{"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\/da\/blog\/news\/uyazvimost-v-tls-dopuskayushhaya-opredelenie-klyucha-dlya-soedinenij-na-baze-shifrov-dh","title":{"rendered":"S\u00e5rbarhed i TLS, der muligg\u00f8r bestemmelse af n\u00f8glen for forbindelser baseret p\u00e5 DH-krypteringsalgoritmer","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"

Afsl\u00f8rede<\/a><\/noindex> information om den nye s\u00e5rbarheder<\/a><\/noindex> (CVE-2020-1968) i TLS-protokollen, der har kodeordet
\nRaccoon<\/a><\/noindex> og som under sj\u00e6ldne omst\u00e6ndigheder muligg\u00f8r bestemmelse af den pre-master hemmelige n\u00f8gle, der kan bruges til at dekryptere TLS-forbindelser, herunder HTTPS, ved at opsnappe transit trafik (MITM). Det bem\u00e6rkes, at angrebet er meget vanskeligt at implementere i praksis og mere har en teoretisk karakter. For at udf\u00f8re angrebet kr\u00e6ves der en specifik konfiguration af TLS-serveren og muligheden for meget pr\u00e6cist at m\u00e5le behandlingstiden for operationer fra serverens side.<\/p>\n

Problemet findes direkte i TLS-specifikationen og p\u00e5virker kun forbindelser, der bruger krypteringsmetoder baseret p\u00e5 DH-n\u00f8gleudvekslingsprotokollen (Diffie-Hellman, TLS_DH_*). Med ECDH-krypteringer viser problemet sig ikke, og de forbliver sikre. Kun TLS-protokoller op til og med version 1.2 er s\u00e5rbare, mens TLS 1.3 ikke er p\u00e5virket af problemet. S\u00e5rbarheden forekommer i TLS-implementeringer, der genbruger den hemmelige DH-n\u00f8gle i forskellige TLS-forbindelser (s\u00e5danne adf\u00e6rd observeres p\u00e5 omtrent 4,4 % af serverne fra Alexa Top 1M).<\/p>\n

I OpenSSL 1.0.2e og tidligere versioner genbruges den prim\u00e6re DH-n\u00f8gle i alle serverforbindelser, medmindre indstillingen SSL_OP_SINGLE_DH_USE eksplicit er sl\u00e5et til. Fra OpenSSL 1.0.2f genbruges den prim\u00e6re DH-n\u00f8gle kun, n\u00e5r der anvendes statiske DH-krypteringer (\"DH-*\", for eksempel \"DH-RSA-AES256-SHA\"). I OpenSSL 1.1.1 viser s\u00e5rbarheden sig ikke, da den prim\u00e6re DH-n\u00f8gle ikke anvendes i denne version, og statiske DH-krypteringer ikke benyttes. <\/p>\n

Ved brug af DH-n\u00f8gleudvekslingsmetoden genererer begge parter i forbindelsen tilf\u00e6ldige private n\u00f8gler (kaldet n\u00f8gle \"a\" og n\u00f8gle \"b\"), hvorp\u00e5 de offentlige n\u00f8gler (ga mod p og gb mod p) beregnes og sendes. Efter modtagelsen af de offentlige n\u00f8gler beregner hver part den f\u00e6lles prim\u00e6re n\u00f8gle (gab mod p), som anvendes til at danne sessionn\u00f8gler. Raccoon-angrebet g\u00f8r det muligt at bestemme den prim\u00e6re n\u00f8gle ved at analysere oplysninger fra sidekanaler, idet det i TLS-specifikationer op til version 1.2 foreskrives at kaste alle indledende null byte i den prim\u00e6re n\u00f8gle f\u00f8r beregningerne.<\/p>\n

Den besk\u00e5rne prim\u00e6re n\u00f8gle overf\u00f8res til sessionn\u00f8glegenereringsfunktionen, som er baseret p\u00e5 hash-funktioner med forskellige forsinkelser ved behandling af forskellige data. Pr\u00e6cise m\u00e5linger af den tid, serveren bruger p\u00e5 operationer med n\u00f8glen, g\u00f8r det muligt for en angriber at bestemme spor (oracle), som giver mulighed for at vurdere, om den prim\u00e6re n\u00f8gle begynder med nul eller ej. For eksempel kan en angriber opsnappe den \u00e5bne n\u00f8gle (ga), der sendes af klienten, videresende den til serveren og bestemme
\nom den resulterende prim\u00e6re n\u00f8gle begynder med nul.<\/p>\n

Selv definitionen af \u00e9n byte af n\u00f8glen giver ikke meget, men ved at opsnappe den v\u00e6rdi, som klienten sender ved aftaler om forbindelsen, kaldet \"ga\", kan angriberen danne et s\u00e6t af andre v\u00e6rdier relateret til \"ga\" og sende dem til serveren i separate sessionsaftaler. Ved at danne og sende v\u00e6rdierne \"gri*ga\", kan angriberen gennem analyse af \u00e6ndringer i serverens svartider bestemme de v\u00e6rdier, der f\u00f8rer til prim\u00e6re n\u00f8gler, der starter med nul. N\u00e5r s\u00e5danne v\u00e6rdier er identificeret, kan angriberen opstille et s\u00e6t ligninger for. l\u00f8sninger<\/a><\/noindex> problemet med skjulte tal<\/a><\/noindex> og beregne den oprindelige prim\u00e6re n\u00f8gle. <\/p>\n

\"S\u00e5rbarhed<\/center><\/p>\n

I OpenSSL er s\u00e5rbarhederne bliver tildelt<\/a><\/noindex> lavt niveau af fare, og rettelsen blev reduceret til at flytte problematiske krypter med «TLS_DH_*» til den som standard deaktiverede kategori af krypter med utilstr\u00e6kkeligt beskyttelsesniveau («weak-ssl-ciphers»). Tilsvarende gjorde udviklerne hos Mozilla, deaktiverede<\/a><\/noindex> s\u00e6ttene af krypteringer DH og DHE i NSS-biblioteket, der anvendes i Firefox. Fra Firefox 78 er de problematiske krypteringer deaktiveret. I Chrome blev underst\u00f8ttelsen af DH stoppet allerede i 2016. Bibliotekerne BearSSL, BoringSSL, Botan, Mbed TLS og s2n er ikke p\u00e5virket af problemet, da de ikke underst\u00f8tter DH-krypteringer eller statiske varianter af DH-krypteringer. <\/p>\n

Derudover bem\u00e6rkes ekstra problemer (CVE-2020-5929<\/a><\/noindex>) i TLS-stakken p\u00e5 F5 BIG-IP-enheder, som g\u00f8r angreb mere realistiske. Is\u00e6r blev der identificeret afvigelser i enhedernes adf\u00e6rd ved tilstedev\u00e6relsen af et nul-byte i begyndelsen af den prim\u00e6re n\u00f8gle, som kan bruges i stedet for at m\u00e5le den pr\u00e6cise tid forsinkelse ved beregninger.<\/p>\n

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

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

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