W3C home > Mailing lists > Public > public-webappsec@w3.org > January 2016

Re: Header size and policy delivery

From: Patrick Toomey <patrick.toomey@github.com>
Date: Thu, 07 Jan 2016 05:08:22 +0000
Message-ID: <CAN4Q8dBs5bFA7zNNxcf28kwTZ8QJHs5fPjzauzhViiCfG048wQ@mail.gmail.com>
To: Martin Thomson <martin.thomson@gmail.com>, Jonathan Kingston <jonathan@jooped.co.uk>
Cc: WebAppSec WG <public-webappsec@w3.org>
We have started customizing our policy per endpoint and have plans to do so
even more in the future. It feels like "CSP as a resource" would be a bit
tricker if one customized their policy per response (maybe I missed someone
already addressing this concern). If I look at our CSP policy (or the
Twitter one someone showed in the original thread), the bulk of the size is
taken up with various source lists. What if the cacheable CSP resource was
mostly used to provide a place to collect/label/cache sets of these values.
For example, rather than having to send down something like "connect-src
'self' foo.com bar.com foobar.com" on each response, it could be a
reference to a "source set" from the cacheable resource. So, the policy
would be more like "connect-src csp-manifest-my-connect-srcs", where
"my-connect-srcs" would be a labeled set of sources from the cached CSP
resource. I guess there is inevitably a point where a sufficient number of
CSP directives overwhelms the header, but maybe there is a way to handle
that too. I haven't thought about it much, but maybe one could also use the
CSP resource to collect/label/cache collections of commonly used directives
too. Even though we customize our policy per response, the majority of the
policy stays the same. So, there could be a reference in the CSP header
response that pulls in a collection of directives that you intend to have
on any page...something like "base-policy csp-manifest-my-base-policy",
where "my-base-policy" would have the parts of your CSP policy that don't
really change across the site.
On Wed, Jan 6, 2016 at 9:31 PM Martin Thomson <martin.thomson@gmail.com>
wrote:

> A CSP resource sounds appealing, but I'm not sure about the latency
> situation: are people OK with the notion that this is a separate
> fetch?  We could use HTTP/2 server push to address the latency
> problem.
>
> On 7 January 2016 at 13:48, Jonathan Kingston <jonathan@jooped.co.uk>
> wrote:
> > Creating a new tread for discussion of a solution to header bloat size
> if at
> > all possible.
> >
> > Also relevant is:
> > https://lists.w3.org/Archives/Public/public-webappsec/2015Mar/0148.html
> >
> > Taken out from the discussion in: [CSP] "sri" source expression to
> enforce
> > SRI
> >
> > On Tue, Jan 5, 2016 at 1:59 AM Nottingham, Mark <mnotting@akamai.com>
> wrote:
> >>
> >> Catching up after holidays -- I've been wanting to talk about this.
> >>
> >> In HTTP/2, the default of SETTINGS_HEADER_TABLE_SIZE is 4k.
> >>
> >> From what I've seen, Chrome and Firefox both stick with the default.
> >>
> >> While 4k of header compression context can help performance
> considerably,
> >> it's important to understand that HPACK's compression scheme is
> >> coarse-grained, so when the encoder is faced with a large header, it
> has to
> >> choose between putting it into the dynamic table -- thereby denying use
> of
> >> that space to other headers -- or repeatedly putting it out onto the
> wire.
> >>
> >> For example, Twitter's response headers already get close to this limit,
> >> mostly thanks to CSP:
> >> https://redbot.org/?id=w5yLyD
> >>
> >> Their server has to choose between putting that ~3K CSP header into the
> >> dynamic table, leaving them only about 1k to play with for other
> headers per
> >> connection, or leave it out, and send it verbatim on EVERY response.
> They'll
> >> get small benefit from static Huffman coding (which reduces the numbers
> >> above a bit), but that's it.
> >>
> >> If a single header value exceeds SETTINGS_HEADER_TABLE_SIZE, it can't be
> >> encoded by reference, and the sender has no choice but to emit it on
> every
> >> message.
> >>
> >> Things get even nastier if there are several large versions of CSP on a
> >> single connection.
> >>
> >> Clients could start advertising a larger SETTINGS_HEADER_TABLE_SIZE, but
> >> that means a larger state commitment (both client-side and server-side,
> >> where it can hurt a lot more, offers more DoS exposure, etc.).
> >>
> >> Given that we're already seeing popular sites brush up against this,
> >> PLEASE don't assume that HTTP/2 == free compression, and that we can
> >> continue to merrily add headers.
> >>
> >> Also - when a header is both large and monolithic like CSP (i.e., it
> >> doesn't allow multiple values to be combined into a comma-separated
> value),
> >> it makes it much harder to optimise for compression, because of HPACK's
> >> granularity (again). I realise that there are security motivations
> behind
> >> this for CSP, but I wonder if the cost is justified (because once
> somebody
> >> can append headers, there's a lot of other damage they can do).
> >>
> >> Cheers,
> >
> >
> > On Tue, Jan 5, 2016 at 11:29 AM Mike O'Neill <
> michael.oneill@baycloud.com>
> > wrote:
> >>
> >> I don’t know if this has already been talked about, but maybe long
> headers
> >> like CSP can be could be put in a well-known resource. It would cost
> >> another
> >> roundtrip but save bandwidth in the end  because the resource would be
> >> cached. The CSP header would only need to contain a hash of the resource
> >> to
> >> confirm
> >>
> >
> > On Tue, Jan 5, 2016 at 11:52 AM Jonathan Kingston <jonathan@jooped.co.uk
> >
> > wrote:
> >>
> >> Yup Mike I had suggested the use of SRI in the header and pointing to
> some
> >> form of manfest file.
> >>
> >> I think this addresses some of Marks concerns about header size however
> >> creates other issues such as cache management and extra round trips.
> >>
> >> The advantage of the manifest also would allow separation of concerns
> >> between CSP and SRI within the policy.
> >>
> >
> > Kind regards
> > Jonathan
>
>
Received on Thursday, 7 January 2016 05:09:06 UTC

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