curl / Docs / Security Problems / re-use of wrong HTTP NTLM connection

re-use of wrong HTTP NTLM connection

Project curl Security Advisory, January 29th 2014 permalink


libcurl can in some circumstances re-use the wrong connection when asked to do an NTLM-authenticated HTTP or HTTPS request.

libcurl features a pool of recent connections so that subsequent requests can re-use an existing connection to avoid overhead.

When re-using a connection a range of criterion must first be met. Due to a logical error in the code, a request that was issued by an application could wrongfully re-use an existing connection to the same server that was authenticated using different credentials. One underlying reason being that NTLM authenticates connections and not requests, contrary to how HTTP is designed to work and how other authentication methods work.

An application that allows NTLM and another auth method (the bug only triggers if more than one auth method is asked for) to a server (that responds wanting NTLM) with user1:password1 and then does another operation to the same server with user2:password2 (when the previous connection was left alive) - the second request will re-use the same connection and since it'll then see that the NTLM negotiation is already made, it will just send the request over that connection thinking it uses 'user2' credentials when it is in fact still using the connection authenticated for user1...

The set of auth methods to use is set with CURLOPT_HTTPAUTH.

Two common auth defines in libcurl are CURLAUTH_ANY and CURLAUTH_ANYSAFE. Both of them ask for NTLM and other methods and can therefore trigger this problem.

Applications can disable libcurl's re-use of connections and thus mitigate this problem, by using one of the following libcurl options to alter how connections are or aren't re-used: CURLOPT_FRESH_CONNECT, CURLOPT_MAXCONNECTS and CURLMOPT_MAX_HOST_CONNECTIONS (if using the curl_multi API).

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2014-0015 to this issue.

CWE-305: Authentication Bypass by Primary Weakness


This flaw has existed ever since libcurl started to support NTLM, although the code has been restructured a few times over the years so the mistake has altered shape over the years.

libcurl is used by many applications, but not always advertised as such!


libcurl 7.35.0 makes sure that connections that may use NTLM cannot re-use another NTLM-using connection unless the credentials match.

A patch for this problem (that applies to libcurl >= 7.28.0) is [available](

A patch that applies for 7.27.0 is available

This fix is already committed to the public source code repository because the full security impact wasn't properly realized until after the fact.


We suggest you take one of the following actions immediately, in order of preference:

A - Upgrade to curl and libcurl 7.35.0

B - Apply the patch and rebuild libcurl

C - Avoid using HTTP NTLM in your application, or make sure to only specify NTLM as the requested authentication method.


It was reported to the curl project on January 7th 2014. We contacted distros@openwall on January 14th.

libcurl 7.35.0 was released on January 29th 2014, coordinated with the publication of this advisory.


Bug originally reported by Paras Sethia. The security impact of it was flagged by Yehezkel Horowitz.

Thanks a lot!