a short timeframe of user deactivation events
• Create alerts for API responses containing team data sent to sessions belonging to deactivated users
• Review access logs for timing patterns consistent with race condition exploitation attempts

Monitoring Recommendations

• Enable detailed audit logging for all team-related API endpoints including /common_teams
• Implement real-time correlation rules between user lifecycle events and API access patterns
• Configure alerting thresholds for rapid sequential requests to sensitive endpoints during account state transitions
• Regularly audit session validity checks and ensure they are enforced atomically with data retrieval

How to Mitigate CVE-2026-20796

Immediate Actions Required

• Upgrade Mattermost to a version newer than 10.11.9 that addresses this vulnerability
• Review access logs for any evidence of exploitation attempts targeting the /common_teams endpoint
• Ensure all deactivated user sessions are immediately invalidated upon account deactivation
• Implement additional authorization checks at the data retrieval layer for sensitive endpoints

Patch Information

Mattermost has addressed this vulnerability in versions beyond 10.11.9. Organizations running affected versions should upgrade to the latest available release. For detailed patch information and security advisories, refer to the Mattermost Security Updates page. The vulnerability is tracked under Mattermost Advisory ID MMSA-2025-00549.

Workarounds

• Implement network-level rate limiting on the /common_teams API endpoint to reduce race condition exploitation windows
• Configure immediate session termination upon user deactivation to minimize the attack window
• Add additional middleware authorization checks that validate user status atomically with data access
• Consider implementing a short delay or queue for deactivation processing to ensure all authorization caches are properly invalidated before completing the operation

# Configuration example - Implement session invalidation on user deactivation
# Add to Mattermost config.json to enforce strict session management
# Note: Consult Mattermost documentation for your specific version

# Review and invalidate sessions for deactivated users
mmctl user session list --all | grep "deactivated" | awk '{print $1}' | xargs -I {} mmctl user session revoke {}

# Enable enhanced logging for API access monitoring
# In config.json, ensure audit logging is enabled:
# "ExperimentalAuditSettings": {
#   "FileEnabled": true,
#   "FileName": "mattermost_audit.log"
# }