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Threat Model Analysis for phpoffice/phpexcel

Threat: Zip Slip Vulnerability

  • Description: An attacker crafts a malicious Excel file in .xlsx or .ods format containing archive entries with filenames that include path traversal sequences (e.g., ../../). When PHPExcel extracts files from these archives, it may write files outside the intended extraction directory if filename sanitization is insufficient. This allows an attacker to potentially overwrite critical system or application files. The attacker achieves this by manipulating the filenames within the ZIP archive structure.
    • Impact:
      • File Overwrite: Overwriting critical system or application files, potentially leading to application malfunction, system compromise, or even remote code execution if executable files are targeted.
    • PHPExcel Component Affected: ZIP Archive Handling (within PHPExcel_Reader_Excel2007 and .ods readers). Specifically, the file extraction mechanism responsible for processing filenames within ZIP archives.
    • Risk Severity: Critical
    • Mitigation Strategies:
      • PHPExcel/PhpSpreadsheet Level: Ensure PHPExcel (or preferably PhpSpreadsheet) uses secure file extraction methods that rigorously prevent path traversal during ZIP archive processing. Verify robust filename sanitization within the library's ZIP handling code. Upgrade to the latest version of PhpSpreadsheet, as it is more likely to have addressed this type of vulnerability.
      • Application Level: Restrict file upload directories and permissions to the minimum necessary. Run file processing operations in a least-privilege environment to limit the impact of potential file overwrites. Regularly update PHPExcel/PhpSpreadsheet to benefit from security patches.

Threat: XML External Entity (XXE) Injection

  • Description: An attacker crafts a malicious Excel file (e.g., .xlsx) containing an XXE payload. When PHPExcel parses this file, it processes the external entity, potentially allowing the attacker to read local files on the server, perform Server-Side Request Forgery (SSRF) attacks, or trigger a Denial of Service. The attacker embeds malicious XML within the Excel file that instructs the XML parser to access external or local resources when the file is processed by PHPExcel.
    • Impact:
      • Confidentiality Breach: Disclosure of sensitive local files from the server's filesystem.
      • Server-Side Request Forgery (SSRF): Ability to make requests to internal or external resources from the server, potentially compromising internal systems or external services.
      • Denial of Service: Resource exhaustion due to excessive entity expansion, making the application unavailable.
    • PHPExcel Component Affected: XML Parser (used in PHPExcel_Reader_Excel2007 and potentially other readers handling XML-based formats like .ods). Specifically, the XML parsing functionality used to process XML structures within Excel files.
    • Risk Severity: High
    • Mitigation Strategies:
      • PHPExcel/PhpSpreadsheet Level: Ensure the XML parser used by PHPExcel is configured to disable external entity resolution by default. Verify this configuration in the specific PHPExcel version being used. Upgrade to PhpSpreadsheet, which is more likely to have secure XML parsing configurations.
      • Application Level: Implement strict validation of uploaded file types to only accept expected Excel formats. Implement file size limits to mitigate potential DoS attacks. Consider processing uploaded files in a sandboxed environment to limit the impact of potential vulnerabilities. Regularly update PHPExcel/PhpSpreadsheet to benefit from security updates.

Threat: Billion Laughs Attack (XML Bomb)

  • Description: An attacker crafts a malicious Excel file (e.g., .xlsx) containing a "billion laughs" XML bomb. This bomb consists of deeply nested, recursively defined XML entities. When PHPExcel parses this file, the XML parser attempts to expand these entities, leading to exponential memory consumption and CPU usage, resulting in a Denial of Service. The attacker leverages the XML entity expansion mechanism to create a small XML file that expands to consume vast resources when parsed.
    • Impact: Denial of Service: The application becomes unresponsive or crashes due to severe server resource exhaustion (CPU and memory), effectively disrupting service availability.
    • PHPExcel Component Affected: XML Parser (used in PHPExcel_Reader_Excel2007 and potentially other readers handling XML-based formats like .ods). Specifically, the XML parsing functionality and its handling of entity expansion.
    • Risk Severity: High
    • Mitigation Strategies:
      • PHPExcel/PhpSpreadsheet Level: The XML parser used by PHPExcel should have built-in limits on entity expansion depth and size to prevent excessive resource consumption. Verify if the PHPExcel version in use has such protections. Newer versions of PhpSpreadsheet are expected to have improved defenses against XML bomb attacks.
      • Application Level: Implement file size limits for uploaded Excel files to restrict the size of potentially malicious files. Set timeouts for file processing operations to prevent long-running parsing from consuming resources indefinitely. Implement resource monitoring to detect and mitigate DoS attempts by observing CPU and memory usage.