Cache timing attacks use the time some operation takes to learn whether or not a datum is in a cache. They’re inherently hard to fix, because the entire point of a cache is to speed up access to data that was used recently. They’ve been studied as far back as Multics.  In the context of the Web, the cache in question (usually) is the browser’s cache of resources retrieved from the network, and the attacker (usually) is a malicious website that wants to discover something about the victim’s activity on other websites.   This paper gives examples of using cache timing to learn information needed for phishing, discover the victim’s location, and monitor the victim’s search queries. It’s also known to be possible to use the victim’s browsing habits as an identifying fingerprint .
The possibility of this attack is well-known, but to date it has been dismissed as an actual risk for two reasons: it was slow, and probing the cache was a destructive operation, i.e. the attacker could only probe any given resource once, after which it would be cached whether or not the victim had ever loaded it. This paper overcomes both of those limitations. It uses Web Workers to parallelize cache probing, and it sets a very short timeout on all its background network requests—so short that the request can only succeed if it’s cached. Otherwise, it will be cancelled and the cache will not be perturbed. (Network requests will always take at least a few tens of milliseconds, whereas the cache can respond in less than a millisecond.) In combination, these achieve two orders of magnitude speedup over the best previous approach, and, more importantly, they render the attack repeatable.
What do we do about it? Individual sites can mark highly sensitive data not to be cached at all, but this slows the web down, and you’ll never get everyone to do it for everything that could be relevant. Browsers aren’t about to disable caching; it’s too valuable. A possibility (not in this paper) is that we could notice the first time a resource was loaded from a new domain, and deliberately delay satisfying it from the cache by approximately the amount of time it took to load off the network originally. I’d want to implement and test that to make sure it didn’t leave a hole, but it seems like it would allow us to have the cake and eat it.
In closing, I want to point out that this is a beautiful example of the maxim that