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+Source: https://github.com/attackercan/regexp-security-cheatsheet/blob/master/README.md <br>
+
+# Regexp Security Cheatsheet
+
+Research was done to find "weak places" in regular expressions of Web Application Firewalls (WAFs).  
+Repository contains SAST, which can help you to find security vulnerabilities in custom regular expressions in own projects.  
+Contribution is highly welcomed.  
+
+### High severity issues:
+|#| Requirement  | Vulnerable regex example  | Bypass example |
+|---|---|---|---|
+|1|  Regexp should avoid using `^` (alternative: `\A`) and `$` (alternative: `\Z`) symbols, which are metacharacters for start and end of a string. It is possible to bypass regex by inserting any symbol in front or after regexp. | `(^a|a$)`  |   `%20a%20`
+|2| Regexp should be case-insensitive: `(?i:` or `/regex/i`. It is possible to bypass regex using upper or lower cases in words. [Modsecurity transformation commands](https://github.com/SpiderLabs/ModSecurity/wiki/Reference-Manual#cmdLine) (which are applied on string before regex pattern is applied) can also be included in tests to cover more regexps.  |  `http` | `hTtP`
+|3| In case modifier `/m` is not (globally) specified, regexp should avoid using dot `.` symbol, which means every symbol except newline (`\n`). It is possible to bypass regex using [newline injection](https://www.htbridge.com/blog/bypassing-bitrix-web-application-firewall-via-tiny-regexp-error.html).  |  `a.*b` | `a%0Ab`
+|4|  Regexp should not be vulnerable to ReDoS. [OWASP ReDoS article](https://www.owasp.org/index.php/Regular_expression_Denial_of_Service_-_ReDoS) 1. Find various evil patterns.  2. Generate evil string using e.g. “SDL Regex Fuzzer”  |  `(a+)+`  |  `aaaaaaaaaaaaaaaaaaaa!`
+|5| Number of repetitions of set or group `{}` should be carefully used, as one can bypass such limitation by lowering or increasing specified numbers.  |  `a{1,5}` | `aaaaaa (6 times)`
+|6| Nonstandard ranges (almost everything except a-z, 0-9, a-f, etc)  |  ``[A-z] = [a-zA-Z] + [\]^_` `` | ``aaa[\]^_`aaa`` 
+|7| Regexp should only use plus “`+`” metacharacter in places where it is necessary, as it means “one or more”. Alternative metacharacter star “`*`”, which means “zero or more” is generally preferred. |  `a'\s+\d` | `a'5`
+|8| Usage of newline wildcards should be reasonable. `\r\n` characters can often be bypassed by either substitution, or by using newline alternative `\v`, `\f` and others. Wildcard `\b` has different meanings while using it in square brackets (“backspace”) and in plain regex (“word boundary”) - [RegexLib](http://regexlib.com/CheatSheet.aspx) | `a[^\n]*$`  | `a\n`? `a\r`?
+|9| Regexp should be applied to right scope of inputs: `Cookies names and values`, `Argument names and values`, `Header names and values`, `Files argument names and content`. Modsecurity: `grep -oP 'SecRule(.*?)"' -n` Other WAFs: manual observation. |  Argument values  | Cookie names and values
+|10| Regular expression writers should be careful while using only whitespace character (`%20`) as separators. Rule can be bypassed e.g. with newline character, tabulation, by skipping whitespace, or alternatives.  |  `a\s(not[whitespace]|and)\sb` | `a not b`
+|11| Nonstandard combinations of operators  |  `a||b` | `any_string`
+|12| Special cases: whitespaces before operators |  `(a |b)c` | `ac`
+|13| Usage of wrong syntax in POSIX character classes |  `a[digit]b` | `aab`
+|14| Opposite usage of brackets [], () and {} | `[SYSTEM|PUBLIC]` or `(a-z123)` | `SYSTEM` or `abcdef`
+
+### Medium severity issues (non-expected behaviour: manual observation needed):
+|#| Requirement  | Vulnerable regex example  | Bypass example |
+|---|---|---|---|
+|15| Check backlinks, and bear in mind that [`\11` can be backlink -OR- 0x09](http://php.net/manual/en/regexp.reference.escape.php) |  `(\d{1})=\1` | `1!=2`
+|16| Unsafe usage of comments | `a(?#some comment about wildcards:\)(\w*)b` | `afffb`
+|17| Excessive usage of metacharacters in [] | `[\w+]` | ` `
+|18| Rarely used [wildcards](http://php.net/manual/en/regexp.reference.escape.php). All wildcards except popular: A,Z,b,r,n,t,wW,sS,dD,u,x  | `\a = 0x07; \e = 0x1B; \R = \r|\n|\r\n; \xXX = 0xXX; \ddd = 0oddd; \cX, \x{XXXX}, \H, \V, \G` | ` `
+|19| Excessive escaping, e.g. escaping symbol which is not a wildcard | `\q` | ` `
+|20| Unsafe usage of [recursion](http://php.net/manual/ru/regexp.reference.recursive.php), IF statements, etc | `(?R`, `(?(id)true|false)`, ... | ` `
+|21| Unsafe usage of ranges | `[\0-9]` = `\0\1\2\3...$%&'...789` | ` `
+
+##### Experimental rules (probably to be removed):
+|#| Requirement  | Vulnerable regex example  | Bypass example |
+|---|---|---|---|
+|X| Greediness of regular expressions should be considered. Highlight of this topic is well done in [Chapter 9 of Jan Goyvaert’s tutorial](https://www.princeton.edu/~mlovett/reference/Regular-Expressions.pdf). While greediness itself does not create bypasses, bad implementation of regexp Greediness can raise False Positive rate. This can cause excessive log-file flooding, forcing vulnerable rule or even whole WAF to be switched off.  |   |
+|X| Best Practice from [slides of Ivan Novikov](http://www.slideshare.net/d0znpp/lie-tomephd2013): Modsecurity should avoid using t:base64Decode function (t:base64DecodeExt instead).  |  `t:base64Decode` | `detected=bypassed` 
+
+Vladimir Ivanov
+@httpsonly