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attack-surface.md

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Attack Surface Analysis for phpoffice/phpexcel

Attack Surface: Untrusted File Upload and Processing

  • Description: Accepting and processing spreadsheet files from untrusted sources, where malicious files can exploit vulnerabilities in PHPExcel's parsing logic.
  • PHPExcel Contribution: PHPExcel is designed to parse and process various spreadsheet file formats. Vulnerabilities in its parsing logic are directly exploited by malicious files processed by PHPExcel.
  • Example: A user uploads a specially crafted XLSX file containing a buffer overflow vulnerability in PHPExcel's XML parsing. Processing this file with PHPExcel leads to remote code execution on the server.
  • Impact: Remote Code Execution (RCE), Denial of Service (DoS), Information Disclosure.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Strict File Type Validation: Only allow uploads of necessary file types and rigorously validate file extensions and MIME types on the server-side.
    • File Size Limits: Implement strict file size limits to mitigate DoS attacks.
    • Sandboxed Processing: Process uploaded files in a highly isolated, sandboxed environment with severely limited resource access to contain any potential exploit.
    • Regularly Update PHPExcel (or migrate to PhpSpreadsheet): Immediately update to the latest stable version of PHPExcel or, ideally, migrate to PhpSpreadsheet to benefit from critical security patches.

Attack Surface: XML External Entity (XXE) Injection (XLSX format)

  • Description: Exploiting vulnerabilities in PHPExcel's XML parsing of XLSX files to access local files or internal network resources.
  • PHPExcel Contribution: PHPExcel parses XLSX files, which are XML-based. Vulnerable XML parsing within PHPExcel directly allows for XXE attacks through malicious XLSX files.
  • Example: An attacker crafts a malicious XLSX file with an external entity definition pointing to /etc/passwd. When PHPExcel parses this file, it reads and exposes the contents of the sensitive /etc/passwd file.
  • Impact: Information Disclosure (access to sensitive local files, internal network resources).
  • Risk Severity: High
  • Mitigation Strategies:
    • Disable External Entity Resolution in XML Parsing: Forcefully disable external entity resolution in the XML parsing configuration used by PHPExcel. This is a critical security configuration.
    • Use a Secure XML Parser (if configurable within PHPExcel): Ensure the underlying XML parser used by PHPExcel is robust and patched against XXE vulnerabilities.
    • Regularly Update PHPExcel (or migrate to PhpSpreadsheet): Apply security updates for PHPExcel and its dependencies promptly.

Attack Surface: Zip Slip Vulnerability (XLSX, ODS formats)

  • Description: Exploiting vulnerabilities in PHPExcel's ZIP archive extraction for XLSX and ODS files to write files outside the intended extraction directory, potentially leading to arbitrary file write and remote code execution.
  • PHPExcel Contribution: PHPExcel handles ZIP-based formats. Insecure ZIP extraction within PHPExcel allows malicious ZIP archives to write files to arbitrary locations.
  • Example: An attacker crafts a malicious XLSX file containing a file entry with a path like ../../../malicious.php. PHPExcel's extraction writes malicious.php to a web-accessible directory, enabling remote code execution.
  • Impact: Arbitrary File Write, potentially Remote Code Execution (RCE).
  • Risk Severity: High
  • Mitigation Strategies:
    • Secure ZIP Extraction: Implement and enforce secure file extraction functions that strictly prevent path traversal. Thoroughly validate and sanitize extracted file paths to ensure they remain within the intended directory.
    • Principle of Least Privilege (File System Access): Run the PHP process with minimal file system write permissions to limit the damage from Zip Slip.
    • Regularly Update PHPExcel (or migrate to PhpSpreadsheet): Apply updates that address Zip Slip vulnerabilities in PHPExcel or its dependencies.