Google’s decision to change the default setting for Gmail to show images by default has been criticised by security researchers as opening the door to sophisticated forms of message tracking.
Until late last week, Gmail would ask users whether they wished to display images in an email and for a good reason; booby-trapped images have in the past been used to route users to external malicious content while their use by marketers and spammers to track whether a user has opened a communication were implicit.
Worse, spammers could use http image requests to verify that a random address was in use, causing even more spam to be sent to that identity.
Aware of these issues, Google announced that as part of the change it will now proxy all email images, transcoding them to avoid such obvious abuses.
“Your images are checked for known viruses or malware, and you’ll never have to press that pesky ‘display images below’ link again. With this new change, your email will now be safer, faster and more beautiful than ever.
Senders would not be able to determine a receiver’s IP address or set or read cookies, Google insisted.
Critics have pointed out that although Google caching images should make it more difficult than at present to track a user’s email interaction, marketers are spammers could simply send each user the same image with a unique URL. Because these will now load automatically through Google’s proxy system, this could actually make tracking even more powerful than at present.
“If Gmail does start to display images automatically and this occurs only when a user views the message, it will enable ‘read tracking’ by default for all Gmail users. This would allow a stalker or other malicious entity to determine whether the email they sent to a target is being read,” argued Rapid7 chief research officer and Metasploit architect, HD Moore.
It could also open the door to spammers bombarding random Gmail addresses to find out which ones were being used, he said.
What’s unclear is why beyond speeding up image loading and avoiding some user annoyance Google thought that the default image handling setting needed to be changed in the first place. Proxying could have been introduced as an extra safety layer for those users wishing to enable images for specific senders. Gmail users can already set image display to 'on' for trusted senders.
Judging from forum discussion, the most likely motivation is simply that many users don’t understand why Gmail asks them to allow images in emails, seeing it as an unnecessary hindrance. As Google points out, it is still possible to reset the new default so that Gmail continues to ask whether images should be opened through the settings/general/ ‘ask before displaying external content’ setting.
But it's also true that many non-technical users will not understand the privacy and security downsides of making image handling more convenient.
“If Gmail starts to cache images as email is received and *BEFORE* the user reads the message, the tracking aspect will be resolved, but it does open the door to malicious request proxying in a much more aggressive form,” said Moore.