URI validation failure on SVG parsing. Bypass of CVE-2023-23924

Summary

Due to the difference in the attribute parser of Dompdf and php-svg-lib, an attacker can still call arbitrary URLs with arbitrary protocols.

Details

Dompdf parses the href attribute of image tags with the following code:

src/Image/Cache.php line 135-150

function ($parser, $name, $attributes) use ($options, $parsed_url, $full_url) {
    if (strtolower($name) === "image") {
        $attributes = array_change_key_case($attributes, CASE_LOWER);
        $url = $attributes["xlink:href"] ?? $attributes["href"];
        if (!empty($url)) {
            $inner_full_url = Helpers::build_url($parsed_url["protocol"], $parsed_url["host"], $parsed_url["path"], $url);
            if ($inner_full_url === $full_url) {
                throw new ImageException("SVG self-reference is not allowed", E_WARNING);
            }
            [$resolved_url, $type, $message] = self::resolve_url($url, $parsed_url["protocol"], $parsed_url["host"], $parsed_url["path"], $options);
            if (!empty($message)) {
                throw new ImageException("This SVG document references a restricted resource. $message", E_WARNING);
            }
        }
    }
},

As you can see from the code snippet above, it respects xlink:href even if href is specified.

$url = $attributes["xlink:href"] ?? $attributes["href"];

However, php-svg-lib, which is later…