In a blog post, Google tells Iranians to change their Gmail password and check that their Google accounts have not been compromised.
“the compromise of a Dutch company involved with verifying the authenticity of websites could have put the Internet communications of many Iranians at risk, including their Gmail. While Google’s internal systems were not compromised, we are directly contacting possibly affected users and providing similar information below because our top priority is to protect the privacy and security of our users.”
This action has been taken because hackers created fake SSL certificate credentials for Google.com and many other domains. These fake Google credentials were used to run man-in-the-middle attacks against Gmail users in Iran, according to an examination of authentication look-ups logs at DigiNotar and other evidence.