^{CVE-2021-3838:}PHAR Deserialization Vulnerability in DomPDF before version 2.0.0

Overview

This vulnerability (CVE-2021-3838) affects DomPDF library versions prior to 2.0.0. The issue stems from a lack of protocol validation before passing user-controlled input to PHP's file_get_contents() function. When an attacker can upload files to the server (of any type), they can leverage the phar:// protocol wrapper to trigger PHP object deserialization. This is particularly dangerous because when the deserialization occurs, PHP will instantiate objects defined in the PHAR file, potentially executing malicious code through magic methods like __destruct(), __wakeup(), etc. The vulnerability is especially critical when DomPDF is used alongside frameworks like Laravel that have documented PHP Object Injection chains, or when used in custom code with vulnerable class structures.

Remediation

1. Update DomPDF to version 2.0.0 or later, which includes the security patch for this vulnerability.
2. If immediate updating is not possible, implement additional validation for any file paths or URLs passed to DomPDF:
   • Explicitly whitelist allowed protocols (e.g., http://, https://, file://)
   • Block dangerous protocols like phar://, php://, zip://, data://
3. Implement proper file upload validation to restrict file types and content
4. Consider implementing a wrapper function around file_get_contents() that validates the protocol
5. Apply the principle of least privilege to file system permissions for uploaded files
6. Review your codebase for other instances where user input might influence file paths

References

1. Patch: https://github.com/dompdf/dompdf/commit/99aeec1efec9213e87098d42eb09439e7ee0bb6a
2. Vulnerability Report and Exploit: https://huntr.com/bounties/0bdddc12-ff67-4815-ab9f-6011a974f48e
3. PHAR Deserialization Documentation: https://blog.secarma.co.uk/labs/near-phar-dangerous-unserialization-wherever-you-are
4. PHP Object Injection: https://owasp.org/www-community/vulnerabilities/PHP_Object_Injection
5. CWE-915: Improperly Controlled Modification of Dynamically-Determined Object Attributes