Path Traversal Pattern Regex for Python

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching path traversal pattern, ported and verified for Python. In security-sensitive code, using an unverified regex can open the door to both false positives and denial-of-service attacks. The snippet below is ready to drop into your Python project — whether you're validating in a Django view, a FastAPI endpoint, or a standalone data processing script.

Python Implementation

# Path Traversal Pattern
# ReDoS-safe | RegexVault — Security > Injection Patterns

import re

path_traversal_pattern_pattern = re.compile(r'(?:\.\./|\.\.\\|%2e%2e%2f|%2e%2e/|\.%2e/|%2e\./|%252e%252e|\.\.%c0%af|\.\.%c1%9c|\.\./|%00|\.php\x00|\.asp\x00)')

def validate_path_traversal_pattern(value: str) -> bool:
    return bool(path_traversal_pattern_pattern.fullmatch(value))

# Example
print(validate_path_traversal_pattern("../../etc/passwd"))  # True

Test Cases

When to use this pattern

This pattern is drawn from the Security > Injection Patterns category and carries a ReDoS-safe certification. That matters for Python developers because particularly important in Python web servers where CPU-bound regex operations can stall concurrent request handling. RegexVault audits patterns against known backtracking attack vectors, ensuring you have the necessary context before using this regex in a high-stakes production environment.

Common Pitfalls

Technical Notes

Path traversal (directory traversal) allows attackers to read files outside the intended directory. Double encoding (%252e = %) is used to bypass naive decoders. Null byte (%00) terminates strings in some C-based systems. Real defense: use path canonicalization and compare against the allowed base directory.