DROWN: Breaking TLS using SSLv2

Christoph Paar
David Adrian
J. Alex Halderman
Jens Steube
Juraj Somorovsky
Luke Valenta
Maik Dankel
Nadia Heninger
Nimrod Aviram
Sebastian Schinzel
Shaanan Cohney
Susanne Engels
Viktor Dukhovni
Yuval Shavitt
25th USENIX Security Symposium(2016)
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We present DROWN, a novel cross-protocol attack that can decrypt passively collected TLS sessions from upto-date clients by using a server supporting SSLv2 as a Bleichenbacher RSA padding oracle. We present two versions of the attack. The more general form exploits a combination of thus-far unnoticed protocol flaws in SSLv2 to develop a new and stronger variant of the Bleichenbacher attack. A typical scenario requires the attacker to observe 1,000 TLS handshakes, then initiate 40,000 SSLv2 connections and perform 2 50 offline work to decrypt a 2048-bit RSA TLS ciphertext. (The victim client never initiates SSLv2 connections.) We implemented the attack and can decrypt a TLS 1.2 handshake using 2048- bit RSA in under 8 hours using Amazon EC2, at a cost of $440. Using Internet-wide scans, we find that 33% of all HTTPS servers and 22% of those with browser-trusted certificates are vulnerable to this protocol-level attack, due to widespread key and certificate reuse. For an even cheaper attack, we apply our new techniques together with a newly discovered vulnerability in OpenSSL that was present in releases from 1998 to early 2015. Given an unpatched SSLv2 server to use as an oracle, we can decrypt a TLS ciphertext in one minute on a single CPU—fast enough to enable man-in-the-middle attacks against modern browsers. 26% of HTTPS servers are vulnerable to this attack. We further observe that the QUIC protocol is vulnerable to a variant of our attack that allows an attacker to impersonate a server indefinitely after performing as few as 225 SSLv2 connections and 265 offline work. We conclude that SSLv2 is not only weak, but actively harmful to the TLS ecosystem.