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

Critical severity GitHub Reviewed Published Feb 7, 2023 in dompdf/dompdf • Updated Feb 24, 2023

Package

composer dompdf/dompdf (Composer)

Affected versions

= 2.0.2

Patched versions

2.0.3

Description

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 used to parse the svg file, parses the href attribute with the following code:

src/Svg/Tag/Image.php line 51-57

if (isset($attributes['xlink:href'])) {
    $this->href = $attributes['xlink:href'];
}

if (isset($attributes['href'])) {
    $this->href = $attributes['href'];
}

Since href is respected if both xlink:href and href is specified, it's possible to bypass the protection on the Dompdf side by providing an empty xlink:href attribute.

Impact

An attacker can exploit the vulnerability to call arbitrary URLs with arbitrary protocols if they provide an SVG file to the Dompdf. In PHP versions before 8.0.0, it leads to arbitrary unserialize, which will lead, at the very least, to arbitrary file deletion and might lead to remote code execution, depending on available classes.

References

@bsweeney bsweeney published to dompdf/dompdf Feb 7, 2023
Published to the GitHub Advisory Database Feb 7, 2023
Reviewed Feb 7, 2023
Published by the National Vulnerability Database Feb 7, 2023
Last updated Feb 24, 2023

Severity

Critical

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

EPSS score

0.451%
(76th percentile)

Weaknesses

CVE ID

CVE-2023-24813

GHSA ID

GHSA-56gj-mvh6-rp75

Source code

Credits

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