CVE-2026-55671: ZITADEL: Server-Side Request Forgery (SSRF) and Denylist Bypass in Outgoing HTTP Components

A Server-Side Request Forgery (SSRF) vulnerability was discovered in Zitadel affecting:

• HTTP Notification Channels: Used as an alternative to SMTP/Twilio configurations, sending payloads to user-defined URLs via HTTP POST webhooks.
• OIDC BackChannel Logout: Terminates sessions across different applications. When a session ends, the Zitadel server sends an HTTP POST request to configured endpoints.
• SAML Metadata URL Fetches: Fetches SAML metadata configurations from user-provided external URLs.

User-defined URLs in these components were not properly validated against an internal denylist, allowing potentially malicious URLs to bypass restrictions. Furthermore, the existing denylist mechanism previously introduced for Actions was found to be vulnerable to DNS rebinding, HTTP redirects, and protocol downgrades (HTTPS to HTTP), and it missed several common local network default entries.

Because an attacker can supply arbitrary URLs—including loopback addresses, internal IPs, or cloud link-local addresses—they could potentially gather internal network architecture details, scan internal ports, or interact with unauthorized internal services and infrastructure.