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:
```php
if (0 === \strpos($filename, 'phar://')) {
throw new \InvalidArgumentException('The output file cannot be a phar archive.');
}
```
PHP stream wrappers are **case-insensitive**, so `PHAR://`, `Phar://`, etc. bypass the check and reach `fileExists()` (`file_exists()`) in `prepareOutput()`. On PHP 7 (which the library still supports — PHP 7.4+), this triggers deserialization of a crafted PHAR archive's metadata, leading to remote code execution. This is the patch-bypass of CVE-2023-28115.
The same issue and fix were handled upstream in KnpLabs/snappy ([GHSA-92rv-4j2h-8mjj](https://github.com/KnpLabs/snappy/
security/advisories/GHSA-92rv-4j2h-8mjj)).
### Affected versions
`pontedilana/php-weasyprint` versions `<= 2.5.1` (the case-sensitive guard was introduced in commit `eb8accc`, "Implement countermeasures for CVE-2023-28115").
Patched in: `2.6.0`.
### Privilege required
A caller able to control the output filename passed to `generate()` / `generateFromHtml()`, plus the ability to place a PHAR archive on the filesystem (e.g. via an upload). Exploitation of the deserialization requires the server to run PHP < 8.
### Vulnerable code
`src/AbstractGenerator.php`, `prepareOutput()`:
```php
if (0 === \strpos($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
```bash
# Craft a PHAR with a fast-destruct gadget chain
phpggc -f Monolog/RCE1 exec 'touch /tmp/exploit' -p phar -o exploit.phar
```
```php
generateFromHtml('
POC
', 'PHAR://exploit.phar');
// On PHP /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:
```php
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](https://github.com/KnpLabs/snappy/security/advisories/GHSA-92rv-4j2h-8mjj)); identified as applicable to `pontedilana/php-weasyprint`, which mirrors the same code.
