NTLM credentials not-checked for proxy connection re-use
========================================================

Project cURL Security Advisory, January 27th 2016 -
[Permalink](http://curl.haxx.se/docs/adv_20160127A.html)

VULNERABILITY
-------------

libcurl will reuse NTLM-authenticated proxy connections without properly
making sure that the connection was authenticated with the same credentials as
set for this transfer.

libcurl maintains a pool of connections after a transfer has completed. The
pool of connections is then gone through when a new transfer is requested and
if there's a live connection available that can be reused, it is preferred
instead of creating a new one.

Since NTLM-based authentication is *connection oriented* instead of *request
oriented* as other HTTP based authentication, it is important that only
connections that have been authenticated with the correct username + password
are reused. This was done properly for server connections already, but libcurl
failed to do it properly for proxy connections using NTLM.

A libcurl application can easily switch user credentials used for a proxy
connection between two requests, and that subsequent transfer then MUST make
libcurl use another connection. libcurl previously failed to do so.

The effects of this flaw, is that the application could be reusing a proxy
connection using the previously used credentials and thus it could be given to
or prevented access from resources that it wasn't intended to.

This problem is very similar to
[CVE-2014-0015][http://curl.haxx.se/docs/adv_20140129.html], which was for
direct server connections while this is for proxy connections.

We are not aware of any exploit of this flaw.

INFO

----
This flaw can also affect the curl command line tool if a similar operation
series is made with that.
The Common Vulnerabilities and Exposures (CVE) project has assigned the name
CVE-2016-0755 to this issue.
AFFECTED VERSIONS
-----------------
This flaw is relevant for
- Affected versions: libcurl 7.10.7 to and including 7.46.0
- Not affected versions: libcurl < 7.10.7 and libcurl >= 7.47.0
libcurl is used by many applications, but not always advertised as such!
THE SOLUTION
------------
In version 7.47.0, libcurl properly verifies the credentials for NTLM proxies
and only reuses a connection if there there is a match.
A patch for this problem that changes the default is available at:
     http://curl.haxx.se/CVE-2016-0755.patch
RECOMMENDATIONS
---------------
We suggest you take one of the following actions immediately, in order of
preference:
  A - Upgrade curl and libcurl to version 7.47.0
  B - Apply the patch to your version and rebuild
  C - Avoid NTLM with proxies, or if you use NTLM with proxies, make sure you
      close all the libcurl handles if you ever change proxy credentials so that
      libcurl won't get a chance to reuse the wrong connection.
TIME LINE
---------
It was first reported to the curl project on January 13 2016. We contacted
distros_at_openwall on January 21.
libcurl 7.47.0 was released on January 27 2016, coordinated with the
publication of this advisory.
CREDITS
-------
Reported and patched by Isaac Boukris.
Thanks a lot!
-- 
  / daniel.haxx.se
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