{"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\/pt\/blog\/news\/uyazvimost-v-tls-dopuskayushhaya-opredelenie-klyucha-dlya-soedinenij-na-baze-shifrov-dh","title":{"rendered":"Uma vulnerabilidade no TLS permite a defini\u00e7\u00e3o da chave para conex\u00f5es baseadas em cifras DH","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"

Revelado<\/a><\/noindex> informa\u00e7\u00f5es sobre o novo no aplicativo para Windows, que permitia ao usu\u00e1rio elevar seus privil\u00e9gios no sistema para administrador (a vulnerabilidade foi causada pela intera\u00e7\u00e3o inadequada entre o cliente GUI sem privil\u00e9gios e o servi\u00e7o de sistema).<\/a><\/noindex> (CVE-2020-1968) no protocolo TLS, codinome
\nRaccoon<\/a><\/noindex> e que permite, em circunst\u00e2ncias raras, identificar a chave prim\u00e1ria preliminar (pre-master), que pode ser usada para decifrar conex\u00f5es TLS, incluindo HTTPS, ao interceptar tr\u00e1fego transit\u00f3rio (MITM). Observa-se que o ataque \u00e9 muito dif\u00edcil de ser implementado na pr\u00e1tica e tem mais car\u00e1ter te\u00f3rico. Para realizar o ataque, \u00e9 necess\u00e1ria uma configura\u00e7\u00e3o espec\u00edfica do servidor TLS e a capacidade de medir com precis\u00e3o o tempo de processamento das opera\u00e7\u00f5es pelo servidor.<\/p>\n

O problema est\u00e1 presente diretamente na especifica\u00e7\u00e3o do TLS e afeta apenas as conex\u00f5es que utilizam criptografia baseada no protocolo de troca de chaves DH (Diffie-Hellman, TLS_DH_*). Com as criptografias ECDH, o problema n\u00e3o se manifesta, e elas permanecem seguras. Apenas os protocolos TLS at\u00e9 a vers\u00e3o 1.2 inclusiva s\u00e3o vulner\u00e1veis, enquanto o protocolo TLS 1.3 n\u00e3o \u00e9 afetado. A vulnerabilidade se manifesta em implementa\u00e7\u00f5es do TLS que reutilizam a chave secreta DH em diferentes conex\u00f5es TLS (esse comportamento foi observado em cerca de 4,4% dos servidores do ranking Alexa Top 1M).<\/p>\n

Nas vers\u00f5es do OpenSSL 1.0.2e e anteriores, a chave prim\u00e1ria DH \u00e9 reutilizada em todas as conex\u00f5es do servidor, a menos que a op\u00e7\u00e3o SSL_OP_SINGLE_DH_USE seja explicitamente definida. A partir do OpenSSL 1.0.2f, a chave prim\u00e1ria DH \u00e9 reutilizada apenas ao usar criptografias DH est\u00e1ticas (\"DH-*\", por exemplo, \"DH-RSA-AES256-SHA\"). No OpenSSL 1.1.1, a vulnerabilidade n\u00e3o se manifesta, pois nesta vers\u00e3o n\u00e3o \u00e9 utilizada a chave prim\u00e1ria DH e n\u00e3o aplicam-se criptografias DH est\u00e1ticas. <\/p>\n

Ao utilizar o m\u00e9todo de troca de chaves DH, ambas as partes da conex\u00e3o geram chaves secretas aleat\u00f3rias (chave \"a\" e chave \"b\"), a partir das quais s\u00e3o calculadas e enviadas as chaves p\u00fablicas (ga mod p e gb mod p). Ap\u00f3s receber as chaves p\u00fablicas, cada parte calcula a chave prim\u00e1ria compartilhada (gab mod p), que \u00e9 utilizada para a forma\u00e7\u00e3o das chaves de sess\u00e3o. O ataque Raccoon permite determinar a chave prim\u00e1ria atrav\u00e9s da an\u00e1lise de informa\u00e7\u00f5es por canais externos, com base no fato de que nas especifica\u00e7\u00f5es do TLS at\u00e9 a vers\u00e3o 1.2 \u00e9 prescrito descartar todos os bytes iniciais nulos da chave prim\u00e1ria antes de realizar c\u00e1lculos que envolvem essa chave.<\/p>\n

