mirror of
https://github.com/swisskyrepo/PayloadsAllTheThings.git
synced 2024-12-23 21:05:27 +00:00
74 lines
2.8 KiB
Markdown
74 lines
2.8 KiB
Markdown
# 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
|
||
|
||
* [Tools](#tools)
|
||
* [Evil Regex](#evil-regex)
|
||
* [Backtrack Limit](#backtrack-limit)
|
||
* [References](#references)
|
||
|
||
|
||
## Tools
|
||
|
||
* [tjenkinson/redos-detector](https://github.com/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](https://github.com/doyensec/regexploit) - Find regular expressions which are vulnerable to ReDoS (Regular Expression Denial of Service)
|
||
* [devina.io/redos-checker](https://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:
|
||
|
||
```php
|
||
$pattern = '/(a+)+$/';
|
||
$subject = str_repeat('a', 1000) . 'b';
|
||
|
||
if (preg_match($pattern, $subject)) {
|
||
echo "Match found";
|
||
} else {
|
||
echo "No match";
|
||
}
|
||
```
|
||
|
||
|
||
## References
|
||
|
||
- [Intigriti Challenge 1223 - Hackbook Of A Hacker - December 21, 2023](https://simones-organization-4.gitbook.io/hackbook-of-a-hacker/ctf-writeups/intigriti-challenges/1223)
|
||
- [MyBB Admin Panel RCE CVE-2023-41362 - SorceryIE - September 11, 2023](https://blog.sorcery.ie/posts/mybb_acp_rce/)
|
||
- [OWASP Validation Regex Repository - OWASP - March 14, 2018](https://wiki.owasp.org/index.php/OWASP_Validation_Regex_Repository)
|
||
- [PCRE > Installing/Configuring - PHP Manual - May 3, 2008](https://www.php.net/manual/en/pcre.configuration.php#ini.pcre.recursion-limit)
|
||
- [Regular expression Denial of Service - ReDoS - Adar Weidman - December 4, 2019](https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS) |