CVE-2026-1629: Mattermost Server Information Disclosure

CVE-2026-1629 Overview

CVE-2026-1629 is a medium-severity information disclosure vulnerability in Mattermost Server that allows users to continue viewing private channel content after losing access. The vulnerability exists in Mattermost versions 10.11.x through 10.11.10, where the application fails to properly invalidate cached permalink preview data when a user's channel access is revoked.

When users access permalink previews in Mattermost, the application caches this data for performance optimization. However, the cache invalidation mechanism does not account for access control changes, allowing previously authorized users to continue viewing sensitive private channel content through their browser or client cache until a cache reset or re-login occurs.

Critical Impact

Users who have lost access to private channels can continue viewing confidential information through cached permalink previews, potentially exposing sensitive organizational communications and data.

Affected Products

• Mattermost Server versions 10.11.x through 10.11.10
• Mattermost Server deployments using permalink preview functionality

Discovery Timeline

• 2026-03-16 - CVE-2026-1629 published to NVD
• 2026-03-18 - Last updated in NVD database

Technical Details for CVE-2026-1629

Vulnerability Analysis

This vulnerability represents a classic Operation on a Resource after Expiration or Release issue (CWE-672). The core problem lies in the disconnect between Mattermost's access control system and its caching layer for permalink previews.

When a user accesses a permalink to a message in a private channel, Mattermost generates a preview of that content and caches it client-side for improved performance. The access check occurs at the time of the initial request, and the cached data persists beyond any subsequent access revocations.

The vulnerability is exploitable over the network and requires low-privileged authenticated access. An attacker needs only basic user credentials and prior legitimate access to the target channel. Once access is revoked—whether due to leaving a team, being removed from a private channel, or role changes—the cached preview data remains accessible.

Root Cause

The root cause is improper cache management in Mattermost's permalink preview system. Specifically, the application does not implement cache invalidation hooks tied to access control changes. When the removeUserFromChannel or similar access revocation functions execute, they do not trigger corresponding cache purge operations for affected preview data.

This design flaw means that cached permalink previews operate independently of the real-time permission system, creating a window where stale authorization decisions continue to grant access to sensitive content.

Attack Vector

The attack vector requires an authenticated user who previously had legitimate access to a private channel. The exploitation scenario unfolds as follows:

1. User A is a member of a private channel and accesses permalink previews for messages in that channel
2. User A's access to the private channel is revoked by an administrator
3. User A can continue viewing the cached permalink preview data despite no longer having channel access
4. The cached data remains accessible until User A manually clears their cache or logs out and back in

This vulnerability is particularly concerning in scenarios involving employee offboarding, contractor access termination, or organizational restructuring where timely access revocation is critical.

Detection Methods for CVE-2026-1629

Indicators of Compromise

• Unusual permalink preview access patterns from users who no longer have channel membership
• Audit log entries showing permalink access after access revocation events
• Cache hit metrics for preview data associated with revoked user sessions

Detection Strategies

• Monitor Mattermost audit logs for permalink access events correlated with recent access revocation changes
• Implement alerting on cache access patterns that occur after removeUserFromChannel API calls
• Review session activity for users whose channel memberships have been recently modified

Monitoring Recommendations

• Enable verbose logging for permalink preview requests and cache operations
• Cross-reference channel membership changes with subsequent preview access events
• Implement periodic cache integrity checks that validate user permissions against cached preview data

How to Mitigate CVE-2026-1629

Immediate Actions Required

• Update Mattermost Server to a patched version beyond 10.11.10
• Force all users to re-authenticate to clear cached session data
• Review recent access revocations and assess potential data exposure
• Clear server-side caches if applicable to your deployment configuration

Patch Information

Mattermost has released security updates addressing this vulnerability. Organizations should upgrade to the latest available version. Detailed patch information is available at the Mattermost Security Updates page. The associated advisory ID is MMSA-2026-00580.

Workarounds

• Implement a policy requiring users to log out and back in after access changes
• Configure shorter cache TTL values to reduce the exposure window
• Educate administrators to manually trigger cache invalidation after sensitive access revocations
• Consider disabling permalink previews in environments with strict data compartmentalization requirements

# Force cache invalidation by requiring re-authentication
# In Mattermost System Console or config.json
# Reduce session length to minimize cache exposure window
"ServiceSettings": {
    "SessionLengthWebInHours": 24,
    "SessionCacheInMinutes": 10
}