Path Traversal Detection

Path Traversal Detection

You are performing a focused security assessment to find path traversal vulnerabilities in a codebase. This skill uses a three-phase approach with subagents: recon (find file-loading sinks with dynamic paths), batched verify (trace user input and check mitigations in parallel batches of 3), and merge (consolidate batch results into one report).

Prerequisites: sast/architecture.md must exist. Run the analysis skill first if it doesn't.

What is Path Traversal

Path traversal (also called directory traversal) occurs when user-supplied input is incorporated into a file path that is then used to read, write, or serve files from the filesystem — without properly constraining the resulting path to an intended base directory. An attacker can supply sequences like ../ or encoded variants (%2e%2e%2f, ..%2f, %2e%2e/) to escape the intended directory and access arbitrary files such as /etc/passwd, application source code, credentials, or private keys.

The core pattern: unvalidated user input reaches a filesystem operation and the resolved path is not verified to remain within the intended base directory.

What Path Traversal IS

• Serving a user-requested filename directly from a base directory without canonicalizing and checking the resulting path: open(os.path.join(BASE_DIR, user_filename))
• Constructing a file path from a URL parameter and passing it to a file-read function: fs.readFile(path.join(__dirname, req.query.file), ...)
• Template rendering or include directives driven by user input: include($_GET['page'] . '.php')
• Archive extraction (ZipFile, tarfile, zipslip) where entry names are used as output paths without stripping ../ components
• Using send_file() / send_from_directory() / res.sendFile() with an unsanitized user-controlled path
• Reading a file whose path is derived from a user-controlled database value that was stored without sanitization

What Path Traversal is NOT

Do not flag these as path traversal:

• SSRF: Fetching a remote URL from user input — that is Server-Side Request Forgery, a separate class
• RCE via file write: Writing attacker-controlled content to an arbitrary path — related but a different impact class (flag as RCE or File Upload)
• Static file serving: Serving files from a path that is entirely hardcoded with no user influence
• Safe path joins followed by realpath + prefix check: The code computes realpath() and verifies it starts with the intended base directory
• basename() before join: Using only the filename component strips traversal sequences (though note this prevents directory selection, not just traversal)

Patterns That Prevent Path Traversal

When you see these mitigations applied before the file operation, the code is likely not vulnerable: