Ambient light sensors can steal sensitive browser data

APIs can be used to figure out visited links and QR codes

Sensors used in smartphones, tablets and laptops to detect ambient light levels can used to steal data from browsers, according to a security researcher.

These sensors have been growing in use over the last decade to detect surrounding light levels and adjust display brightness to save battery power. However, in a blog post, independent researcher and W3C Invited Expert Lukasz Olejnik said that an API developed by the World Wide Web Consortium (W3C) to enable websites to interact with these sensors could be used to gather information from users. This API does not ask permission from a user to access sensors.

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Olejnik said that the colour of the user's screen can carry useful information which websites are prevented from directly accessing for security reasons. As well as that, light sensor readings allow an attacker to distinguish between different screen colours.

He said that a hacker could find out what URLs a victim has visited in the past (as these are normally a different colour) by detecting any variations the sensors pick up.

"Potentially more troubling is the fact that attackers can extract pixel-perfect representations of cross-origin images and frames: essentially, discover how a given site or image looks for the attacked user," he said.

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"In extreme cases, for example on sites which use account recovery QR codes for emergency access to an account," he added. "This could allow the attacker to hijack the victim's account." 

At present, such sensor readings are blocked in Chrome, but supported in Firefox. He said that mitigating such attacks would be a case of requiring browsers to limit both the frequency of sensor readings (to much less than 60Hz) and the precision of sensor output (quantize the result). While these wouldn't completely stop such attacks, they would make them take longer to execute, making them impractical to carry out.

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"Perhaps the most obvious solution is to require the user to grant permission to the website requesting access to the sensor, as is already the case for other features such as geolocation. It would also be prudent to expand the security and privacy considerations section in the Ambient Light Sensor API specifications to document the risk of attacks such as this one," he said.

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