{"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\/ro\/blog\/news\/uyazvimost-v-tls-dopuskayushhaya-opredelenie-klyucha-dlya-soedinenij-na-baze-shifrov-dh","title":{"rendered":"Vulnerabilitate \u00een TLS care permite determinarea cheii pentru conexiuni bazate pe criptarea DH","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"

Dezv\u0103luite<\/a><\/noindex> informa\u021bii despre noua vulnerabilit\u0103\u021bi<\/a><\/noindex> (CVE-2020-1968) \u00een protocolul TLS, cunoscut sub numele de cod
\nRaccoon<\/a><\/noindex> \u0219i care permite, \u00een condi\u021bii rare, determinarea cheii primare preliminare (pre-master), care poate fi utilizat\u0103 pentru decriptarea conexiunilor TLS, inclusiv HTTPS, \u00een timpul intercept\u0103rii traficului de tranzit (MITM). Se remarc\u0103 faptul c\u0103 atacul este foarte complex pentru implementare practic\u0103 \u0219i este mai mult de natur\u0103 teoretic\u0103. Pentru a efectua atacul, este nevoie de o configura\u021bie specific\u0103 a serverului TLS \u0219i de capacitatea de a m\u0103sura cu precizie timpul de procesare a opera\u021biunilor de c\u0103tre server.<\/p>\n

Problema este prezent\u0103 direct \u00een specifica\u021bia TLS \u0219i afecteaz\u0103 doar conexiunile care utilizeaz\u0103 cipher-uri pe baz\u0103 de protocol de schimb de chei DH (Diffie-Hellman, TLS_DH_*). Cu cipher-urile ECDH, problema nu se manifest\u0103 \u0219i acestea r\u0103m\u00e2n sigure. Sunt vulnerabile doar protocoalele TLS p\u00e2n\u0103 la versiunea 1.2 inclusiv, protocolul TLS 1.3 nu este afectat. Vulnerabilitatea se manifest\u0103 \u00een implement\u0103rile TLS care reutilizeaz\u0103 cheia secret\u0103 DH \u00een diferite conexiuni TLS (un comportament similar se observ\u0103 pe aproximativ 4.4% din serverele din clasamentul Alexa Top 1M).<\/p>\n

\u00cen OpenSSL 1.0.2e \u0219i versiunile anterioare, cheia principal\u0103 DH este reutilizat\u0103 \u00een toate conexiunile serverului, cu excep\u021bia cazului \u00een care op\u021biunea SSL_OP_SINGLE_DH_USE este setat\u0103 explicit. \u00cencep\u00e2nd cu OpenSSL 1.0.2f, cheia principal\u0103 DH este reutilizat\u0103 doar atunci c\u00e2nd se utilizeaz\u0103 cipher-uri DH static (\"DH-*\", de exemplu \"DH-RSA-AES256-SHA\"). \u00cen OpenSSL 1.1.1, vulnerabilitatea nu se manifest\u0103, deoarece \u00een aceast\u0103 ramur\u0103 nu se utilizeaz\u0103 cheia principal\u0103 DH \u0219i nu se aplic\u0103 cipher-uri DH statice. <\/p>\n

Atunci c\u00e2nd se utilizeaz\u0103 metoda de schimb de chei DH, ambele p\u0103r\u021bi ale conexiunii genereaz\u0103 chei private aleatorii (denumite \u00een continuare cheia \u201ea\u201d \u0219i cheia \u201eb\u201d), pe baza c\u0103rora se calculeaz\u0103 \u0219i se trimit chei publice (ga mod p \u0219i gb mod p). Dup\u0103 primirea cheilor publice de c\u0103tre fiecare parte, se calculeaz\u0103 cheia principal\u0103 comun\u0103 (gab mod p), care este utilizat\u0103 pentru a forma cheile de sesiune. Atacul Raccoon permite determinarea cheii principale prin analizarea informa\u021biilor de pe canale secundare, av\u00e2nd \u00een vedere c\u0103 \u00een specifica\u021biile TLS p\u00e2n\u0103 la versiunea 1.2 se prevede s\u0103 se elimine to\u021bi byte-ii ini\u021biali zero ai cheii principale \u00eenainte de calculele care implic\u0103 aceasta.<\/p>\n

Inclusiv un cheie primar\u0103 redus\u0103 este transmis la func\u021bia de generare a cheii de sesiune, bazat\u0103 pe func\u021bii hash cu \u00eent\u00e2rzieri diferite \u00een procesarea datelor. M\u0103surarea exact\u0103 a timpului opera\u021biunilor efectuate de server cu cheia permite atacatorului s\u0103 determine sugestii (oracle), care ofer\u0103 posibilitatea de a judeca dac\u0103 cheia primar\u0103 \u00eencepe cu zero sau nu. De exemplu, atacatorul poate intercepta cheia public\u0103 (ga) trimis\u0103 de client, o poate redirec\u021biona c\u0103tre server \u0219i poate determina
\ndac\u0103 cheia primar\u0103 rezultat\u0103 \u00eencepe cu zero.<\/p>\n

