=====================================================================

                              CERT-Renater

                   Note d'Information No. 2023/VULN044

_____________________________________________________________________

DATE                : 02/02/2023

HARDWARE PLATFORM(S): /

OPERATING SYSTEM(S): Systems running dompdf versions 2.0.1.

=====================================================================
https://github.com/dompdf/dompdf/security/advisories/GHSA-3cw5-7cxw-v5qg
_____________________________________________________________________

URI validation failure on SVG parsing
Critical
bsweeney published GHSA-3cw5-7cxw-v5qg


Package
dompdf (Composer)

Affected versions
2.0.1

Patched versions
None


Description

Summary

The URI validation on dompdf 2.0.1 can be bypassed on SVG
parsing by passing <image> tags with uppercase letters.
This might leads to arbitrary object unserialize on
PHP < 8, through the phar URL wrapper.


Details

The bug occurs during SVG parsing of <image> 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);
     xml_set_element_handler(
         $parser,
         function ($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 <image> 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 :

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

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 $name before
the check :

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

PoC

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

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


Impact

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.


Severity
Critical

10.0/ 10

CVSS base metrics

Attack vector
Network

Attack complexity
Low

Privileges required
None

User interaction
None

Scope
Changed

Confidentiality
High

Integrity
Low

Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:H

CVE ID
CVE-2023-23924

Weaknesses
CWE-551

Credits

     @Blaklis Blaklis

=========================================================
+ CERT-RENATER        |    tel : 01-53-94-20-44         +
+ 23/25 Rue Daviel    |    fax : 01-53-94-20-41         +
+ 75013 Paris         |   email:cert@support.renater.fr +
=========================================================