{"id":93917,"date":"2020-09-11T01:41:59","date_gmt":"2020-09-10T23:41:59","guid":{"rendered":"https:\/\/prohoster.info\/blog\/novosti-interneta\/uyazvimost-v-tls-dopuskayushhaya-opredelenie-klyucha-dlya-soedinenij-na-baze-shifrov-dh"},"modified":"2020-09-11T01:41:59","modified_gmt":"2020-09-10T23:41:59","slug":"uyazvimost-v-tls-dopuskayushhaya-opredelenie-klyucha-dlya-soedinenij-na-baze-shifrov-dh","status":"publish","type":"post","link":"https:\/\/prohoster.info\/hr\/blog\/news\/uyazvimost-v-tls-dopuskayushhaya-opredelenie-klyucha-dlya-soedinenij-na-baze-shifrov-dh","title":{"rendered":"Ranljivost u TLS-u koja omogu\u0107ava odre\u0111ivanje klju\u010da za veze zasnovane na DH \u0161iframa","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"

Otkriveno<\/a><\/noindex> informacije o novoj ranjivosti<\/a><\/noindex> (CVE-2020-1968) u TLS protokolu, koji je dobio ime
\nRaccoon<\/a><\/noindex> i omogu\u0107uje u vrlo rijetkim okolnostima odre\u0111ivanje preliminarnog glavnog klju\u010da (pre-master) koji se mo\u017ee koristiti za de\u0161ifriranje TLS veza, uklju\u010duju\u0107i HTTPS, prilikom presretanja tranzitnog prometa (MITM). Napominje se da je napad vrlo slo\u017een za prakti\u010dnu provedbu i vi\u0161e je teoretske prirode. Za provo\u0111enje napada potrebna je specifi\u010dna konfiguracija TLS poslu\u017eitelja i mogu\u0107nost vrlo preciznog mjerenja vremena obrade operacija od strane poslu\u017eitelja.<\/p>\n

Problem se osje\u0107a direktno u TLS specifikacijama i utje\u010de isklju\u010divo na veze koje koriste \u0161ifre temeljene na protokolu razmjene klju\u010deva DH (Diffie-Hellman, TLS_DH_*). S ECDH \u0161iframa problem se ne manifestira, a one ostaju sigurne. Vulnerabilne su samo TLS verzije do 1.2 uklju\u010divo; protokol TLS 1.3 nije podlo\u017ean ovom problemu. Vulnerabilnost se manifestira u TLS implementacijama koje ponovo koriste tajni DH klju\u010d u razli\u010ditim TLS vezama (takvo pona\u0161anje zabilje\u017eeno je na otprilike 4.4% servera s Alexa Top 1M liste).<\/p>\n

U OpenSSL 1.0.2e i ranijim izdanjima, primarni DH klju\u010d ponovo se koristi u svim server ovim vezama, osim ako nije izri\u010dito postavljena opcija SSL_OP_SINGLE_DH_USE. Od OpenSSL 1.0.2f, primarni DH klju\u010d ponovo se koristi samo prilikom kori\u0161tenja stati\u010dkih DH \u0161ifara (\"DH-*,\" npr. \"DH-RSA-AES256-SHA\"). U OpenSSL 1.1.1, vulnerabilnost se ne manifestira, jer se u ovoj grani ne koristi primarni DH klju\u010d ni stati\u010dke DH \u0161ifre. <\/p>\n

Kori\u0161tenjem DH metode razmjene klju\u010deva, obje strane veze generiraju nasumi\u010dne privatne klju\u010deve (dalje klju\u010d \"a\" i klju\u010d \"b\"), na temelju kojih se izra\u010dunavaju i \u0161alju javni klju\u010devi (ga mod p i gb mod p). Nakon primanja javnih klju\u010deva, svaka strana izra\u010dunava zajedni\u010dki primarni klju\u010d (gab mod p), koji se koristi za formiranje sesijskih klju\u010deva. Raccoon napad omogu\u0107ava odre\u0111ivanje primarnog klju\u010da analizom informacija putem sporednih kanala, oslanjaju\u0107i se na to da se u TLS specifikacijama do verzije 1.2 propisuje odbacivanje svih po\u010detnih nultih bajtova primarnog klju\u010da prije izra\u010duna koji uklju\u010duju taj klju\u010d.<\/p>\n

Uklju\u010duju\u0107i skra\u0107eni primarni klju\u010d, koji se \u0161alje u funkciju za generiranje sesijskog klju\u010da, baziranu na hash funkcijama s razli\u010ditim ka\u0161njenjima prilikom obrade razli\u010ditih podataka. To\u010dno mjerenje vremena operacija servera s klju\u010dem omogu\u0107ava napada\u010du da odredi naznake (oracle) koje daju mogu\u0107nost da se zaklju\u010di po\u010dinje li primarni klju\u010d s nulom ili ne. Na primjer, napada\u010d mo\u017ee presresti otvoreni klju\u010d (ga) koji je poslao klijent, ponovno ga poslati serveru i odrediti
\npo\u010dinje li s nulom rezultantni primarni klju\u010d.<\/p>\n

