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attack-surface.md

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Attack Surface Analysis for phpoffice/phpspreadsheet

Attack Surface: XML External Entity (XXE) Injection

Description: An attacker can inject malicious XML code into a spreadsheet file (like XLSX) that, when parsed by phpspreadsheet, allows them to access local files, perform Server-Side Request Forgery (SSRF), or cause Denial of Service (DoS). How phpspreadsheet contributes: phpspreadsheet uses XML parsers to process XLSX and other XML-based spreadsheet formats. If these parsers are not configured to disable external entity processing, they become vulnerable to XXE. Example: A malicious XLSX file contains XML code that instructs the parser to read /etc/passwd on the server. When phpspreadsheet parses this file, it reads the file and potentially exposes its contents. Impact: Confidentiality breach (reading local files), Server-Side Request Forgery (SSRF), Denial of Service (DoS). Risk Severity: High to Critical Mitigation Strategies:

  • Disable external entity processing in XML parsers: Configure the XML parser used by phpspreadsheet (often underlying PHP XML extensions) to disable external entity resolution. This is usually done by setting specific parser options when loading XML data. Consult PHP documentation and phpspreadsheet documentation for specific configuration details.

Attack Surface: Zip Slip/Path Traversal (during ZIP archive extraction)

Description: An attacker crafts a malicious spreadsheet file (like XLSX or ODS, which are ZIP archives) containing filenames with path traversal sequences (e.g., ../../) that, when extracted by phpspreadsheet, allows writing files outside the intended extraction directory, potentially overwriting system files or application files. How phpspreadsheet contributes: phpspreadsheet extracts ZIP archives to process XLSX and ODS files. If the extraction process doesn't properly sanitize filenames within the archive, it's vulnerable to Zip Slip. Example: A malicious XLSX file contains a file entry named ../../../../tmp/evil.php. When phpspreadsheet extracts this archive, it attempts to write evil.php to /tmp/evil.php on the server, potentially overwriting or creating files in unintended locations. Impact: Arbitrary file write, potentially leading to code execution, data corruption, or denial of service. Risk Severity: Critical Mitigation Strategies:

  • Secure ZIP Extraction: Ensure the ZIP extraction process used by phpspreadsheet (or underlying libraries) properly sanitizes filenames and prevents path traversal. This typically involves validating and normalizing file paths extracted from the archive to ensure they remain within the intended extraction directory.

Attack Surface: Denial of Service (DoS) via Malformed or Large Files

Description: An attacker uploads a specially crafted spreadsheet file (very large, deeply nested structures, or malformed content) that consumes excessive server resources (CPU, memory, disk I/O) when parsed by phpspreadsheet, leading to application slowdown or crash. How phpspreadsheet contributes: phpspreadsheet must parse and process potentially complex spreadsheet files. Inefficient parsing or lack of resource limits can make it vulnerable to DoS attacks via resource exhaustion. Example: An attacker uploads an XLSX file containing millions of rows or columns, or deeply nested styles. When phpspreadsheet attempts to load this file, it consumes all available memory, causing the application to crash. Impact: Application unavailability, service disruption. Risk Severity: High Mitigation Strategies:

  • File Size Limits: Implement limits on the size of uploaded spreadsheet files.
  • Resource Limits (Memory, CPU, Timeouts): Configure PHP and the web server to enforce resource limits (memory limits, execution time limits) for PHP scripts. This can prevent a single request from consuming excessive resources.
  • Input Validation and Sanitization: While not fully preventing DoS, validate file structure and content to reject obviously malformed or excessively complex files before full parsing.

Attack Surface: Memory Exhaustion Vulnerabilities (within phpspreadsheet or dependencies)

Description: Specific file formats or structures within spreadsheets might trigger memory leaks or inefficient memory management within phpspreadsheet or its underlying libraries, leading to gradual or rapid memory exhaustion and application failure. How phpspreadsheet contributes: Complexity of spreadsheet formats and parsing logic within phpspreadsheet and its dependencies can introduce memory management issues. Example: Processing a specific type of chart in an XLSX file triggers a memory leak in phpspreadsheet. Repeatedly uploading files with this chart type eventually exhausts server memory and crashes the application. Impact: Application instability, service disruption, denial of service. Risk Severity: High Mitigation Strategies:

  • Regular Updates: Keep phpspreadsheet and its dependencies updated to benefit from bug fixes and memory management improvements.
  • Memory Monitoring: Monitor application memory usage to detect potential memory leaks or excessive consumption.
  • Resource Limits (Memory Limits): Set appropriate memory limits for PHP processes to prevent a single process from consuming all available memory and crashing the server.