CVE-2025-62690: 
vulnerability analysis and mitigation

CVE-2025-62690 is an open redirect vulnerability in Mattermost Server affecting versions 10.11.0 through 10.11.4 (inclusive). The /error page fails to validate redirect URLs, allowing an unauthenticated attacker to craft a malicious link that redirects a victim to an untrusted external site when opened in a new tab. The vulnerability was published on December 17, 2025. It carries a CVSS v3.1 base score of 3.1 (Low) per the GitHub Advisory, though NVD rates it 6.1 (Medium) due to a differing scope assessment (GitHub Advisory, Red Hat CVE).

The root cause is classified as CWE-601 (URL Redirection to Untrusted Site / Open Redirect): the application accepts a user-controlled URL parameter on the /error page without validating whether the destination is a trusted or internal resource. An attacker can craft a Mattermost /error page URL containing a malicious external redirect target and distribute it as a seemingly legitimate link. Exploitation requires user interaction — specifically, the victim must open the crafted link in a new tab — and no authentication or special privileges are needed on the attacker's part. The fix was introduced in commit dad6bd7 in the Mattermost repository (GitHub Advisory).

Successful exploitation enables phishing and social engineering attacks by silently redirecting users from a trusted Mattermost domain to an attacker-controlled site, which can be used for credential harvesting or malware delivery. The vulnerability has low confidentiality and low integrity impact with no availability impact, and its scope is limited to user deception rather than direct system compromise. There is no risk of remote code execution, privilege escalation, or lateral movement from this vulnerability alone (GitHub Advisory, Red Hat CVE).

1. Identify target: Confirm the target organization is running Mattermost Server versions 10.11.0–10.11.4.
2. Craft malicious URL: Construct a URL targeting the Mattermost /error page with a redirect parameter pointing to an attacker-controlled site, e.g., https://<mattermost-instance>/error?link=https://attacker.example.com.
3. Distribute link: Send the crafted URL to a victim via email, chat, or another channel, leveraging the trusted Mattermost domain to increase credibility.
4. Victim interaction: The victim opens the link in a new tab, triggering the unvalidated redirect and landing on the attacker's malicious site.
5. Achieve objective: The attacker's site presents a phishing page (e.g., fake login form) to harvest credentials or deliver malware (GitHub Advisory).

• Network: Outbound HTTP redirects from the Mattermost /error endpoint to external, non-organizational domains in web proxy or firewall logs.
• Logs: Mattermost access logs showing requests to /error with unexpected external URL values in query parameters (e.g., link=, redirect=, or similar).
• User Reports: End users reporting unexpected redirects to unfamiliar external sites after clicking Mattermost links.

Upgrade Mattermost Server to version 10.11.5 or later (Go module patched version: 10.11.5-0.20251016131338-dad6bd7a1509), or to version 11.1.0 or later for the v11 branch. No configuration-based workaround has been officially documented; upgrading is the recommended remediation. As an additional defense-in-depth measure, deploying a WAF rule to block or alert on /error page requests containing external URLs can reduce exposure until patching is feasible (GitHub Advisory, Mattermost Security Updates).