Affected versions of the weasyprint package are vulnerable to Server-Side Request Forgery (SSRF) due to insufficient validation of redirect destinations in the URL fetching mechanism. The default_url_fetcher function in weasyprint/urls.py relies on Python's urllib.request.urlopen, which automatically follows HTTP redirects (301, 302, 307) without re-invoking the developer's custom url_fetcher validation logic, creating a Time-of-Check to Time-of-Use (TOCTOU) condition. An attacker can supply an external URL that passes initial security checks but redirects to internal network resources such as localhost services or cloud metadata endpoints, enabling exfiltration of sensitive data, including instance credentials.