CVE-2025-24024: Mjolnir Auth Bypass Vulnerability

CVE-2025-24024 Overview

CVE-2025-24024 is an authorization bypass vulnerability in Mjolnir, a moderation tool for the Matrix decentralized communication protocol. Version 1.9.0 of Mjolnir contains a critical flaw where the bot responds to management commands from any room it is a member of, rather than restricting command execution to authorized operators. This vulnerability allows unauthorized users to execute bot functions, including potentially sensitive server administration components if enabled.

Critical Impact

Unauthorized users can execute administrative commands through the Mjolnir bot, potentially gaining control over server moderation functions and administrative capabilities without proper authorization.

Affected Products

• Mjolnir v1.9.0
• Matrix server deployments using vulnerable Mjolnir versions
• Matrix rooms where Mjolnir bot is a member

Discovery Timeline

• 2025-01-21 - CVE CVE-2025-24024 published to NVD
• 2025-01-21 - Last updated in NVD database

Technical Details for CVE-2025-24024

Vulnerability Analysis

This vulnerability stems from improper control of interaction frequency (CWE-671), where the Mjolnir bot fails to properly validate the source of management commands. In version 1.9.0, a feature was introduced that inadvertently allowed the bot to process commands from any Matrix room it had joined, bypassing the intended authorization model that should restrict command processing to designated management rooms and authorized operators only.

The flaw is particularly severe because Mjolnir is designed to perform privileged moderation actions across Matrix servers, including user bans, room management, and when enabled, server administration tasks. An attacker with knowledge of Mjolnir commands who shares any room with the bot could leverage these capabilities without being a designated operator.

Root Cause

The root cause lies in the introduction of a "forward mentions to management room" feature in version 1.9.0. This feature, implemented via the forwardMentionsToManagementRoom configuration option, created a code path that did not properly validate whether incoming commands originated from authorized sources. The configuration change inadvertently expanded the attack surface by processing commands from any room context.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker needs only to be in a Matrix room where the vulnerable Mjolnir bot is also a member. From there, the attacker can issue management commands that the bot will execute, despite the attacker not being a designated operator. This could enable unauthorized moderation actions, user bans, or access to server administration functions if those capabilities are enabled.

// Security patch in src/config.ts - Revert "forward mentions to management room" feature (#565)
    acceptInvitesFromSpace: string;
    recordIgnoredInvites: boolean;
    managementRoom: string;
-    forwardMentionsToManagementRoom: boolean;
    verboseLogging: boolean;
    logLevel: "DEBUG" | "INFO" | "WARN" | "ERROR";
    syncOnStartup: boolean;

Source: GitHub Commit Updates

Detection Methods for CVE-2025-24024

Indicators of Compromise

• Unexpected moderation actions (bans, kicks, ACL changes) in Matrix rooms not initiated by authorized operators
• Mjolnir command responses appearing in non-management rooms
• Audit logs showing bot commands originating from unauthorized users or rooms
• Unusual bot activity patterns or commands from rooms outside the designated management space

Detection Strategies

• Review Mjolnir bot logs for commands processed from rooms other than the designated management room
• Monitor Matrix server audit logs for moderation actions that don't correlate with authorized operator activity
• Implement alerting on administrative commands executed through the bot
• Compare the room ID of command sources against the configured managementRoom value

Monitoring Recommendations

• Enable verbose logging on Mjolnir instances to capture all command processing events
• Set up alerts for any server administration commands if that feature is enabled
• Monitor for unexpected changes to room ACLs, user bans, or server-level configurations
• Regularly audit which rooms the Mjolnir bot has joined to minimize exposure

How to Mitigate CVE-2025-24024

Immediate Actions Required

• Upgrade Mjolnir to version 1.9.1 or 1.9.2 immediately
• If upgrade is not possible, downgrade to version 1.8.3 which does not contain the vulnerable feature
• Audit bot membership and remove the bot from any unnecessary rooms
• Review recent moderation actions for signs of unauthorized command execution

Patch Information

The Matrix.org Foundation has addressed this vulnerability through two commits. Version 1.9.1 reverts the vulnerable feature entirely, while version 1.9.2 reintroduces the feature with proper authorization controls. Organizations should update to version 1.9.2 for full functionality with security fixes, or 1.9.1 if immediate patching is required without the forwarding feature. The relevant patches are available in the GitHub Security Advisory GHSA-3jq6-xc85-m394.

Workarounds

• Downgrade to Mjolnir version 1.8.3 if upgrading to 1.9.1 or higher is not immediately feasible
• Restrict the rooms that Mjolnir has joined to minimize exposure
• Disable server administration components until patching is complete
• Implement network-level controls to limit access to the Matrix server hosting Mjolnir

# Configuration example
# Update Mjolnir to a patched version
cd /path/to/mjolnir
git fetch --tags
git checkout v1.9.2
npm install
npm run build

# Or downgrade to safe version if upgrade isn't possible
git checkout v1.8.3
npm install
npm run build