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.
fromflaskimportFlask,request,redirectimportreapp=Flask(__name__)UNSAFE_REGEX=re.compile("(www|beta).example.com/")SAFE_REGEX=re.compile(r"(www|beta)\.example\.com/")@app.route('/some/path/bad')defunsafe(request):target=request.args.get('target','')ifUNSAFE_REGEX.match(target):returnredirect(target)@app.route('/some/path/good')defsafe(request):target=request.args.get('target','')ifSAFE_REGEX.match(target):returnredirect(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.
