The compromise of DigiNotar could have put the Internet communications of many Iranians at risk, including their Gmail, it said.

Users have been advised to change their Google account passwords, verify account recovery options, and to check the websites and applications that are allowed to access their account, and revoke any that are unfamiliar.

Google has also provided web links to where users can check their Gmail settings for suspicious forwarding addresses or delegated accounts. Users are also advised to pay attention to warnings that appear in their web browser and not click past them.

A total of 531 fraudulent digital certificates were issued by DigiNotar for domains that included google.com, the CIA, and Israel's Mossad. The list of domains and the fact that 99 percent of the users are in Iran suggest that the objective of the hackers was to intercept private communications in Iran, Fox-IT said.

A hacker styling himself as Comodohacker on Pastebin that he breached DigiNotar, in order to punish the Dutch government for the actions of its soldiers in Srebrenica, where 8,000 Muslims were killed by Serbian forces in 1995 during the Bosnian War.