HTML5 Storage Bug Can Fill Your Hard Drive

Dystopian Rebel writes "A Stanford comp-sci student has found a serious bug in Chromium, Safari, Opera, and MSIE. Feross Aboukhadijeh has demonstrated that these browsers allow unbounded local storage. 'The HTML5 Web Storage standard was developed to allow sites to store larger amounts of data (like 5-10 MB) than was previously allowed by cookies (like 4KB). ... The current limits are: 2.5 MB per origin in Google Chrome, 5 MB per origin in Mozilla Firefox and Opera, 10 MB per origin in Internet Explorer. However, what if we get clever and make lots of subdomains like 1.filldisk.com, 2.filldisk.com, 3.filldisk.com, and so on? Should each subdomain get 5MB of space? The standard says no. ... However, Chrome, Safari, and IE currently do not implement any such "affiliated site" storage limit.' Aboukhadijeh has logged the bug with Chromium and Apple, but couldn't do so for MSIE because 'the page is broken" (see http://connect.microsoft.com/IE). Oops. Firefox's implementation of HTML5 local storage is not vulnerable to this exploit."

Comments

Re:Bug, or exploit?

[DarkRat]

no. it's a bug. the HTML5 spec clearly states that this exact behaviour should be looked out for and blocked

Re:So What's The Point

[Qzukk]

Subdomains are free. With a wildcard DNS record, you have nearly an infinite supply of them.

Re:So What's The Point

[The Mighty Buzzard]

Really? You've never admin'd a dns server then. It's trivial to have one respond to wildcard subdomain names that you could generate dynamically on page load with one line of javascript.

Re:So What's The Point

[arth1]

It doesn't take much work or time to set up a wildcard CNAME entry pointing to a single web server that answers a wildcard. You now have billions of subdomains with a couple of minutes of work.
The web instance serves a short javascript which generates a boatload of data on the client side, and then calls a random subdomain to reload the js with a new domain name.

All this can be linked to a single ad (or blog comment, for vulnerable boards that allow css exploits).

Read the spec: recommendation, not requirement

[DragonWriter]

no. it's a bug. the HTML5 spec clearly states that this exact behaviour should be looked out for and blocked

Its not a bug. While the Web Storage API Candidate Recommendation (related to, but not part, of, the HTML5 spec) both says that user agents should set a per-origin storage limit and should identify and prevent use of "origins of affiliated sites" to circumvent that limit, it doesn't specify either what constitutes an "affiliated site", and neither of those things that it says "should" be done are requirements of the specification. "Should" has a quite specific meaning in the specification (defined by reference in the spec to RFC2119), and its not the same as "must", instead:

SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.

So, its both a recommendation rather than a requirement, and not specified clearly enough to be implemented. There are some cases where origins of the same second-level domain are meaningfully affiliated, and some times where they are not (for a clear case of the latter, consider subdomains of ".co.uk".) Its pretty clear that origins which differ only in protocol are almost always going to be affiliated by any reasonable definition (e.g., http://www.example.com/ and https://www.example.com/ which are different origins), but no automatic identification of origin affiliation by subdomain can be done simply without understanding of per-domain policies from the TLD down to the first level at which all subdomains are affiliated. (And this is a problem which will get worse with the planned explosion of TLDs.) W

Re:Read the spec: recommendation, not requirement

[DragonWriter]

You must be awful fun when talking to customers. They tend not to understand the distinction between "shall" and "should".

There is a reason why internet specifications (whether or not they are from IETF, and often whether or not they are even intended as standards-track) reference the RFC2119 definitions. "MUST" vs. "SHOULD" is an important distinction.

In this particular case, whats even more important is that the recommended functionality at issue isn't defined at all, there is just one example -- and the example doesn't fully specify the origins, so its an incomplete example -- given and no definition of the parameters of the identification of "affiliated origins". So if it was a "MUST", it would be a broken standard (since it would be impossible to assess conformance), and as it is, its impossible to say whether a particular implementation even implements the recommended functionality.

"there may exist valid reasons in particular circumstances to ignore a particular item" - in other words, this is a case where the feature should ALWAYS be applied to generic software because that must deal with all circumstances, not just "particular" ones

Any particular user agent is a "particular circumstance" (it is specific software with a specific use case within the scope of all possible kinds of user agents which might implement the Web Storage API); there is no such thing as an implementation that must deal with "all circumstances".

It really should not be hard to have a popup that says "This web page wants to create local storage on your computer allow/disallow"

Its not at all hard, but that's not related to the recommendation to implement per-origin quotas, or the further recommendation to build on top of the per-origin quotas functionality to detect and limit the use of "affiliated origins" to circumvent the per origin quotas, which is what is at issue here. Per-origin allow/disallow for Web Storage use isn't even a recommendation of the specification. (Though it is explicitly permitted behavior.)

Re:So What's The Point

[Jiro]

That's not true.

"Nearly infinite" means "it's not infinite, but it's large enough that it has most of the same practical effects as it would if it were infinite".

You seem to be interpreting the word "nearly" to mean "has a numerical value close to" rather than "has effects similar to". Obviously it is nonsensical for something to be nearly infinite using that first definition, but that should be a warning sign that you're not using the definition that people mean, not that everyone else is speaking nonsense.