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Re-using connections with wrong client cert

Project curl Security Advisory, August 3rd 2016 - Permalink


libcurl did not consider client certificates when reusing TLS connections.

libcurl supports reuse of established connections for subsequent requests. It does this by keeping a few previous connections "alive" in a connection pool so that a subsequent request that can use one of them instead of creating a new connection will do so.

When using a client certificate for a connection that was then put into the connection pool, that connection could then wrongly get reused in a subsequent request to that same server that either didn't use a client certificate at all or that asked to use a different client certificate thus trying to tell the user that it is a different entity.

This mistakenly using the wrong connection could of course lead to applications sending requests to the wrong realms of the server using authentication that it wasn't supposed to have for those operations.

We are not aware of any exploit of this flaw.


This flaw also affects the curl command line tool.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2016-5420 to this issue.


This flaw is relevant for all versions of curl and libcurl that support SSL/TLS and client certificates.

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


In version 7.50.1, curl will check that re-used connections have the correct client certificate (file name) before used.

A patch for CVE-2016-5420 is available. This patch relies on the CVE-2016-5419 patch already having been applied.


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

A - Upgrade curl and libcurl to version 7.50.1

B - Apply the patch to your version and rebuild

C - Do not use client certificates


This was figured out by curl security team members during our work with the 20160803A flaw during June 2016. We contacted distros@openwall on July 31.

libcurl 7.50.1 was released on August 3 2016, coordinated with the publication of this advisory.


Found by the curl security team. Patch by Daniel Stenberg.

Thanks a lot!