W3C home > Mailing lists > Public > public-webappsec@w3.org > September 2013

Re: Updated script hash proposal (non spec text)

From: Mountie Lee <mountie@paygate.net>
Date: Sat, 21 Sep 2013 11:11:07 +0900
Message-ID: <CAE-+aY+b6iK8JRX1VshjKHz-VMad-RbuMugK6cuf9bN1w0OM3g@mail.gmail.com>
To: Neil Matatall <neilm@twitter.com>
Cc: "public-webappsec@w3.org" <public-webappsec@w3.org>
I have BIG interest for protecting javascript integrity.

at WebCrypto WG, it was metioned out-of-scope
at WebAppSec WG, it was touched but minor than nonce.
at WebApp WG, I found your comment.

many people said SSL/TLS is enough.
but in view of OSI 7 layers, it only protect transport layer.

I think we need a protection mechanism in application layer.

script hash or signature are the possible solutions.

can your suggestions be deliverable?






On Sat, Sep 21, 2013 at 1:46 AM, Neil Matatall <neilm@twitter.com> wrote:

> > 1. what to do after comparing hash?
>
> The policy used determines this, it will generate a warning and
> optionally block depending on the presence of "-Report-Only" in the
> header name.
>
> > 2. page encodings
>
> There has definitely been some discussion on this. ABarth explained
> this one time but I _think_ the consensus that UTF-8 only was OK. All
> valid javascript code characters fall within UTF-8 and non-UTF-8 data
> can be read from elsewhere in the DOM/external scripts. I would
> definitely appreciate extra eyes on this. It breaks some edge cases
> (like script A modifying script B before B is done loading), but still
> works in the common case.
>
> > 3. add tag attributes to hash source
>
> I don't think this gives us any benefit and increases the complexity
> of the hashing significantly. There was talk of including the "type"
> attribute, but I believe that was decided to be ignored. Type
> "application/json" is ignored by CSP enforcement, VBScript/Ruby/Etc
> are not covered by this.
>
> On Fri, Sep 20, 2013 at 12:21 AM, Mountie Lee <mountie@paygate.net> wrote:
> > questions and comments.
> >
> > 1. what to do after comparing hash?
> >
> > if browser calculate and compare the script hash,
> > what to do next? block? warning? get user consent?
> > dependent on browser vendor decision?
> >
> > 2. page encodings
> >
> > scripts are dependent on page encodings.
> > many countries use non unicode encoding (GB2312, BIG5, EUC-KR, EUC-JP,
> > Shift-JIS)
> >
> > is the non-unicode page encodings out of scope?
> > can we use "charset" attribute in script tag?
> >
> > 3. add tag attributes to hash source
> >
> > in your example,
> > <script>
> >   alert(1);
> > </script>
> >
> > "\n  alert(1);\n" is the hash source.
> >
> > if the tag has more attributes like
> > <script language="javascript" charset="EUC-KR">
> >  alert(1);
> > </script>
> >
> > can we add tag attributes ("language...." part) to hash source?
> >
> >
> >
> > On Fri, Sep 20, 2013 at 7:46 AM, Neil Matatall <neilm@twitter.com>
> wrote:
> >>
> >> Sorry for the long period of silence, I've been doing some evangelizing.
> >>
> >> Script hashes will be another source expression. Per script hash, the
> >> algorithm and digest length precede the actual hash value. e.g.:
> >>
> >> script-src 'sha256-0byNO6Svx+EJYSy3Osvd2sBSyTAlqh+ClC7au33rgqE'
> >>
> >> If a script hash source is specified and the user agent understands
> >> it, the browser should ignore the 'unsafe-inline' directive for
> >> backwards compatibility. Any inline script whose computed hash value
> >> does not match a hash specified in the hash sources should not be
> >> executed and an informative error message should be displayed
> >> including the expected hash value.
> >>
> >> If multiple hashing algorithms are specified in the CSP header, the
> >> browser must compute all possible hashes for each inline script block.
> >> If the computed hash matches any computed hash in the header with a
> >> matching algorithm+digest length, the script should execute. There was
> >> talk of limiting this to one algorithm per header, but CDNs complicate
> >> things.
> >>
> >> This is not meant to and should not support dynamic javascript. Hashes
> >> should not be computed dynamically (at least not in production).
> >>
> >> === Computing hash values
> >>
> >> base64encode(<hashing algorithm>(UTF-8(<content of script tag>)))
> >>
> >> <script>
> >>   alert(1);
> >> </script>
> >>
> >> base64encode(sha256(UTF-8("\n  alert(1);\n")))
> >>
> >> === Script-hash unobtrusive workflow (PoC)
> >>
> >> Unfortunately, many online hashing services will strip
> >> leading/trailing whitespace which is not what we want.
> >>
> >> I wrote a quick and dirty method for computing all script-hashes on a
> >> page:
> >>
> >> $.each($('script'), function(index, x) {
> >>
> console.log(CryptoJS.SHA256(x.innerHTML).toString(CryptoJS.enc.Base64));
> >> });
> >>
> >> Here's the equivilent openssl command:
> >>
> >> openssl dgst -sha256 -binary | base64
> >>
> >> I wrote a more thorough rails plugin and explained how it works in [1]
> >> including a (low quality) video on how the developer workflow would
> >> work: [2].
> >>
> >> Essentially:
> >> 1) Find all inline scripts - search through the source code of any
> >> file that could be rendered / displayed to a user.
> >> 2) Extract the content of each inline script, hash according to the algo
> >> above.
> >> 3) Store as Filename -> [hashes] mapping. In a configuration file, for
> >> example.
> >> 4) Any time a file is rendered, the corresponding hashes are added to
> >> the CSP header.
> >>
> >> I believe this can be built in to every framework and be unobtrusive.
> >>
> >> [1]
> http://nmatatal.blogspot.com/2013/09/how-my-script-hash-poc-works.html
> >> [2] http://www.youtube.com/watch?v=Bc2hvziTRxg
> >>
> >> p.s. I support both script nonce and script hash, I think we need to
> >> have both :-/
> >>
> >
> >
> >
> > --
> > Mountie Lee
> >
> > PayGate
> > CTO, CISSP
> > Tel : +82 2 2140 2700
> > E-Mail : mountie@paygate.net
> >
> > =======================================
> > PayGate Inc.
> > THE STANDARD FOR ONLINE PAYMENT
> > for Korea, Japan, China, and the World
> >
>



-- 
Mountie Lee

PayGate
CTO, CISSP
Tel : +82 2 2140 2700
E-Mail : mountie@paygate.net

=======================================
PayGate Inc.
THE STANDARD FOR ONLINE PAYMENT
for Korea, Japan, China, and the World
Received on Saturday, 21 September 2013 02:12:14 UTC

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