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.