CodeQL documentation

Regular expression injection

ID: py/regex-injection
Kind: path-problem
Security severity: 7.5
Severity: error
Precision: high
   - security
   - external/cwe/cwe-730
   - external/cwe/cwe-400
Query suites:
   - python-code-scanning.qls
   - python-security-extended.qls
   - python-security-and-quality.qls

Click to see the query in the CodeQL repository

Constructing a regular expression with unsanitized user input is dangerous as a malicious user may be able to modify the meaning of the expression. In particular, such a user may be able to provide a regular expression fragment that takes exponential time in the worst case, and use that to perform a Denial of Service attack.


Before embedding user input into a regular expression, use a sanitization function such as re.escape to escape meta-characters that have a special meaning regarding regular expressions’ syntax.


The following examples are based on a simple Flask web server environment.

The following example shows a HTTP request parameter that is used to construct a regular expression without sanitizing it first:

from flask import request, Flask
import re

def direct():
    unsafe_pattern = request.args["pattern"], "")

def compile():
    unsafe_pattern = request.args["pattern"]
    compiled_pattern = re.compile(unsafe_pattern)"")

Instead, the request parameter should be sanitized first, for example using the function re.escape. This ensures that the user cannot insert characters which have a special meaning in regular expressions.

from flask import request, Flask
import re

def direct():
    unsafe_pattern = request.args['pattern']
    safe_pattern = re.escape(unsafe_pattern), "")

def compile():
    unsafe_pattern = request.args['pattern']
    safe_pattern = re.escape(unsafe_pattern)
    compiled_pattern = re.compile(safe_pattern)"")


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