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The vulnerability in Mattermost arises from insufficient validation of redirect URLs on the /error page in affected versions. When an error occurs, the application uses a redirect parameter in the URL to send users to another page. However, this parameter is not properly sanitized or validated, allowing an attacker to craft a malicious link with an external URL. If a victim clicks this link and opens it in a new tab, the Mattermost instance will redirect the browser to the attacker-controlled site. This open redirect flaw exploits the trust associated with the Mattermost domain, as users may believe they are on a legitimate page. The attack requires user interaction, such as clicking a phishing link shared via chat or email. The vulnerability is due to missing server-side checks that should restrict redirects to allowed domains only. In code terms, the error handler directly uses user input from the query string without verification. This can lead to HTTP response headers like Location being set to malicious destinations. Attackers can use this to conduct phishing campaigns, steal credentials, or distribute malware by mimicking trusted interfaces. The issue is mitigated in patched versions by adding validation logic to ensure redirects remain within the same origin or approved list. Without the patch, the flaw persists, enabling redirection to any URL, including those with harmful payloads. It highlights common web security oversights in error handling pathways. Administrators should update to fixed versions to prevent exploitation. The low severity reflects the need for social engineering and limited direct access to data.
Platform: Mattermost
Version: 10.11.x, 11.0.x
Vulnerability: Missing URL validation
Severity: Low
Date: Dec 17, 2025
Prediction: Patched Oct 16, 2025
What Undercode Say:
Analytics:
curl -v "https://mattermost-server/error?redirect=https://evil.com"
grep "version" /opt/mattermost/config/config.json
import requests
url = "http://target/error?redirect=http://malicious.net"
resp = requests.get(url, allow_redirects=False)
print(resp.headers.get('Location'))
how Exploit:
Craft phishing link with redirect parameter.
Share link via Mattermost chat.
Victim opens in new tab.
Redirect to malicious site.
Protection from this CVE:
Update to patched versions.
Implement URL allowlisting.
Validate redirect domains.
Impact:
Phishing attacks.
Credential theft.
Reputation damage.
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Sources:
Reported By: github.com
Extra Source Hub:
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