CVE-2026-24692: Mattermost Server Info Disclosure Flaw

CVE-2026-24692 Overview

CVE-2026-24692 is an Improper Authorization vulnerability affecting Mattermost Server that allows guest users to bypass read permissions and access unauthorized posts and files through the search API. This broken access control flaw enables users with limited privileges to retrieve sensitive information from channels they should not have access to by crafting specific search API requests.

Critical Impact

Guest users can access confidential posts and files in restricted channels, potentially exposing sensitive organizational communications and data through unauthorized search queries.

Affected Products

• Mattermost Server versions 11.3.x through 11.3.0
• Mattermost Server versions 11.2.x through 11.2.2
• Mattermost Server versions 10.11.x through 10.11.10

Discovery Timeline

• 2026-03-16 - CVE-2026-24692 published to NVD
• 2026-03-18 - Last updated in NVD database

Technical Details for CVE-2026-24692

Vulnerability Analysis

This vulnerability stems from improper enforcement of read permissions within Mattermost's search API endpoints. The search functionality fails to adequately validate whether the requesting user has the necessary permissions to view content from specific channels before returning search results.

When a guest user executes a search query through the API, the system does not properly filter results based on channel-level read permissions. This allows authenticated guest users—who are typically restricted to specific channels—to retrieve posts and files from channels they have not been granted access to.

The vulnerability is classified under CWE-863 (Incorrect Authorization), indicating a fundamental flaw in the access control logic where authorization checks are either missing or improperly implemented in the search code path. While standard channel access may be properly restricted through the user interface, the search API represents an alternative pathway that bypasses these controls.

Root Cause

The root cause lies in the search API's failure to incorporate channel-level read permission checks when processing and returning search results. The search functionality appears to index and query content without adequately filtering based on the requesting user's actual permission scope. Guest users, who should have limited visibility into organizational content, can leverage this oversight to access data beyond their authorized scope.

Attack Vector

An attacker with guest-level access to a Mattermost instance can exploit this vulnerability by sending crafted search API requests. The attack requires network access and a valid guest account (low privilege level). The attacker can systematically search for keywords, usernames, or file types to extract sensitive information from channels they cannot normally access through the standard user interface.

The attack does not require any user interaction and can be executed programmatically to harvest large amounts of data. While the confidentiality impact is limited to read-only access, this can expose sensitive business communications, credentials shared in private channels, or confidential documents.

Detection Methods for CVE-2026-24692

Indicators of Compromise

• Unusual search API request volume from guest user accounts
• Search queries from guest users returning results from channels outside their assigned permissions
• API access logs showing guest users querying restricted channel content
• Anomalous data access patterns where guest accounts retrieve significantly more content than expected

Detection Strategies

• Monitor Mattermost API logs for search endpoint requests (/api/v4/posts/search, /api/v4/files/search) from guest accounts
• Implement log correlation to identify guest users accessing content from channels not in their permission scope
• Deploy application-layer monitoring to track search query patterns and result sets
• Configure alerts for guest accounts exhibiting elevated search activity compared to baseline behavior

Monitoring Recommendations

• Enable detailed API request logging in Mattermost to capture search queries and responses
• Implement SIEM rules to correlate guest user identities with accessed channel IDs
• Establish baseline metrics for normal guest user search behavior to identify anomalies
• Review access logs periodically for guest accounts with broad search patterns

How to Mitigate CVE-2026-24692

Immediate Actions Required

• Upgrade Mattermost Server to the latest patched version immediately
• Audit guest user accounts and their recent search activity for potential data exposure
• Review sensitive channels for any unauthorized access indicators
• Consider temporarily restricting guest user API access until patching is complete

Patch Information

Mattermost has released security updates addressing this vulnerability. Administrators should upgrade to versions newer than 11.3.0, 11.2.2, and 10.11.10 respectively for their version branch. Refer to the Mattermost Security Updates page for specific patched version numbers and upgrade instructions. The advisory is tracked under Mattermost Advisory ID: MMSA-2025-00554.

Workarounds

• Restrict or disable guest account access to the search API at the reverse proxy or API gateway level
• Implement network segmentation to limit guest user access to the Mattermost API
• Review and minimize guest user permissions across the platform
• Consider disabling guest accounts temporarily if they are not business-critical

# Example: Restrict search API access for guest users via nginx
# Add to your Mattermost nginx configuration
location ~ ^/api/v4/(posts|files)/search {
    # Block requests from guest user sessions
    # Implement session validation logic here
    # This is a temporary workaround until patching is complete
    deny all;
}