Previous posts in this series have covered the SSL/TLS protocol in some detail. Now it’s time to examine some of the more recent vulnerabilities that have been found, and how they were (and can be) dealt with.


Heartbleed is one of the most serious vulnerabilities ever found in SSL/TLS, allowing the theft of server keys, user session ids and user passwords from compromised systems. It was not, however, an SSL protocol flaw, but rather an implementation bug (known as a buffer over-read) in OpenSSL‘s free library, which is widely used across the Internet. Millions of machines were affected, and numerous successful attacks reported.

Software systems not using the relevant versions of OpenSSL were not affected. OpenSSL was rapidly patched, but patching millions of machines takes time. Not only did machines need to be patched, but server private keys must be updated, user passwords changed and certificates re-issued. A year later, it is likely that there are still compromised machines on the Internet that have not been suitably modified.

The total cost of Heartbleed is, by one estimate, in the range of hundreds of millions of dollars.


POODLE is a vulnerability in an older SSL protocol, SSL 3.0. While most systems use TLS 1.0, 1.1 or 1.2, the TLS protocol has a fallback provision to allow interoperability with older software still using SSL 3.0. So POODLE attacks use this fallback provision to fool servers into downgrading to SSL 3.0.

The most simple fix is to disable SSL 3.0 in clients and servers. SSL 3.0 was published in 1996, it has long been superseded, and there should be no need to support it after almost 20 years.

POODLE is a far less serious vulnerability than Heartbleed.


RC4 is a widely used TLS cipher that is no longer regarded as secure. RC4 is also known as ARC4 or ARCFOUR (because RC4 is trademarked). Its speed and simplicity made RC4 popular, but recently (February 2015) RFC7465 recommended that it no longer be used.


TLS is a mature, widely used secure network protocol that will be securing transactions on the Internet for many years to come. Like any secure protocol, vulnerabilities will continue to be discovered, and it is important to keep software that utilises TLS up-to-date so that the latest security patches are applied.

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