This rule is deprecated; use S5852, S2631 instead.
Using regular expressions is security-sensitive. It has led in the past to the following vulnerabilities:
Evaluating regular expressions against input strings is potentially an extremely CPU-intensive task. Specially crafted regular expressions such as
(a+)+s will take several seconds to evaluate the input string aaaaaaaaaaaaaaaaaaaaaaaaaaaaabs. The problem is that with
every additional a character added to the input, the time required to evaluate the regex doubles. However, the equivalent regular
expression, a+s (without grouping) is efficiently evaluated in milliseconds and scales linearly with the input size.
Evaluating such regular expressions opens the door to Regular expression Denial of Service (ReDoS) attacks.
In the context of a web application, attackers can force the web server to spend all of its resources evaluating regular expressions thereby making
the service inaccessible to genuine users.
This rule flags any execution of a hardcoded regular expression which has at least 3 characters and at least two instances of any of the following
characters: *+{.
Example: (a+)*
Ask Yourself Whether
- the executed regular expression is sensitive and a user can provide a string which will be analyzed by this regular expression.
- your regular expression engine performance decrease with specially crafted inputs and regular expressions.
There is a risk if you answered yes to any of those questions.
Recommended Secure Coding Practices
Check whether your regular expression engine (the algorithm executing your regular expression) has any known vulnerabilities. Search for
vulnerability reports mentioning the one engine you’re are using.
Use if possible a library which is not vulnerable to Redos Attacks such as Google Re2.
Remember also that a ReDos attack is possible if a user-provided regular expression is executed. This rule won’t detect this kind of injection.
Sensitive Code Example
Django
from django.core.validators import RegexValidator
from django.urls import re_path
RegexValidator('(a*)*b') # Sensitive
def define_http_endpoint(view):
re_path(r'^(a*)*b/$', view) # Sensitive
re module
import re
from re import compile, match, search, fullmatch, split, findall, finditer, sub, subn
input = 'input string'
replacement = 'replacement'
re.compile('(a*)*b') # Sensitive
re.match('(a*)*b', input) # Sensitive
re.search('(a*)*b', input) # Sensitive
re.fullmatch('(a*)*b', input) # Sensitive
re.split('(a*)*b', input) # Sensitive
re.findall('(a*)*b', input) # Sensitive
re.finditer('(a*)*b',input) # Sensitive
re.sub('(a*)*b', replacement, input) # Sensitive
re.subn('(a*)*b', replacement, input) # Sensitive
regex module
import regex
from regex import compile, match, search, fullmatch, split, findall, finditer, sub, subn, subf, subfn, splititer
input = 'input string'
replacement = 'replacement'
regex.subf('(a*)*b', replacement, input) # Sensitive
regex.subfn('(a*)*b', replacement, input) # Sensitive
regex.splititer('(a*)*b', input) # Sensitive
regex.compile('(a*)*b') # Sensitive
regex.match('(a*)*b', input) # Sensitive
regex.search('(a*)*b', input) # Sensitive
regex.fullmatch('(a*)*b', input) # Sensitive
regex.split('(a*)*b', input) # Sensitive
regex.findall('(a*)*b', input) # Sensitive
regex.finditer('(a*)*b',input) # Sensitive
regex.sub('(a*)*b', replacement, input) # Sensitive
regex.subn('(a*)*b', replacement, input) # Sensitive
Exceptions
Some corner-case regular expressions will not raise an issue even though they might be vulnerable. For example: (a|aa)+,
(a|a?)+.
It is a good idea to test your regular expression if it has the same pattern on both side of a "|".
See
