W3C home > Mailing lists > Public > public-web-security@w3.org > April 2011

Re: policy-uri is slow

From: Adam Barth <w3c@adambarth.com>
Date: Thu, 14 Apr 2011 20:26:10 -0700
Message-ID: <BANLkTi=cniJfJvWHh-ZnDZg+nREqMhaJzg@mail.gmail.com>
To: Mark Nottingham <mnot@mnot.net>
Cc: public-web-security@w3.org
On Thu, Apr 14, 2011 at 7:55 PM, Mark Nottingham <mnot@mnot.net> wrote:
> On 15/04/2011, at 10:47 AM, Adam Barth wrote:
>> To confirm my understanding, if a document has a CSP policy consisting
>> of a policy-uri, then the user agent is supposed to block processing
>> of the document until it finishes fetching the policy-uri, right?
>> That seems very bad for performance.
> It can be, but so can putting verbose policy statements on every response you send.
> If the policy-uri is cacheable for a decent amount of time, the performance impact is only there when the cache is empty or expired.
> The effects can be further mitigated by;
>  - adjusting the client's cache eviction algorithm to be conservative about kicking out policy files
>  - implementing connection pre-heating and HTTP pipelining (as Mozilla is currently) to decrease the impact of making that request
>  - implementing stale-while-revalidate (RFC5861) in the client's cache so that when the cache is expired, the update happens in the background, in a controlled way.
> If anything, I'd like to see this mechanism further optimised; i.e., instead of communicating the URI in each response (with the associated overhead in response size as well as the chance for introducing errors), have a well-known location (RFC5785) for policy files that the client can check proactively while it makes its initial request -- that way, it's much less likely to block anything. And even less bytes are in responses this way, of course.

You're suggesting that we request the well-known policy file before we
know whether the site is using CSP at all?  That seems like a lot of
spam requests, plus added latency for ever page on the web...


>> Should we drop support for policy-uri?
> No.
> Cheers,
> --
> Mark Nottingham   http://www.mnot.net/
Received on Friday, 15 April 2011 03:27:08 UTC

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