Regular Expression

Regular Expression Denial of Service (ReDoS) is a type of attack that exploits the fact that certain regular expressions can take an extremely long time to process, causing applications or services to become unresponsive or crash.

Summary

Denial of Service - ReDoS
- Evil Regex
- Backtrack Limit
References

Denial of Service - ReDoS

tjenkinson/redos-detector - A CLI and library which tests with certainty if a regex pattern is safe from ReDoS attacks. Supported in the browser, Node and Deno.
doyensec/regexploit - Find regular expressions which are vulnerable to ReDoS (Regular Expression Denial of Service)
devina.io/redos-checker - Examine regular expressions for potential Denial of Service vulnerabilities

Evil Regex

Evil Regex contains:

Grouping with repetition
Inside the repeated group:
- Repetition
- Alternation with overlapping

Examples

(a+)+
([a-zA-Z]+)*
(a|aa)+
(a|a?)+
(.*a){x} for x > 10

These regular expressions can be exploited with aaaaaaaaaaaaaaaaaaaaaaaa!

Backtrack Limit

Backtracking in regular expressions occurs when the regex engine tries to match a pattern and encounters a mismatch. The engine then backtracks to the previous matching position and tries an alternative path to find a match. This process can be repeated many times, especially with complex patterns and large input strings.

PHP PCRE configuration options

Name	Default	Note
pcre.backtrack_limit	1000000	100000 for `PHP < 5.3.7`
pcre.recursion_limit	100000	/
pcre.jit	1	/

Sometimes it is possible to force the regex to exceed more than 100 000 recursions which will cause a ReDOS and make preg_match returning false:

$pattern = '/(a+)+$/';
$subject = str_repeat('a', 1000) . 'b';

if (preg_match($pattern, $subject)) {
    echo "Match found";
} else {
    echo "No match";
}

2.9 KiB Raw Permalink Blame History

Regular Expression

Summary

Denial of Service - ReDoS

Evil Regex

Backtrack Limit

References

2.9 KiB

Raw Permalink Blame History