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

PhpWeasyPrint is a PHP library allowing PDF generation from a URL or an HTML page. Prior to version 2.6.0, 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. PhpWeasyPrint version 2.6.0 contains a patch for the issue.