Nikita Sveshnikov

Every day, thousands of web services generate PDF (Portable Document Format) files—bills, contracts, reports. This step is often treated as a technical routine, “just convert the HTML,” but in practice it’s exactly where a trust boundary is crossed. The renderer parses HTML, downloads external resources, processes fonts, SVGs, and images, and sometimes has access to the network and the file system. Risky behavior can occur by default, without explicit options or warnings. That is enough for a PDF converter to become an SSRF proxy, a data leak channel, or even cause denial of service.

We therefore conducted a targeted analysis of popular HTML-to-PDF libraries written in the PHP, JavaScript, and Java languages: TCPDF, html2pdf, jsPDF, mPDF, snappy, dompdf, and OpenPDF. During the research, the PT Swarm team identified 13 vulnerabilities, demonstrated 7 intentional behaviors, and highlighted 6 potential misconfigurations. These included vulnerability classes such as Files or Directories Accessible to External Parties, Deserialization of Untrusted Data, Server-Side Request Forgery, and Denial of Service.

PDF generation is increasingly common across e‑commerce, fintech, logistics, and SaaS. Such services are often deployed inside the perimeter, next to sensitive data, where network controls are looser. This means that even a seemingly harmless bug in the renderer can escalate into a serious incident: leakage of documents, secrets, or internal URLs.

In this article, we present a threat model for an HTML-to-PDF library, walk through representative findings for each library, and provide PoC snippets.

Cross-Origin Resource Sharing (CORS) is a web protocol that outlines how a web application on one domain can access resources from a server on a different domain. By default, web browsers have a Same-Origin Policy (SOP) that blocks these cross-origin requests for security purposes. However, CORS offers a secure way for servers to specify which origins are allowed to access their assets, thereby enabling a structured method of relaxing this policy.

In CORS, the server sends HTTP headers to instruct the browser on rules for making cross-origin requests. These rules define whether a particular HTTP request (such as GET or POST) from a certain origin is allowed. By managing the CORS headers, a server can control its resource accessibility on a case-by-case basis. This maintains the flexibility of cross-origin sharing without compromising overall security.

Blind trust: what is hidden behind the process of creating your PDF file?

Bypassing browser tracking protection for CORS misconfiguration abuse