CVE-2025-55035
📋 TL;DR
This vulnerability in Mattermost Desktop App allows attackers to create a denial-of-service condition by tricking users into configuring a malicious server. When users connect to a malicious server using basic authentication, an unclosable modal popup appears, preventing them from using the desktop application. This affects all Mattermost Desktop App users running versions 5.13.0 or earlier.
💻 Affected Systems
- Mattermost Desktop App
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete denial-of-service for the Mattermost Desktop App, preventing legitimate users from accessing their workspace through the desktop client until the malicious server configuration is removed.
Likely Case
Temporary disruption of desktop app access requiring users to manually remove the malicious server configuration or reinstall the application.
If Mitigated
Minimal impact if users only connect to trusted servers and avoid configuring unknown server URLs.
🎯 Exploit Status
Exploitation requires social engineering to trick users into configuring a malicious server URL. The attacker must host a server that triggers the modal issue.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 5.14.0 or later
Vendor Advisory: https://mattermost.com/security-updates
Restart Required: No
Instructions:
1. Download Mattermost Desktop App version 5.14.0 or later from the official website. 2. Install the update over your existing installation. 3. No server restart required.
🔧 Temporary Workarounds
Remove malicious server configuration
allManually remove the malicious server configuration from the Mattermost Desktop App settings
Navigate to Settings > Servers, select the malicious server, and click 'Remove'
Use web client temporarily
allAccess Mattermost through the web browser instead of the desktop app until patched
🧯 If You Can't Patch
- Restrict users to only connecting to approved, trusted server URLs
- Educate users about the risks of configuring unknown servers and implement policies against adding unapproved servers
🔍 How to Verify
Check if Vulnerable:
Check the Mattermost Desktop App version in Settings > About. If version is 5.13.0 or earlier, the system is vulnerable.Check Version:
In Mattermost Desktop App: Settings > About > VersionVerify Fix Applied:
After updating, verify the version shows 5.14.0 or later in Settings > About. Test connecting to servers to ensure modals function properly.📡 Detection & Monitoring
Log Indicators:
- Multiple failed authentication attempts from desktop clients
- Users reporting unclosable modal windows
Network Indicators:
- Desktop clients connecting to unknown or suspicious server domains
SIEM Query:
source="mattermost-desktop" AND (event="authentication_failure" OR event="server_connection_error")