- From: Neil Matatall <neilm@twitter.com>
- Date: Fri, 20 Sep 2013 09:46:07 -0700
- To: Mountie Lee <mountie@paygate.net>
- Cc: "public-webappsec@w3.org" <public-webappsec@w3.org>
> 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 >
Received on Friday, 20 September 2013 16:46:35 UTC