It was found that an OpenSSL server would, under certain conditions, accept Diffie-Hellman client certificates without the use of a private key. An attacker could use a user's client certificate to authenticate as that user, without needing the private key.
OpenSSL released security advisory  which fixes the below issue:
An OpenSSL server will accept a DH certificate for client authentication
without the certificate verify message. This effectively allows a client
to authenticate without the use of a private key. This only affects servers
which trust a client certificate authority which issues certificates
containing DH keys: these are extremely rare and hardly ever encountered.
Upstream commit that looks to fix the problem:
Created openssl tracking bugs for this issue:
Affects: fedora-all [bug 1181013]
openssl-1.0.1k-1.fc21 has been pushed to the Fedora 21 stable repository. If problems still persist, please make note of it in this bug report.
This issue does not affect the version of openssl and openssl097a as shipped with Red Hat Enterprise Linux 5. This issue does not affect the version of openssl098e as shipped with Red Hat Enterprise Linux 6 and 7.
openssl-1.0.1e-41.fc20 has been pushed to the Fedora 20 stable repository. If problems still persist, please make note of it in this bug report.
This issue has been addressed in the following products:
Red Hat Enterprise Linux 6
Red Hat Enterprise Linux 7
Via RHSA-2015:0066 https://rhn.redhat.com/errata/RHSA-2015-0066.html