Browser Watch: Firefox to Require HTTPS for Application Cache (AppCache)
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Browser Watch: Firefox to Require HTTPS for Application Cache (AppCache)

Firefox 62 to deny AppCache access to HTTP pages

In line with its recent announcement to restrict all-web exposed features to HTTPS, Firefox has announced that AppCache must be served over HTTPS, too. Application Cache (AppCache) facilitates a caching mechanism for the website admins who want to run their sites offline. Right now, it can be availed for HTTP as well as HTTPS pages. However, with the launch of Firefox 62 in September 2018, only the HTTPS pages will be able to include this feature. In other words, Firefox has restricted AppCache to Secure Contexts.

Not only to run the sites offline, but many website admins also implement AppCache to increase loading speed and reduce server load.

This is why it’s happening

If AppCache is run on HTTP sites, it could pose some serious security concerns. That’s because “AppCache has limitations in revalidating its cache, which allows attackers to trick the browser into never revalidate the cache by setting a manifest to a malformed cache file” – in Mozilla’s terms.

If that sounded like a load of jargon, I wouldn’t blame you. Let me put it in simpler words.

When your browser loads an AppCache-enabled site, it stores that website’s data in the form of cache. The AppCache API has its own limitations when it comes to verifying the cache files. So when a user opens such a site, the browser would serve data from the stored cache without validating that the stored cache is in the same format.

I hope this has made the picture a bit clearer to you. Let me make it even more transparent.

Let’s say you’re connected to a Wi-Fi network, and an attacker is connected as well. Your browser must have a stored cache if you had visited a site with AppCache. Now as the attacker is connected to the same network, he/she could somehow manipulate this stored cache, and the browser (AppCache) wouldn’t be able to notice it as it cannot validate the cache. So, if the attacker manages to maneuver the cache and add iframes (which is kind of like embedding), he/she could potentially trick users into giving their confidential information – even after the user is disconnected from that network.

Imagine an AppCache-enabled, HTTP site named and a perpetrator manages to insert a fake Facebook login page. If you type in your e-mail id and password, it’s going to be captured. That’s why Firefox is going to restrict this AppCache feature to HTTPS.

Bottom Line: HTTP is dying a slow, painful death

If you keep up with the latest tech updates, you must have heard that Google Chrome is going to mark all HTTP sites as “Not Secure.” Similarly, all other major browsers are upping their game in the hunt to deprecate the insecure protocol that is HTTP. This move to restrict AppCache to Secure Contexts marks another milestone in this direction.


Jay Thakkar

After graduating from university with an engineering degree, Jay found his true passion as a writer…specifically, a cybersecurity writer. He’s now a Hashed Out staff writer covering encryption, privacy, cybersecurity best practices, and related topics.