[Bug 61433] New: Crashes when trying to hash caps containing pathological data forms

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=61433

          Priority: medium
             Group: Telepathy Security
            Bug ID: 61433
          Assignee: telepathy-bugs at lists.freedesktop.org
           Summary: Crashes when trying to hash caps containing
                    pathological data forms
        QA Contact: telepathy-bugs at lists.freedesktop.org
          Severity: normal
    Classification: Unclassified
                OS: All
          Reporter: will.thompson at collabora.co.uk
          Hardware: Other
            Status: NEW
           Version: git master
         Component: gabble
           Product: Telepathy

Calculating a XEP-0115 hash involves sorting the fields of data forms in the
contact's capabilities according to their field name.
wocky_caps_hash_compute_from_lists() does this using this function:

  gint
  wocky_data_form_field_cmp (const WockyDataFormField *left,
      const WockyDataFormField *right)
  {
    return strcmp (left->var, right->var);
  }

This will obviously crash if either ->var is NULL, but WockyDataForm validates
forms it parses so this shouldn't happen, right? Unfortunately,
http://xmpp.org/extensions/xep-0004.html#protocol-field says:

> If the <field/> element type is anything other than "fixed" (see below), it
> MUST possess a 'var' attribute that uniquely identifies the field in the 
> context of the form (if it is "fixed", it MAY possess a 'var' attribute).

And WockyDataForm implements this correctly:

  var = wocky_node_get_attribute (node, "var");
  if (var == NULL && type != WOCKY_DATA_FORM_FIELD_TYPE_FIXED)
    {
      DEBUG ("field node doesn't have a 'var' attribute; ignoring");
      return FALSE;
    }

So we have a remotely-triggered DoS: send Gabble a <presence> with a caps hash;
include a form with an anonymous fixed field in the reply; boom. Since anyone
can send presence to anyone else, and Gabble always looks up any caps it sees
in any presences it receives. (Note that this is a presence leak, too; another
bug, I think.)

I have some fixes; patches to follow. I will also suggest that
http://xmpp.org/extensions/xep-0115.html be updated to mention this.

-- 
You are receiving this mail because:
You are the QA Contact for the bug.
You are the assignee for the bug.