CVE-2015-4000

El protocolo TLS 1.2 y anteriores, cuando una suite de cifrado DHE_EXPORT está habilitada en un servidor pero no en un cliente, no transporta una elección DHE_EXPORT, lo que permite a atacantes man-in-the-middle realizar ataques de degradación del cifrado mediante la rescritura de un ClientHello con DHE remplazado por DHE_EXPORT y posteriormente la rescritura de un ServerHello con DHE_EXPORT remplazado por DHE, también conocido como el problema 'Logjam'.

A flaw was found in the way the TLS protocol composes the Diffie-Hellman exchange (for both export and non-export grade cipher suites). An attacker could use this flaw to downgrade a DHE connection to use export-grade key sizes, which could then be broken by sufficient pre-computation. This can lead to a passive man-in-the-middle attack in which the attacker is able to decrypt all traffic.

USN-2656-1 fixed vulnerabilities in Firefox for Ubuntu 14.04 LTS and later releases. This update provides the corresponding update for Ubuntu 12.04 LTS. Karthikeyan Bhargavan discovered that NSS incorrectly handled state transitions for the TLS state machine. If a remote attacker were able to perform a man-in-the-middle attack, this flaw could be exploited to skip the ServerKeyExchange message and remove the forward-secrecy property. Looben Yan discovered 2 use-after-free issues when using XMLHttpRequest in some circumstances. If a user were tricked in to opening a specially crafted website, an attacker could potentially exploit these to cause a denial of service via application crash, or execute arbitrary code with the privileges of the user invoking Firefox. Various other issues were also addressed.