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Threat Model Analysis for phpoffice/phpspreadsheet

Threat: XML External Entity (XXE) Injection via XLSX/ODS

  • Threat: XML External Entity (XXE) Injection via XLSX/ODS
    • Description: An attacker uploads a maliciously crafted .xlsx or .ods file (which are XML-based) containing external entity references. These references could point to local files on the server, internal network resources, or external URLs. The attacker aims to read sensitive files, perform server-side request forgery (SSRF), or cause a denial of service.
    • Impact:
      • Information Disclosure: Exposure of sensitive files (e.g., /etc/passwd, configuration files).
      • SSRF: Access to internal services or network resources.
      • DoS: Consumption of server resources.
    • Affected Component: XML Parsers within PhpOffice\PhpSpreadsheet\Reader\Xlsx and PhpOffice\PhpSpreadsheet\Reader\Ods. Specifically, the underlying XML parsing libraries used (e.g., SimpleXML, XMLReader) are the primary targets.
    • Risk Severity: Critical
    • Mitigation Strategies:
      • Disable External Entity Loading: Explicitly disable external entity loading in the XML parser. This is the most important mitigation. For example: 
        libxml_disable_entity_loader(true);
      • Use a Secure XML Parser: Ensure that the underlying XML parser used by PhpSpreadsheet is configured securely and is up-to-date.
      • Input Validation: While not a primary defense against XXE, validate the overall structure of the uploaded file to detect anomalies.

Threat: Denial of Service (DoS) via "Billion Laughs" (XML Bomb)

  • Threat: Denial of Service (DoS) via "Billion Laughs" (XML Bomb)
    • Description: An attacker uploads a specially crafted .xlsx file (XML-based) that exploits nested entity expansion. The file contains a small number of entities that reference each other recursively, leading to exponential expansion when parsed. The attacker aims to consume excessive memory and CPU, causing the server to crash or become unresponsive.
    • Impact:
      • Server Unavailability.
      • Resource Exhaustion.
    • Affected Component: PhpOffice\PhpSpreadsheet\Reader\Xlsx (specifically, the XML parsing components).
    • Risk Severity: High
    • Mitigation Strategies:
      • Limit XML Entity Expansion: Configure the XML parser to limit the depth and size of entity expansion. This may be a configuration option of the underlying XML library.
      • File Size Limits: Enforce strict file size limits for uploaded spreadsheets.
      • Memory Limits: Set memory limits for PHP processes.
      • Timeouts: Implement timeouts for spreadsheet processing.

Threat: Denial of Service (DoS) via Resource Exhaustion (General)

  • Threat: Denial of Service (DoS) via Resource Exhaustion (General)
    • Description: An attacker uploads a spreadsheet with an extremely large number of rows, columns, complex formulas, embedded objects, or styles. The attacker aims to consume excessive server resources (CPU, memory, disk I/O) during processing.
    • Impact:
      • Server Unavailability.
      • Resource Exhaustion.
    • Affected Component: All reader and writer components (PhpOffice\PhpSpreadsheet\Reader\*, PhpOffice\PhpSpreadsheet\Writer\*), as well as cell and style handling components.
    • Risk Severity: High
    • Mitigation Strategies:
      • Limit Rows/Columns: Set reasonable limits on the maximum number of rows and columns that will be processed.
      • Limit Formula Complexity: Consider using a whitelist of allowed formula functions or restricting the nesting depth of formulas.
      • Limit Embedded Objects: Restrict the number and size of embedded objects (images, files) allowed in spreadsheets.
      • Memory and Time Limits: Set appropriate memory and time limits for PHP processes.
      • Input Validation (Preliminary Checks): Before fully loading a spreadsheet, perform preliminary checks to estimate its size and complexity. For example, for .xlsx files, you could examine the [Content_Types].xml file within the ZIP archive to get an idea of the number of sheets and their potential size.

Threat: Code Injection via Unpatched Vulnerability

  • Threat: Code Injection via Unpatched Vulnerability
    • Description: An attacker exploits a known or unknown (zero-day) vulnerability in PhpSpreadsheet itself to inject and execute arbitrary PHP code on the server. This could be due to a flaw in how the library handles specific file formats, formulas, or other input.
    • Impact:
      • Complete Server Compromise.
      • Data Theft.
      • Data Modification.
    • Affected Component: Potentially any component of PhpSpreadsheet, depending on the specific vulnerability.
    • Risk Severity: Critical
    • Mitigation Strategies:
      • Keep PhpSpreadsheet Updated: This is the most crucial mitigation. Regularly update to the latest version to patch known vulnerabilities.
      • Least Privilege: Run the PHP process with the lowest possible privileges.
      • Security Audits and Penetration Testing: Regularly conduct security assessments.
      • Input Validation (Defense in Depth): While not a primary defense against code injection, thorough input validation can help to reduce the attack surface.

Threat: Zip Slip Vulnerability

  • Threat: Zip Slip Vulnerability
    • Description: An attacker crafts a malicious .xlsx file (which is a ZIP archive) that contains files with directory traversal characters (e.g., ../) in their filenames. When PhpSpreadsheet extracts the archive, these files could be written to arbitrary locations on the server's file system, potentially overwriting critical system files or injecting malicious code.
    • Impact:
      • File Overwrite
      • Code Execution (if attacker overwrites executable files or configuration files)
      • Denial of Service
    • Affected Component: PhpOffice\PhpSpreadsheet\Reader\Xlsx, specifically the code that handles the extraction of the ZIP archive.
    • Risk Severity: High
    • Mitigation Strategies:
      • Validate File Paths within Archive: Before extracting files from the .xlsx archive, validate that the file paths do not contain directory traversal characters (../, ..\). Reject any files that violate this rule.
      • Use a Secure Extraction Library: Ensure that the underlying ZIP extraction library used by PhpSpreadsheet is secure and up-to-date.
      • Extract to a Sandboxed Directory: Extract the contents of the .xlsx file to a temporary, isolated directory with limited permissions. This prevents the attacker from writing files to sensitive locations.