RegexInjection.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>
<overview>
<p>
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.
</p>
</overview>

<recommendation>
<p>
Before embedding user input into a regular expression, use a sanitization function such as
<code>re.escape</code> to escape meta-characters that have a special meaning regarding 
regular expressions' syntax.
</p>
</recommendation>

<example>
<p>
The following examples are based on a simple Flask web server environment.
</p>
<p>
The following example shows a HTTP request parameter that is used to construct a regular expression
without sanitizing it first:
</p>
<sample src="re_bad.py" />
<p>
Instead, the request parameter should be sanitized first, for example using the function
<code>re.escape</code>. This ensures that the user cannot insert characters which have a
special meaning in regular expressions.
</p>
<sample src="re_good.py" />
</example>

<references>
<li>OWASP: <a href="https://www.owasp.org/index.php/Regular_expression_Denial_of_Service_-_ReDoS">Regular expression Denial of Service - ReDoS</a>.</li>
<li>Wikipedia: <a href="https://en.wikipedia.org/wiki/ReDoS">ReDoS</a>.</li>
<li>Python docs: <a href="https://docs.python.org/3/library/re.html">re</a>.</li>
<li>SonarSource: <a href="https://rules.sonarsource.com/python/type/Vulnerability/RSPEC-2631">RSPEC-2631</a>.</li>
</references>
</qhelp>