Google is planning to add a new security feature to the Chrome web browser designed to block potential attackers from launching side-channel attacks and tracking a user’s activity by abusing the browser’s HTTP cache.

The new feature will partition Chrome’s HTTP Cache “using top frame origin (and possibly subframe origin) to prevent documents from one origin from knowing whether a resource from a cross-origin was cached.”

This approach will prevent attackers from launching side-channel attacks employing malicious sites they control to detect if another site their target has visited is in their web browser’s cache.

Cache attacks can trigger the following types of information leaks according to the ‘Partition the HTTP Cache’ security feature’s Chrome Platform Status entry:

In addition, the browser’s cache can also be used as a user fingerprinting method by storing cross-site super-cookies which require the targets to completely clear the browser’s cache to delete them.

Attacks abusing Chrome’s HTTP cache

This example HTTP cache-based attack shows how a bad actor can easily collect sensitive information about a Chrome user by abusing Google vulnerable endpoints. As explained by the researcher who documented this specific attack, “the impact varies from retrieving the private book collections to obtaining all the user’s contacts char by char.”

Vulnerable sites that could be used in these attacks range from Google’s Mail and Search to Google Books and YouTube. Users who would visit a maliciously designed website after browsing these Google websites could have their sensitive information exposed.

The types of data attackers could get their hands on in the event of a successful side-channel cache attack are a user’s:

Changes needed to mitigate cache attacks

According to the feature’s design spec the “solution is to partition the disk cache by the origin of the top-frame of the page (e.g., what shows up in the omnibox) or by the combination of top-frame and sub-frame origins. This way resources loaded for one origin can’t be read by another and both problems are resolved.”

The current option to be used for cache isolation and cache attacks mitigation are:
• (top frame origin, request URL) as the double key
• (frame origin, request URL) as the double key
• (top frame origin + frame origin, request URL) as a triple key

The Chrome developers have currently proposed the “top-frame-origin for sub-resource requests and document initiator for shared/service worker requests” as the mitigation solution of choice, but they’re still collecting more data to make the final decision.

“Chrome hasn’t addressed this problem in the past due to concerns that the cache hit rate would be dramatically reduced, causing large increases in network usage and longer page loads,” says Google Software Engineer Shivani Sharma on the feature’s HTTP Cache Threat Model document.

“Recent experiments in canary and dev channels show that contrary to our earlier thinking, the losses would be acceptable.”

The end goal of the planned partitioned HTTP cache feature is to “prevent documents from one origin from knowing if a resource from a cross-origin document load was cached or not,” adds Sharma.

Out of all major browsers, Safari is the only one which has already implemented a version of this feature more than six years ago, while Mozilla already has shown that it publicly supports the new security enhancement and Microsoft hasn’t yet given any public signals of any plans to implement cache partitioning in their Edge browser.

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