CVE-2025-55035: Mattermost Desktop App DoS Vulnerability

CVE-2025-55035 Overview

CVE-2025-55035 affects Mattermost Desktop App versions <=5.13.0. The application fails to properly manage modal dialogs when a user connects to a server that uses basic authentication. An attacker who provides a malicious server can force the user to configure it, then trigger a modal popup that cannot be dismissed. This blocks the user from accessing legitimate servers and renders the Desktop App unusable. The flaw is categorized as improper check for unusual or exceptional conditions [CWE-754].

Critical Impact
A malicious server operator can trigger an undismissable authentication modal in the Mattermost Desktop App, denying the user access to the application until manual remediation.

Affected Products

Mattermost Desktop App versions <=5.13.0
Windows, macOS, and Linux Desktop clients built on affected releases
Deployments where users can be directed to attacker-controlled Mattermost servers

Discovery Timeline

2025-10-16 - CVE-2025-55035 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-55035

Vulnerability Analysis

The Mattermost Desktop App presents modal dialogs when a configured server requires basic authentication. In affected versions, the modal lifecycle does not handle exceptional server responses correctly. Once a malicious server triggers the authentication prompt, the modal cannot be closed, dismissed, or bypassed through normal UI actions. The user loses the ability to reach other configured servers or application settings within the same client window.

Exploitation requires user interaction. The victim must add the attacker-controlled server URL to the Desktop App. After the server is configured, the malicious endpoint issues an authentication challenge that produces the persistent modal. The confidentiality and integrity of user data are not directly affected, but availability of the Desktop App is denied.

Root Cause

The root cause is missing handling of exceptional conditions in the modal management logic for basic-auth flows [CWE-754]. The client assumes the auth modal will always resolve through user submission or cancellation. When the server response or dialog state does not follow the expected path, no fallback closes the modal, leaving the UI blocked.

Attack Vector

The attack vector is network-based with required user interaction. An attacker hosts a malicious Mattermost-compatible server that requires basic authentication. Through phishing, social engineering, or an untrusted invite link, the attacker convinces the target to add the server to their Desktop App. Upon connection, the server returns an authentication challenge that the client renders as a modal, which then cannot be closed.

No verified public exploit code is available for this issue. See the Mattermost Security Updates advisory for vendor-provided technical details.

Detection Methods for CVE-2025-55035

Indicators of Compromise

User reports of a Mattermost Desktop App window locked on an authentication prompt that cannot be dismissed.
Recently added Mattermost server entries pointing to unfamiliar or untrusted domains in user configuration files.
Outbound connections from the Mattermost Desktop process to non-corporate Mattermost server hostnames.

Detection Strategies

Inventory installed Mattermost Desktop App versions across the fleet and flag any at or below 5.13.0.
Monitor endpoint telemetry for the Mattermost Desktop process initiating HTTP basic-auth handshakes against unapproved external hosts.
Correlate helpdesk tickets referencing frozen Mattermost windows with recent server configuration changes.

Monitoring Recommendations

Alert on new Mattermost server URLs added to user profiles that do not match approved corporate domains.
Log DNS queries and HTTPS connections from Mattermost Desktop clients to detect connections to attacker-controlled infrastructure.
Track application crash and hang events tied to the Mattermost Desktop binary in endpoint logs.

How to Mitigate CVE-2025-55035

Immediate Actions Required

Upgrade Mattermost Desktop App to a version later than 5.13.0 as published in the vendor security advisory.
Instruct users to only add Mattermost servers provided by IT or trusted collaboration partners.
Terminate the Mattermost Desktop process from the operating system task manager when a locked modal is observed, then remove the offending server entry.

Patch Information

Mattermost has released fixed builds addressed in the Mattermost Security Updates advisory. Administrators should review the advisory for the exact fixed version and roll out the update through standard software distribution channels.

Workarounds

Restrict the list of allowed Mattermost server URLs through managed configuration or group policy where supported.
Block outbound connections from managed endpoints to non-corporate Mattermost server domains at the network perimeter.
If a modal lock occurs, close the Desktop App via the OS process manager and edit the local Mattermost configuration file to remove the malicious server before restarting.

bash

# Example: remove a malicious server entry from Mattermost Desktop config on Linux
# Close the app first, then edit the config file
cp ~/.config/Mattermost/config.json ~/.config/Mattermost/config.json.bak
jq 'del(.teams[] | select(.url == "https://malicious.example.com"))' \
  ~/.config/Mattermost/config.json.bak > ~/.config/Mattermost/config.json