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

Summary

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.

Impact

When a user-supplied URL points to a local host or internal IP address, an adversary can perform a Server-Side Request Forgery (SSRF) attack. This allows them to map internal network structures and exploit exposed internal services.

By leveraging DNS rebinding, an attacker could also bypass standard DNS-level checks, creating Time-of-Check to Time-of-Use (TOCTOU) gaps to access restricted internal endpoints. Additionally, vulnerabilities to HTTP redirects and protocol downgrades could allow attackers to manipulate the request flow or intercept sensitive communication.

Notably, if Zitadel is deployed within cloud environments (such as AWS, GCP, or Azure) that still permit legacy IMDSv1 or unauthenticated cloud metadata endpoints (169.254.169.254), an attacker could theoretically attempt to target these metadata services.

While Zitadel expects specific schemas or response formats for these features (which inherently limits data exfiltration capabilities and reduces the severe execution of the threat vector), users are strongly advised to patch immediately.

Affected Versions

Systems running one of the following versions are affected:

• 4.x: 4.0.0 through 4.15.1 (including RC versions)
• 3.x: 3.0.0 through (including RC versions)