Incluindo a chave prim\u00e1ria reduzida que \u00e9 passada para a fun\u00e7\u00e3o de gera\u00e7\u00e3o da chave de sess\u00e3o, baseada em fun\u00e7\u00f5es de hash com diferentes lat\u00eancias ao processar diferentes dados. A medi\u00e7\u00e3o exata do tempo das opera\u00e7\u00f5es realizadas pelo servidor com a chave permite que um atacante identifique dicas (oracle) que possibilitam concluir se a chave prim\u00e1ria come\u00e7a com zero ou n\u00e3o. Por exemplo, o atacante pode interceptar a chave p\u00fablica enviada pelo cliente (ga), reenvi\u00e1-la ao servidor e determinar
\nse a chave prim\u00e1ria resultante come\u00e7a com zero.<\/p>\n

A defini\u00e7\u00e3o de um \u00fanico byte da chave, por si s\u00f3, n\u00e3o fornece resultados, mas ao interceptar o valor \"ga\" transmitido durante o estabelecimento da conex\u00e3o pelo cliente, o atacante pode formar um conjunto de outros valores relacionados a \"ga\" e envi\u00e1-los ao servidor em sess\u00f5es separadas de estabelecimento de conex\u00e3o. Ao formar e enviar os valores \"gri*ga\", o atacante pode, atrav\u00e9s da an\u00e1lise das varia\u00e7\u00f5es de lat\u00eancia na resposta do servidor, determinar valores que resultam em chaves prim\u00e1rias que come\u00e7am com zero. Com a determina\u00e7\u00e3o desses valores, o atacante pode montar um conjunto de equa\u00e7\u00f5es para. Um mesmo<\/a><\/noindex> o problema dos n\u00fameros ocultos<\/a><\/noindex> e calcular a chave prim\u00e1ria original. <\/p>\n

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

No OpenSSL, as vulnerabilidades recebeu<\/a><\/noindex> n\u00edvel baixo de risco, e a corre\u00e7\u00e3o foi reduzida ao deslocamento na vers\u00e3o 1.0.2w dos cifradores problem\u00e1ticos «TLS_DH_*» para a categoria de cifradores desativada por padr\u00e3o com n\u00edvel de prote\u00e7\u00e3o insuficiente («weak-ssl-ciphers»). Os desenvolvedores da Mozilla tomaram uma abordagem semelhante, desativaram<\/a><\/noindex> na biblioteca NSS, usada no Firefox, os conjuntos de cifradores DH e DHE. A partir do Firefox 78, os cifradores problem\u00e1ticos est\u00e3o desativados. No Chrome, o suporte a DH foi encerrado ainda em 2016. As bibliotecas BearSSL, BoringSSL, Botan, Mbed TLS e s2n n\u00e3o s\u00e3o vulner\u00e1veis ao problema, pois n\u00e3o suportam cifradores DH ou variantes est\u00e1ticas de cifradores DH. <\/p>\n

Adicionalmente, s\u00e3o mencionados problemas (CVE-2020-5929<\/a><\/noindex>) no stack TLS dos dispositivos F5 BIG-IP, tornando o ataque mais realista. Em particular, foram identificadas discrep\u00e2ncias no comportamento dos dispositivos na presen\u00e7a de um byte nulo no in\u00edcio da chave prim\u00e1ria, que podem ser utilizadas em vez de medir o tempo exato de lat\u00eancia durante os c\u00e1lculos.<\/p>\n

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

Fonte: 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 \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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