Microsoft Outlook users are in danger of being snooped on by the Chinese internet authority, according to research by censorship watchdog GreatFire.
Those using email clients to access the service were said to be vulnerable to man-in-the-middle (MitM) attacks, allowing hackers to log emails, contact information and passwords by placing themselves between the sender and receiver of messages.
The author of the report, identified only as Percy, said: "Because of the similarity between this attack and previous, recent MitM attacks in China (on Google, Yahoo and Apple), we once again suspect that [internet minister] Lu Wei and the Cyberspace Administration of China have orchestrated this attack or have willingly allowed the attack to happen."
"If our accusation is correct, this new attack signals that the Chinese authorities are intent on further cracking down on communication methods that they cannot readily monitor."
Users were warned of the attack through a message saying that the client could not verify the email server’s certificate, which Percy warned might be interpreted as a sign of network problems rather than hacking.
"The authorities are most likely continuing to test their MITM technology. The authorities may also be gauging user response," he added.
"By keeping track of how many users ignore the certificate warnings, the authorities will be able to determine the effectiveness of this type of attack."
GreatFire previously accused China of attacking Apple’s iCloud storage service last year using similar methods, prompting Apple chief executive Tim Cook to fly to the country to discuss cybersecurity with Chinese officials.