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

Smarty is a template engine for PHP, facilitating the separation of presentation (HTML/CSS) from application logic. Prior to 4.5.7 and 5.8.2, depending on the release line, Smarty's {fetch} handling in libs/plugins/function.fetch.php and src/FunctionHandler/Fetch.php used Security::isTrustedUri() to validate only the initial remote URL against trusted_uri when a security policy was active. For resources handled by file_get_contents(), including HTTPS URLs, PHP followed HTTP redirects by default. An attacker who could supply or influence a fetch target and had an open redirect on a trusted host could redirect the request to an attacker-chosen internal endpoint, bypass the trusted_uri allowlist, and perform server-side request forgery. This issue is fixed in versions 4.5.7 and 5.8.2.

The server follows an attacker-influenced destination, allowing requests to systems or services the attacker cannot reach directly.

An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.