Heartbleed (CVE-2014-0160)

Heartbleed was a bug in OpenSSL, the library behind a large share of the web's HTTPS. TLS has a “heartbeat” feature: one side sends a small message plus its length, and the other side echoes it back to show the connection is alive. OpenSSL trusted the length it was told. Send two bytes but claim 64 KB, and the server copied back your two bytes plus up to 64 KiB of whatever sat next to them in memory.

That memory could hold passwords, session cookies, other users' requests and even the server's private TLS keys. Reading it left no trace in normal logs, and an attacker could simply ask again and again. The bug arrived with OpenSSL 1.0.1 in March 2012. Neel Mehta of Google and the Finnish firm Codenomicon found it independently, and it was disclosed on 7 April 2014, the same day OpenSSL 1.0.1g fixed it.

Attacks followed within a day. About 900 Canadian social insurance numbers were taken from the Canada Revenue Agency on 8 April, and a Canadian student was arrested. US hospital operator Community Health Systems later lost 4.5 million patient records in a breach reported to have started through Heartbleed. Patching was only step one: because keys might have leaked, sites also had to revoke and reissue certificates and ask users to change passwords.