Dompdf vulnerable to URI validation failure on SVG parsing

Original text by Blaklis


The URI validation on dompdf 2.0.1 can be bypassed on SVG parsing by passing 

 tags with uppercase letters. This might leads to arbitrary object unserialize on PHP < 8, through the 
 URL wrapper.


The bug occurs during SVG parsing of 

 tags, in src/Image/Cache.php :

if ($type === "svg") {
    $parser = xml_parser_create("utf-8");
    xml_parser_set_option($parser, XML_OPTION_CASE_FOLDING, false);
        function ($parser, $name, $attributes) use ($options, $parsed_url, $full_url) {
            if ($name === "image") {
                $attributes = array_change_key_case($attributes, CASE_LOWER);

This part will try to detect 

 tags in SVG, and will take the href to validate it against the protocolAllowed whitelist. However, the `$name comparison with «image» is case sensitive, which means that such a tag in the SVG will pass :

    <Image xlink:href="phar:///foo"></Image>

As the tag is named «Image» and not «image», it will not pass the condition to trigger the check.

A correct solution would be to strtolower the 

 before the check :

if (strtolower($name) === "image") {


Parsing the following SVG file is sufficient to reproduce the vulnerability :

    <Image xlink:href="phar:///foo"></Image>


An attacker might be able to exploit the vulnerability to call arbitrary URL with arbitrary protocols, if they can provide a SVG file to dompdf. In PHP versions before 8.0.0, it leads to arbitrary unserialize, that will leads at the very least to an arbitrary file deletion, and might leads to remote code execution, depending on classes that are available.