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Re: Subresource Integrity Length Extension?

From: Frederik Braun <fbraun@mozilla.com>
Date: Tue, 28 Jan 2014 09:43:53 +0100
Message-ID: <52E76DC9.1010301@mozilla.com>
To: public-webappsec@w3.org
On 27.01.2014 17:50, Andrew wrote:
> http://en.wikipedia.org/wiki/Length_extension_attack
> 
> One solution would to be use a HMAC construction where the 'key'
> material is composed from resource meta data, including the verified
> Content-Length, or to mandate a hash function immune to such attacks,
> such as SHA-3.

Thank you for bringing this to our attention.
I actually had someone come up to me yesterday, to raise this very same
point.

It would be an interesting challenge to set oneself, to make a working
lengh extension that not only contains valid JavaScript, but also does
something evil (one could imagine appending "eval(name);//" and then
append some more to fix up the hash - maybe that'll work. But I'd rather
we don't take the chances ;)


So maybe an HMAC makes more sense - who'd pick the key? Is it just
something encoded within the attribute by the website author then?

What do the others think?
Frederik
Received on Tuesday, 28 January 2014 08:44:26 UTC

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