Smarty: SSRF via redirect bypass of trusted_uri using {fetch}

When a Security policy is active, {fetch} validates the requested remote URL against the trusted_uri allowlist via Security::isTrustedUri(). For non-http:// schemes (e.g. https://) the resource was then read with file_get_contents(), which follows HTTP redirects by default. Because isTrustedUri() only validates the initial URL, an open redirect on an otherwise-trusted host could be used to redirect the request to a non-trusted, internal target — bypassing the trusted_uri policy.

Impact

An attacker who can supply a fetch target (or influence one) and who has an open redirect available on a trusted host can cause the server to issue requests to attacker-chosen internal endpoints, defeating the trusted_uri allowlist (server-side request forgery).

Patches

Fixed in 5.8.2. When a security policy is active, {fetch} now passes a stream context that disables redirect following (follow_location => 0, max_redirects => 1) to file_get_contents() for remote resources. Behavior is unchanged when no security policy is set, since there is no trusted_uri to bypass.

Workarounds

Avoid fetching remote resources from within templates under untrusted control; ensure hosts listed in trusted_uri do not expose open redirects.