CVE-2025-55073
📋 TL;DR
This vulnerability allows attackers to edit arbitrary posts in Mattermost by exploiting an improper validation flaw in the MSTeams plugin OAuth flow. Attackers can craft malicious OAuth redirect URLs to modify posts they shouldn't have access to. This affects Mattermost instances running vulnerable versions with the MSTeams plugin enabled.
💻 Affected Systems
- Mattermost
📦 What is this software?
Mattermost Server by Mattermost
Mattermost Server by Mattermost
Mattermost Server by Mattermost
⚠️ Risk & Real-World Impact
Worst Case
An attacker could edit critical posts, spread misinformation, delete important communications, or manipulate sensitive information across all channels and teams.
Likely Case
Attackers edit posts in accessible channels to spread phishing links, malware, or misinformation, potentially leading to credential theft or social engineering attacks.
If Mitigated
With proper access controls and monitoring, impact is limited to unauthorized post edits in channels the attacker already has some access to.
🎯 Exploit Status
Exploitation requires knowledge of post IDs and access to initiate OAuth flow. Attacker must have some level of access to the Mattermost instance.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 10.11.4, 10.5.12, 10.12.1 or later
Vendor Advisory: https://mattermost.com/security-updates
Restart Required: Yes
Instructions:
1. Backup your Mattermost instance. 2. Download and install the patched version from Mattermost downloads page. 3. Restart the Mattermost service. 4. Verify the update was successful.
🔧 Temporary Workarounds
Disable MSTeams Plugin
allTemporarily disable the MSTeams plugin to prevent exploitation until patching is complete.
mmctl plugin disable msteams-sync
Restrict OAuth Access
allLimit OAuth access to trusted IP addresses or networks only.
🧯 If You Can't Patch
- Disable the MSTeams plugin completely until patching is possible
- Implement strict network segmentation to limit access to Mattermost OAuth endpoints
🔍 How to Verify
Check if Vulnerable:
Check Mattermost version via System Console > About or run: mattermost versionCheck Version:
mattermost versionVerify Fix Applied:
Verify version is 10.11.4+, 10.5.12+, or 10.12.1+ and test MSTeams plugin functionality📡 Detection & Monitoring
Log Indicators:
- Unusual post edit patterns from MSTeams plugin users
- Multiple failed OAuth authorization attempts
- Posts edited via MSTeams plugin with unusual timing
Network Indicators:
- Unusual traffic to /oauth/msteams/callback endpoint
- Multiple redirects to MSTeams OAuth URLs
SIEM Query:
source="mattermost" AND (event="post_edited" AND plugin="msteams-sync")