Gravatar is a service that provides users with a profile image that can appear on many sites across the Net.
It is integrated with WordPress.com (The version of WordPress hosted by Automattic) and is also integrated into WordPress.org, the self hosted version of WordPress.
Gravatar is also used by many other popular services on the web like StackOverflow.com.
Using Gravatar and GPU cracking to steal email addresses
In 2013 Dominique Bongard presented a talk at PasswordsCon in Las Vegas where he demonstrated that he could reverse engineer 45% of Gravatar hashes into email addresses. He targeted a well known political forum in France which uses Gravatar for user profile pictures.
The big difference in Dominique’s approach is that he used Hashcat, which is a password cracking tool. He repurposed it so that he could reverse engineer Gravatar hashes into email addresses. The reason this is important is that Hashcat executes significantly faster because it uses consumer graphics processing units, or GPUs, which are used by gamers to accelerate game graphics performance. Cracking hashes with GPU acceleration increases performance by a factor of several thousand.
When you consider that 2 years ago a single researcher reverse engineered 45% of gravatar profile photos into email addresses, it’s quite possible that a criminal group armed with a modern GPU cluster, as shown above, could reverse engineer a far higher percentage today. The problem will only get worse.
What to do to protect your email address and identity
To solve the identity and spam problem that Gravatar presents, the most effective option is to use a unique email address to register on each website you are a member of. The email address should be hard to reverse engineer.
If you use an @gmail.com address, Gmail provides a feature whereby you can append a plus sign to your email address and anything after it is ignored. If your email address is email@example.com, you can change it to yourname+junkGoesHere@gmail.com and you will still receive the email.
What we suggest you do is use a unique gmail address on any Gravatar enabled website when you register. Therefore firstname.lastname@example.org would become: yourname+2h4J1q9ZuU9@gmail.com. Gmail has documented this feature here. The feature also works with hosted Gmail addresses where you use your own domain. Outlook.com also provides this feature.
Using this technique makes it much harder for a spammer to reverse engineer your email address from a Gravatar hash. Try to make your email address at least 20 characters long and include upper and lower-case letters and numbers in the suffix after the plus sign. If you have uploaded a custom Gravatar profile image, you should note that this has the side effect of not displaying that image on the websites where you make this change. Instead you will get a default profile image.
Receiving extra spam is an inconvenience. It can be a minor inconvenience if you have an excellent spam filter in place. However, having your identity exposed on a website where you assumed your identity was private can be embarrassing at best and have far worse consequences. We therefore suggest that you switch to using a plus-suffix on any website where it is important to maintain your personal privacy.