CVE-2025-12756 Overview
CVE-2025-12756 is an authorization bypass vulnerability affecting Mattermost Server that allows authenticated users with the editor role to delete comments created by other users in Boards. The vulnerability stems from improper validation of user permissions when processing comment deletion requests, enabling unauthorized modification of collaborative workspace content.
This Broken Access Control vulnerability (CWE-863) allows attackers with legitimate but limited access to exceed their intended permissions and impact the integrity of team collaboration data.
Critical Impact
Authenticated users with editor permissions can delete any comment in Boards, regardless of the original author, potentially leading to data loss, manipulation of audit trails, and disruption of team collaboration workflows.
Affected Products
- Mattermost Server versions 11.0.x through 11.0.2
- Mattermost Server versions 10.12.x through 10.12.1
- Mattermost Server versions 10.11.x through 10.11.4
- Mattermost Server versions 10.5.x through 10.5.12
Discovery Timeline
- 2025-12-01 - CVE-2025-12756 published to NVD
- 2025-12-05 - Last updated in NVD database
Technical Details for CVE-2025-12756
Vulnerability Analysis
The vulnerability exists in the Mattermost Boards feature, specifically in the comment deletion functionality. With a CVSS v3.1 score of 4.3 (Medium) and vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N, this vulnerability requires low privileges and no user interaction to exploit over a network connection.
The EPSS (Exploit Prediction Scoring System) score is 0.025% (6.28th percentile), indicating a relatively low probability of exploitation in the wild. However, the ease of exploitation once authenticated makes this a concern for organizations with many users.
The attack complexity is low, meaning that once an attacker has editor-level access, exploitation is straightforward. The scope is unchanged, affecting only the vulnerable component without impacting other system components.
Root Cause
The root cause is classified as CWE-863: Incorrect Authorization. The Mattermost Server fails to properly validate whether the requesting user has ownership or administrative rights over a comment before processing the deletion request. The authorization logic does not adequately distinguish between a user's ability to delete their own comments versus comments authored by other users.
When processing a comment deletion request in the Boards module, the server verifies that the user has editor permissions on the board but does not perform an additional check to confirm the user is the comment's author or has elevated administrative privileges.
Attack Vector
The attack is network-based and requires low-privilege authenticated access. An attacker would:
- Authenticate to the Mattermost instance with valid credentials
- Navigate to a Board where they have editor-level access
- Identify target comments created by other users
- Issue deletion requests for those comments through the Boards API
The deletion request would be processed successfully despite the user not being the comment author, as the permission validation only checks for editor-level board access rather than comment ownership.
Detection Methods for CVE-2025-12756
Indicators of Compromise
- Unusual patterns of comment deletions in Boards audit logs
- High volume of comment deletions from a single user account
- Comment deletions where the deleting user differs from the comment author
- Anomalous API activity targeting the Boards comment deletion endpoint
Detection Strategies
Organizations should implement monitoring for the Boards comment deletion API endpoint. Specifically, correlation of delete operations with comment authorship can identify exploitation attempts. Database query logging that tracks DELETE operations on comment tables, compared against the authenticated user session, can reveal unauthorized deletions.
Security Information and Event Management (SIEM) systems should be configured to alert on patterns where users delete comments they did not author, particularly when this occurs at scale or during unusual hours.
Monitoring Recommendations
Enable comprehensive audit logging for all Mattermost Boards operations. Configure alerts for:
- Multiple comment deletions within a short time window
- Comment deletions where metadata shows different user IDs for author and deleter
- Unusual access patterns to Boards by service accounts or recently created accounts
Review Mattermost server logs for API requests to comment deletion endpoints and cross-reference with user permission levels and comment ownership data.
How to Mitigate CVE-2025-12756
Immediate Actions Required
- Upgrade Mattermost Server to the latest patched version immediately
- Audit recent comment deletion activity in Boards for signs of exploitation
- Review and restrict editor-level permissions to trusted users only
- Consider temporarily disabling Boards functionality if upgrade is not immediately possible
Patch Information
Mattermost has released security updates to address this vulnerability. Organizations should upgrade to versions beyond the affected ranges:
- Version 11.0.x - Upgrade to 11.0.3 or later
- Version 10.12.x - Upgrade to 10.12.2 or later
- Version 10.11.x - Upgrade to 10.11.5 or later
- Version 10.5.x - Upgrade to 10.5.13 or later
Refer to the official Mattermost security advisory at https://mattermost.com/security-updates for complete patch information and upgrade instructions.
Workarounds
If immediate patching is not feasible, organizations can implement the following temporary mitigations:
Restrict editor-level access in Boards to only essential personnel and implement additional manual review processes for board membership changes. Organizations may also consider temporarily disabling the Boards feature until the patch can be applied.
# Review current Boards permissions via Mattermost CLI
mattermost permissions list --plugin boards
# Audit recent comment deletions (requires database access)
# Check Mattermost audit logs for comment deletion events
grep -i "delete" /opt/mattermost/logs/mattermost.log | grep -i "comment"
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
