Encrypted email provider ProtonMail says its service has been blocked in Turkey, but can still be accessed via a VPN, DNS, or Tor.
The secure messaging provider said Thursday that it believes the government is behind the block that has left its services partially or fully unreachable in Turkey.
"After investigating the issue along with members of the ProtonMail community in Turkey, we have confirmed this is a government-ordered block rather than a technical glitch," said ProtonMail CEO Andy Yen.
"Internet censorship in Turkey tends to be fluid so the situation is constantly evolving. Sometimes ProtonMail is accessible, and sometimes it is unreachable. For the first time ever though, we have confirmed that ProtonMail was subject to a block, and could face further issues in the future."
According to Yen, the block is being carried out by ISPs on the order of the government. The service providers have directed their DNS servers not to resolve requests for the ProtonMail service, leaving their customers unable to access the site.
This isn't the first time the controlling party in Turkey has moved to shut down web services as a political maneuver. In 2016, the hacking of government officials prompted a nationwide block on file and code-sharing sites, and that same year an emergency block was placed on social networking sites during a coup attempt.
According to ProtonMail, this is the first attempt by the government to cut off its service in Turkey.
Because the block is based on DNS servers, however, there are a number of ways to circumvent it. Yen notes that measures including the use of a VPN or running traffic through Tor will allow users to get around the block, as will simply changing their DNS server to one that resolves the ProtonMail.com address.
"We view this as a serious threat, and we focus a lot of development effort on technologies to circumvent such blocks," Yen said.
"Because the situation is constantly changing, some of these recommendations could stop working at any time."
The move comes just days after another secure email service, Tutanota, reported Comcast had blocked its customers from access earlier this month. That block was not believed to have been intentional, and the cause is still under investigation. ®