Re: [Acme] Benjamin Kaduk's Discuss on draft-ietf-acme-acme-14: (with DISCUSS and COMMENT)

[Felix Fontein <[e-mail]>]

Hi,

> > >    [...] Secondly, the entropy requirement
> > >    prevents ACME clients from implementing a "naive" validation
> > > server that automatically replies to challenges without
> > > participating in the creation of the initial authorization
> > > request.
> > >
> > > IMPORTANT: I'm not sure I see how this applies to the HTTP
> > > mechanism -- couldn't you write a script to reply
> > > to .well-known/acme-challenge/<foo> with <foo>.<key thumbprint>
> > > for a fixed key thumbprint?  The validation server would ned to
> > > know about the ACME account in question, but not about any
> > > individual authorization request.  
> > 
> > It would also need to know the <foo> value, which is not provided
> > in the validation request specifically to avoid this.  
> 
> As I said above, "[w]ell now I'm really confused."  In the example
> HTTP challenge exchange (duplicated here):
> 
> GET /.well-known/acme-challenge/LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowyjxAjEuX0
> Host: example.org
> 
> HTTP/1.1 200 OK
> Content-Type: application/octet-stream
> 
> LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowyjxAjEuX0.9jg46WB3rR_AHD-EBXdN7cBkH1WOu0tA3M9fm21mqTI
> 
> Doesn't the path in the GET include the <foo>?

Yes, and some people are already using this to add a generic
authorization replier (which needs the thumbprint of the account key).
For example, the acme.sh client supports this "stateless mode" (as it
is called there):
https://github.com/Neilpang/acme.sh/wiki/Stateless-Mode

(There currently is also a discussion in the Let's Encrypt community
forum about this, see
https://community.letsencrypt.org/t/xss-via-acme-implementations/72295;
this is because some implementations do not restrict the allowed input
and thus allow XSS attacks, as described here:
https://labs.detectify.com/2018/09/04/xss-using-quirky-implementations-of-acme-http-01/)

Cheers,
Felix