Samo po sebi, odre\u0111ivanje jednog bajta klju\u010da ne donosi ni\u0161ta, ali presretanjem vrijednosti \"ga\" poslane od strane klijenta tijekom pregovora o vezi, napada\u010d mo\u017ee generirati skup drugih vrijednosti povezanih s \"ga\" i poslati ih serveru u odvojenim sesijama pregovora o vezi. Formiraju\u0107i i \u0161alju\u0107i vrijednosti \"gri*ga\", napada\u010d mo\u017ee putem analize promjena ka\u0161njenja u odgovoru servera odrediti vrijednosti koje vode do primarnih klju\u010deva koji po\u010dinju s nulom. Utvr\u0111ivanjem takvih vrijednosti, napada\u010d mo\u017ee sastaviti skup jednad\u017ebi za odluke<\/a><\/noindex> problem skrivenih brojeva<\/a><\/noindex> i izra\u010dunati izvorni primarni klju\u010d. <\/p>\n

\"Ranljivost<\/center><\/p>\n

U OpenSSL-u, ranjivosti dodijeljen<\/a><\/noindex> nizak nivo opasnosti, a ispravka se svodila na preme\u0161tanje u izdanju 1.0.2w problemati\u010dnih \u0161ifri «TLS_DH_*» u kategoriju \u0161ifri sa nedovoljnim nivoom za\u0161tite («weak-ssl-ciphers»), koja je prema zadatku isklju\u010dena. Sli\u010dan pristup su imali i developeri Mozille, koji isklju\u010dili<\/a><\/noindex> u NSS biblioteci koja se koristi u Firefoxu, skupove \u0161ifri DH i DHE. Od Firefox 78 problemati\u010dne \u0161ifre su isklju\u010dene. U Chromeu je podr\u0161ka za DH prekinuta jo\u0161 2016. godine. Biblioteke BearSSL, BoringSSL, Botan, Mbed TLS i s2n nisu pogo\u0111ene problemom jer ne podr\u017eavaju DH \u0161ifre ili stati\u010dke varijante DH \u0161ifri. <\/p>\n

Posebno se isti\u010du dodatni problemi (CVE-2020-5929<\/a><\/noindex>) u TLS staku F5 BIG-IP ure\u0111aja, \u0161to \u010dini napad realisti\u010dnijim. Konkretno, otkrivene su devijacije u pona\u0161anju ure\u0111aja pri prisutnosti nultog bajta na po\u010detku primarnog klju\u010da, koje se mogu koristiti umjesto mjerenja to\u010dnog ka\u0161njenja prilikom izra\u010duna.<\/p>\n

<\/a><\/noindex><\/p>\n

Izvor: opennet.ru<\/a><\/p>","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"excerpt":{"rendered":"

\u0420\u0430\u0441\u043a\u0440\u044b\u0442\u044b \u0441\u0432\u0435\u0434\u0435\u043d\u0438\u044f \u043e \u043d\u043e\u0432\u043e\u0439 \u0443\u044f\u0437\u0432\u0438\u043c\u043e\u0441\u0442\u0438 (CVE-2020-1968) \u0432 \u043f\u0440\u043e\u0442\u043e\u043a\u043e\u043b\u0435 TLS, \u043f\u043e\u043b\u0443\u0447\u0438\u0432\u0448\u0435\u0439 \u043a\u043e\u0434\u043e\u0432\u043e\u0435 \u0438\u043c\u044f Raccoon \u0438 \u043f\u043e\u0437\u0432\u043e\u043b\u044f\u044e\u0449\u0435\u0439 \u043f\u0440\u0438 \u0440\u0435\u0434\u043a\u043e\u043c \u0441\u0442\u0435\u0447\u0435\u043d\u0438\u0438 \u043e\u0431\u0441\u0442\u043e\u044f\u0442\u0435\u043b\u044c\u0441\u0442\u0432 \u043e\u043f\u0440\u0435\u0434\u0435\u043b\u0438\u0442\u044c \u043f\u0440\u0435\u0434\u0432\u0430\u0440\u0438\u0442\u0435\u043b\u044c\u043d\u044b\u0439 \u043f\u0435\u0440\u0432\u0438\u0447\u043d\u044b\u0439 \u043a\u043b\u044e\u0447 (pre-master), \u043a\u043e\u0442\u043e\u0440\u044b\u0439 \u043c\u043e\u0436\u043d\u043e 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