fix(security): prevent path traversal in ocr_service filename handling with sanitization and path verification

[Aryanbansal-05]

🔐 Summary

Resolves #3948 by adding strict filename sanitization and path verification to backend/services/ocr_service.py, preventing path traversal attacks (CWE-22) where user-supplied filenames containing ../, NULL bytes, or absolute paths could be used to read arbitrary server files.

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🛠️ Changes Made

backend/services/ocr_service.py

Added two security utility functions:

_sanitize_filename(filename)

• Rejects NULL bytes (\x00)
• Rejects .. traversal sequences
• Rejects absolute paths (/ or \ prefix)
• Extracts os.path.basename() only — strips all directory components
• Raises ValueError for any invalid input

_verify_safe_path(base_dir, filename)

• Resolves full path via os.path.realpath() to follow symlinks
• Verifies resolved path stays inside base_dir using os.path.commonpath()
• Prevents symlink-based escape attacks
• Raises ValueError if path escapes the upload directory

process_document() updated:

• filename parameter now sanitized via _sanitize_filename() before use
• Returns {"success": False, "error": "Invalid filename: ..."} on traversal attempts
• No change to normal behaviour for valid filenames

backend/tests/test_path_traversal.py — New Security Regression Tests

3 test classes, 18 test cases:

• TestSanitizeFilename — traversal sequences, NULL bytes, absolute paths, empty inputs
• TestVerifySafePath — symlink escape, path escape, valid path resolution
• TestProcessDocumentFilenameValidation — integration tests via process_document()

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✅ Security Issues Fixed

• ../../etc/passwd traversal rejected with 400-equivalent error
• NULL byte injection (file\x00.pdf) rejected
• Absolute path filenames rejected
• Symlink-based directory escape prevented via realpath()
• CVSS 8.8 vector mitigated

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Closes #3948

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