mPDF is a popular open-source PHP library designed to convert HTML content into PDF documents, boasting around 70 million installations via Packagist. The reported security issue arises from the way mPDF processes input HTML, specifically due to quirks in its internal logic combined with the use of regular expressions (regex) for sanitization and parsing. These regex patterns and logic flaws can be manipulated by an attacker to craft malicious inputs that bypass sanitization and trigger unintended web requests during PDF generation. Such behavior can lead to remote code execution (RCE) vulnerabilities, where an attacker may execute arbitrary code on the server hosting mPDF. The threat is notable because it can occur even when inputs are sanitized, indicating that the sanitization logic is insufficient or flawed. Currently, there are no known exploits in the wild, and no official patches have been released, but the issue was highlighted recently on Reddit’s NetSec community and linked to a Medium article. The vulnerability affects all versions of mPDF, as no specific affected versions were listed, suggesting a design or logic flaw rather than a version-specific bug. The exploitation does not require user interaction beyond submitting crafted HTML content to the PDF generation endpoint, and no authentication requirements were specified, increasing the attack surface. The complexity of exploitation depends on the attacker’s ability to craft inputs that exploit the regex and logic quirks effectively. This vulnerability is particularly concerning for web applications that accept user-generated HTML content and convert it to PDFs using mPDF, as it could allow attackers to perform server-side request forgery (SSRF), remote code execution, or other malicious actions through triggered web requests.

For European organizations, the impact of this vulnerability can be significant, especially for those relying on mPDF for generating PDFs from user-supplied HTML in web applications, content management systems, or document automation workflows. Exploitation could lead to unauthorized remote code execution, allowing attackers to compromise server confidentiality, integrity, and availability. This could result in data breaches, defacement, ransomware deployment, or lateral movement within the network. The ability to trigger web requests from the server could also facilitate SSRF attacks, potentially exposing internal network resources or cloud metadata services. Given the widespread use of PHP and mPDF in Europe, especially among SMEs and public sector organizations, the risk is amplified. The lack of patches means organizations must rely on mitigations and monitoring to reduce exposure. Additionally, the vulnerability could undermine trust in document processing systems and disrupt business operations dependent on automated PDF generation.

1. Implement strict input validation and sanitization at the application level before passing HTML content to mPDF, using well-maintained libraries that are separate from mPDF’s internal mechanisms. 2. Employ allowlists for HTML tags and attributes to minimize the risk of malicious content reaching mPDF. 3. Isolate the PDF generation process in a sandboxed environment or container with minimal privileges and restricted network access to prevent unauthorized outbound web requests. 4. Monitor and log all outbound network requests from servers running mPDF to detect anomalous or unexpected connections. 5. Apply network-level controls such as egress filtering and firewall rules to limit the server’s ability to make arbitrary web requests. 6. Stay informed about official patches or updates from the mPDF project and apply them promptly once available. 7. Consider alternative PDF generation libraries with a stronger security track record if immediate mitigation is not feasible. 8. Conduct security code reviews and penetration testing focused on PDF generation workflows to identify and remediate potential exploitation paths.