codeguard-0-file-handling-and-uploads

Instruction file imported from avinn1990/acyl-app-sec (.cursor/rules/codeguard-0-file-handling-and-uploads.mdc). Copyright stays with the author.

description: Secure file handling & uploads (validation, storage isolation, scanning, safe delivery) globs: /*.c,/.go,**/.h,/*.java,/.js,**/.jsx,/*.mjs,/.php,**/.py,/*.pyi,/.pyx,**/.rb,/*.ts,/*.tsx version: 1.4.0

rule_id: codeguard-0-file-handling-and-uploads

File Upload Security Guidelines

This rule advises on secure file upload practices to prevent malicious file attacks and protect system integrity:

• Extension Validation

  • List allowed extensions only for business-critical functionality.
  • Ensure input validation is applied before validating extensions.
  • Avoid double extensions (e.g., .jpg.php) and null byte injection (e.g., .php%00.jpg).
  • Use allowlist approach rather than denylist for file extensions.
  • Validate extensions after decoding filename to prevent bypass attempts.

• Content Type and File Signature Validation

  • Never trust client-supplied Content-Type headers as they can be spoofed.
  • Validate file signatures (magic numbers) in conjunction with Content-Type checking.
  • Implement allowlist approach for MIME types as a quick protection layer.
  • Use file signature validation but not as a standalone security measure.

• Filename Security

  • Generate random filenames (UUID/GUID) instead of using user-supplied names.
  • If user filenames required, implement maximum length limits.
  • Restrict characters to alphanumeric, hyphens, spaces, and periods only.
  • Prevent leading periods (hidden files) and sequential periods (directory traversal).
  • Avoid leading hyphens or spaces for safer shell script processing.

• File Content Validation

  • For images, apply image rewriting techniques to destroy malicious content.
  • For Microsoft documents, use Apache POI for validation.
  • Avoid ZIP files due to numerous attack vectors.
  • Implement manual file review in sandboxed environments when resources allow.
  • Integrate antivirus scanning and Content Disarm & Reconstruct (CDR) for applicable file types.

• Storage Security

  • Store files on different servers for complete segregation when possible.
  • Store files outside webroot with administrative access only.
  • If storing in webroot, set write-only permissions with proper access controls.
  • Use application handlers that map IDs to filenames for public access.
  • Consider database storage for specific use cases with DBA expertise.

• Access Control and Authentication

  • Require user authentication before allowing file uploads.
  • Implement proper authorization levels for file access and modification.
  • Set filesystem permissions on principle of least privilege.
  • Scan files before execution if execution permission is required.

• Upload and Download Limits

  • Set proper file size limits for upload protection.
  • Consider post-decompression size limits for compressed files.
  • Implement request limits for download services to prevent DoS attacks.
  • Use secure methods to calculate ZIP file sizes safely.

• Additional Security Measures

  • Protect file upload endpoints from CSRF attacks.
  • Keep all file processing libraries securely configured and updated.
  • Implement logging and monitoring for upload activities.
  • Provide user reporting mechanisms for illegal content.
  • Use secure extraction methods for compressed files.

Summary:
Implement defense-in-depth for file uploads through multi-layered validation, secure storage practices, proper access controls, and comprehensive monitoring. Never rely on single validation methods and always generate safe filenames to prevent attacks.