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Border case: Basic authentication with a comma in the auth realmfails

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January 13, 2018 00:00
Border case: Basic authentication with a comma in the auth realmfails
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Hello libwww,

I've encountered a situation where browsers "just work" but
LWP::UserAgent fails.

This is a border case which apparently went unnoticed for decades, and
for my own problem I've found a ridiculously easy workaround.  So I'm
not sure whether it is a good idea or worth the effort to change
things.  I volunteer to create a test case but am hesitating with a
fix because this might break things in the real world.


   * A web server with basic authentication for an auth realm (chosen
     somewhat, but not completely arbitrary):

       $realm = "data, protected";

   * A LWP::UserAgent which gets passed the correct credentials with:


  No matter what, you'll get a 401 status code for every request.
  The web server sends a WWW-Authenticate header:

    WWW-Authenticate: Basic realm="data, protected"

  On the client side, it turns out that the requests don't contain the
  corresponding Authorization header.


  When processing the WWW-Authenticate header, LWP::UserAgent
  unconditionally changes all commas in this header's value to
    $challenge =~ tr/,/;/;  # "," is used to separate auth-params!!

  But this modifies the realm if there's a comma in it.  Therefore,
  there's no longer a match when the credentials are looked up later
  in the process, so no credentials are sent with the request.

  The code change happened almost exactly 20 years ago, in commit
  c4cefa219297e42ce73a10b6ad1fe4d9a19a9373 (1997-12-01).  Since then,
  RFC 2068 (dated Jan 1997) was obsoleted by RFC 2617 (Jun 1999),
  which in turn has been obsoleted by RFC 7235 (Jun 2014).  But does
  that say that it is reasonably safe to rely on HTTP::Header::Util to
  split words as intended?

  Is it safe to do that replacement only _after_ the string
  "auth-params" or are there other values where that replacment is


  * Either get rid of the comma in the web server's realm definition.
    Can be tricky it isn't _your_ server.

  * Or do the same unconditional modification to the realm before
    passing it to the LWP::UserAgent's credentials method.  (BTW:
    Fixing it in LWP::UserAgent's get_basic_credentials method is
    possible but ugly, and not fully sufficient because you are
    advised to override this method when subclassing).

  * Or, if you're actually using a derived class like WWW::Mechanize
    and talk to just one application, use the credentials method of
    this class which allows to omit $netloc and $realm.


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