PhpWeasyPrint vulnerable to SSRF and local file disclosure via the attachment option

Summary

pontedilana/php-weasyprint fetches the content of option values server-side via file_get_contents() when the value looks like a URL, without restricting the URL scheme. The attachment option of Pdf is the reachable sink: any value that passes isOptionUrl() (filter_var(..., FILTER_VALIDATE_URL)) is downloaded by the PHP process and embedded into the generated PDF. Because FILTER_VALIDATE_URL accepts http, https, ftp, file and PHP stream wrappers such as php://, an attacker who can influence the attachment value reaches both a Server-Side Request Forgery primitive (e.g. internal HTTP endpoints, cloud metadata) and a local file disclosure primitive (file://, php://filter/...), with the fetched bytes exfiltrated as a PDF attachment.

This is the same class of issue KnpLabs/snappy patched for its xsl-style-sheet option in GHSA-c5fp-p67m-gq56. The library is documented as a one-to-one substitute for KnpLabs/snappy and shares the same code shape.

Affected versions

pontedilana/php-weasyprint versions <= 2.5.1.

Patched in: 2.6.0.

Privilege required

Any caller that can influence the attachment option value handed to Pdf::generate() / Pdf::getOutput() / setOption('attachment', ...). Typical reach paths: a value sourced from a request parameter, a per-tenant configuration row, or any user-controllable field that flows into the attachment list.

Vulnerable code

src/Pdf.php — isOptionUrl() accepts any well-formed URL regardless of scheme:

protected function isOptionUrl($option): bool
{
    return false !== \filter_var($option, \FILTER_VALIDATE_URL);
}

src/Pdf.php — handleArrayOptions() fetches the URL content for the attachment option:

$fetchUrlContent = 'attachment' === $option && $this->isOptionUrl($item);
if ($saveToTempFile || $fetchUrlContent) {
    $fileContent = $fetchUrlContent ? \file_get_contents($item) : $item;
    $returnOptions[] = $this->createTemporaryFile($fileContent, $this->optionsWithContentCheck[$option] ?? 'temp');
}

FILTER_VALIDATE_URL returns truthy for http://, https://, ftp://, file://localhost/..., and php://filter/..., so \file_get_contents() is invoked on attacker-chosen schemes with no allow-list.

Proof of concept

<?php
use Pontedilana\PhpWeasyPrint\Pdf;

$pdf = new Pdf('/usr/local/bin/weasyprint');

// Attacker-controlled attachment value (e.g. from a request / tenant config):
//   SSRF:               http://169.254.169.254/latest/meta-data/iam/security-credentials/
//   Local file read:    php://filter/convert.base64-encode/resource=/etc/passwd
$attachment = $_GET['doc'];

$pdf->generate('page.html', 'out.pdf', [
    'attachment' => $attachment,
]);

// The bytes fetched server-side by file_get_contents() are embedded in out.pdf,
// allowing the attacker to read internal HTTP responses or local files.

Impact

• SSRF: the server fetches arbitrary http(s)/ftp URLs, reaching internal-only services, link-local metadata endpoints, etc.
• Local file / wrapper disclosure: php://filter/... (and similar) let an attacker read and exfiltrate local file content inside the generated PDF.
• Affects any consumer that does not fully control the attachment option value.

Note: passing a plain local path (e.g. /etc/passwd) or a file:// path that resolves to an existing file is handled as a normal local attachment and is not the issue addressed here — that is the documented local-attachment feature (callers must not pass untrusted input to the option). The fix specifically removes the server-side fetch amplification through non-http(s) schemes.

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N (6.5, Medium) — adjust PR/S/A to the consuming application's reachability (e.g. PR:N if the attachment value is reachable from an unauthenticated surface).

CWE-918 (Server-Side Request Forgery); secondary CWE-22 (Improper Limitation of a Pathname) for the wrapper-based file read.

Suggested fix

Restrict the schemes the library will fetch to an allow-list (http, https by default), and treat any other scheme as inline content instead of fetching it:

private array $allowedSchemes = ['http', 'https'];

// new optional 4th constructor argument: ?array $allowedSchemes = null

protected function isOptionUrl($option): bool
{
    $url = \parse_url((string)$option);

    return false !== $url
        && isset($url['scheme'])
        && \in_array(\strtolower($url['scheme']), $this->allowedSchemes, true);
}

A value with a non-allowed scheme (file://, php://, ftp://, ...) is then never passed to file_get_contents().

Credit

Reported upstream to KnpLabs/snappy (GHSA-c5fp-p67m-gq56); identified as applicable to pontedilana/php-weasyprint, which mirrors the same code.