Incomplete regular expression for hostnames¶
ID: py/incomplete-hostname-regexp Kind: problem Security severity: 7.8 Severity: warning Precision: high Tags: - correctness - security - external/cwe/cwe-020 Query suites: - python-code-scanning.qls - python-security-extended.qls - python-security-and-quality.qls Click to see the query in the CodeQL repository
Sanitizing untrusted URLs is a common technique for preventing attacks such as request forgeries and malicious redirections. Often, this is done by checking that the host of a URL is in a set of allowed hosts.
If a regular expression implements such a check, it is easy to accidentally make the check too permissive by not escaping the . meta-characters appropriately. Even if the check is not used in a security-critical context, the incomplete check may still cause undesirable behaviors when it accidentally succeeds.
Recommendation¶
Escape all meta-characters appropriately when constructing regular expressions for security checks, and pay special attention to the . meta-character.
Example¶
The following example code checks that a URL redirection will reach the example.com domain, or one of its subdomains.
from flask import Flask, request, redirect import re app = Flask(__name__) UNSAFE_REGEX = re.compile("(www|beta).example.com/") SAFE_REGEX = re.compile(r"(www|beta)\.example\.com/") @app.route('/some/path/bad') def unsafe(request): target = request.args.get('target', '') if UNSAFE_REGEX.match(target): return redirect(target) @app.route('/some/path/good') def safe(request): target = request.args.get('target', '') if SAFE_REGEX.match(target): return redirect(target) The unsafe check is easy to bypass because the unescaped . allows for any character before example.com, effectively allowing the redirect to go to an attacker-controlled domain such as wwwXexample.com.
The safe check closes this vulnerability by escaping the . so that URLs of the form wwwXexample.com are rejected.