CVE-2026-55834

### Summary The OIDC authorization page in the pocket-id frontend redirects the browser to an attacker-controlled URL without consulting the backend redirect_uri allow-list when the request uses prompt=none. An attacker who knows a valid client_id can craft an /authorize link that sends a victim (or a victim's browser doing a silent re-auth) to any external https URL, enabling phishing and OAuth response smuggling. The backend allow-list validation that protects the normal authenticated flow is bypassed because this path is handled entirely client-side. ### Details For an OIDC authorization request with prompt=none, the SvelteKit frontend short-circuits the flow before the backend ever validates the redirect_uri against the client's registered callback list. File: frontend/src/routes/authorize/+page.ts (line 14) parses the raw redirect_uri query parameter from the incoming URL. File: frontend/src/routes/authorize/+page.svelte - in onMount(), when the request carries prompt=none and the user cannot be silently authorized (for example the visitor is not logged in, so login_required must be returned), the page builds the callback URL from the raw redirect_uri and performs: window.location.href = `${callbackURL}?error=login_required...` The callbackURL is taken directly from the user-supplied redirect_uri. The only filtering applied is a scheme check that blocks javascript: and data: URLs; any http: or https: origin passes through. The backend allow-list validator GetCallbackURLFromList (which the authenticated code path uses to confirm the redirect_uri matches one of the client's registered callback URLs) is never invoked on this branch. This means the per-client redirect_uri allow-list, the central control that makes redirect_uri safe in OAuth/OIDC, is not enforced for the prompt=none error-return path. A client_id is not a secret: client metadata is retrievable at /api/oidc/clients/:id/meta, and client_ids appear in any integration's authorization links. C