A serious overrun vulnerability in the OpenSSL cryptographic library affects around 17% of SSL web servers which use certificates issued by trusted certificate authorities. Already commonly known as the Heartbleed bug, a missing bounds check in the handling of the TLS heartbeat extension can allow remote attackers to view up to 64 kilobytes of memory on an affected server. This could allow attackers to retrieve private keys and ultimately decrypt the server's encrypted traffic or even impersonate the server.

The Heartbleed bug write-up mentions Apache and nginx as being the most notable software using OpenSSL, and also points out that these have a combined active site market share of over 66% according to our April 2014 Web Server Survey. However, not all of these servers are running an HTTPS service, nor are they all running vulnerable versions of OpenSSL with heartbeats enabled.

Our most recent SSL Survey found that the heartbeat extension was enabled on 17.5% of SSL sites, accounting for around half a million certificates issued by trusted certificate authorities. These certificates are consequently vulnerable to being spoofed (through private key disclosure), allowing an attacker to impersonate the affected websites without raising any browser warnings.





Most vulnerable servers are using Apache.

Note that a small percentage of Microsoft web servers also appear to support the TLS heartbeat extension; these are actually likely to be vulnerable Linux machines acting as reverse proxy frontends to Windows servers.

Support for heartbeats was added to OpenSSL 1.0.1 (released in 2012) by Robin Seggelmann, who also coauthored the Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) Heartbeat Extension RFC. The new code was committed to OpenSSL's git repository just before midnight on new year's eve 2011.

OpenSSL's security advisory states that only versions 1.0.1 and 1.0.2-beta are affected, including 1.0.1f and 1.0.2-beta1. The vulnerability has been fixed in OpenSSL 1.0.1g, and users who are unable to upgrade immediately can disable heartbeat support by recompiling OpenSSL with the -DOPENSSL_NO_HEARTBEATS flag.

Popular sites which exhibit support for the TLS heartbeat extension include Twitter, GitHub, Yahoo, Tumblr, Steam, DropBox, HypoVereinsbank, PostFinance, Regents Bank, Commonwealth Bank of Australia, and the anonymous search engine DuckDuckGo.

Certificates and keys at risk of compromise should be revoked and replaced, particularly if they are used to protect sensitive data. Certificate Authorities, hosting companies and other interested parties can contact us for assistance in identifying affected certificates.

You can check whether your own HTTPS website might be vulnerable using the form below, and looking for the RFC6520 heartbeat TLS extension.