Definirea unui singur byte de cheie nu ofer\u0103 nimic de la sine, dar prin interceptarea valorii \u201ega\u201d transmise \u00een timpul acordului de conexiune de c\u0103tre client, atacatorul poate forma un set de alte valori asociate cu \u201ega\u201d \u0219i le poate trimite pe server \u00een sesiuni separate de acord. Form\u00e2nd \u0219i trimi\u021b\u00e2nd valori \u201egri*ga\u201d, atacatorul poate, prin analiza modific\u0103rilor \u00eent\u00e2rzierilor \u00een r\u0103spunsul serverului, determina valorile care conduc la ob\u021binerea cheilor principale care \u00eencep cu zero. Odat\u0103 ce atacatorul a determinat astfel de valori, acesta poate construi un set de ecua\u021bii pentru de a livra versiunea LTS a Qt doar sub licen\u021b\u0103 comercial\u0103, compania Qt Company ia \u00een considerare trecerea la un model de distribu\u021bie a Qt, \u00een care toate versiunile din primele 12 luni vor fi disponibile doar pentru utilizatorii cu licen\u021be comerciale. Despre aceast\u0103 inten\u021bie, compania Qt Company a informat organiza\u021bia KDE e.V., care coordoneaz\u0103 dezvoltarea KDE.<\/a><\/noindex> problema numerelor ascunse<\/a><\/noindex> \u0219i calcula cheia primar\u0103 ini\u021bial\u0103. <\/p>\n

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

\u00cen OpenSSL, vulnerabilit\u0103\u021bile a fost clasificat\u0103<\/a><\/noindex> nivel sc\u0103zut de risc, iar corectarea s-a redus la mutarea \u00een versiunea 1.0.2w a cipurilor problematice «TLS_DH_*» \u00een categoria de cifruri dezactivate implicit din cauza nivelului insuficient de protec\u021bie («weak-ssl-ciphers»). O abordare similar\u0103 au avut \u0219i dezvoltatorii Mozilla, care dezactivat<\/a><\/noindex> \u00een biblioteca NSS, utilizat\u0103 \u00een Firefox, au dezactivat seturile de cifre DH \u0219i DHE. \u00cencep\u00e2nd cu Firefox 78, cifrele problematice sunt dezactivate. \u00cen Chrome, suportul pentru DH a fost \u00eentrerupt \u00eenc\u0103 din 2016. Bibliotecile BearSSL, BoringSSL, Botan, Mbed TLS \u0219i s2n nu sunt afectate de problem\u0103, deoarece nu suport\u0103 cifrurile DH sau variantele statice ale cifrurilor DH. <\/p>\n

Sunt men\u021bionate separat probleme suplimentare (CVE-2020-5929<\/a><\/noindex>) \u00een stiva TLS a dispozitivelor F5 BIG-IP, ceea ce face atacul mai realist. \u00cen special, au fost identificate abateri \u00een comportamentul dispozitivelor atunci c\u00e2nd exist\u0103 un byte zero la \u00eenceputul cheii primare, care pot fi utilizate \u00een loc de m\u0103surarea timpului exact al \u00eent\u00e2rzierilor \u00een timpul calculelor.<\/p>\n

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

Sursa: opennet.ro<\/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 \u0438\u0441\u043f\u043e\u043b\u044c\u0437\u043e\u0432\u0430\u0442\u044c \u0434\u043b\u044f \u0440\u0430\u0441\u0448\u0438\u0444\u0440\u043e\u0432\u043a\u0438 TLS-\u0441\u043e\u0435\u0434\u0438\u043d\u0435\u043d\u0438\u0439, \u0432 \u0442\u043e\u043c \u0447\u0438\u0441\u043b\u0435 HTTPS, \u043f\u0440\u0438 \u043f\u0435\u0440\u0435\u0445\u0432\u0430\u0442\u0435 \u0442\u0440\u0430\u043d\u0437\u0438\u0442\u043d\u043e\u0433\u043e \u0442\u0440\u0430\u0444\u0438\u043a\u0430 (MITM). \u041e\u0442\u043c\u0435\u0447\u0430\u0435\u0442\u0441\u044f, \u0447\u0442\u043e \u0430\u0442\u0430\u043a\u0430 \u043e\u0447\u0435\u043d\u044c \u0441\u043b\u043e\u0436\u043d\u0430 \u0434\u043b\u044f \u043f\u0440\u0430\u043a\u0442\u0438\u0447\u0435\u0441\u043a\u043e\u0439 \u0440\u0435\u0430\u043b\u0438\u0437\u0430\u0446\u0438\u0438 \u0438 \u0431\u043e\u043b\u044c\u0448\u0435 \u043d\u043e\u0441\u0438\u0442 \u0442\u0435\u043e\u0440\u0435\u0442\u0438\u0447\u0435\u0441\u043a\u0438\u0439 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