curl, as shipped in Red Hat Enterprise Linux 6 before version 7.19.7-53, did not correctly backport the fix for CVE-2015-3148 because it did not reflect the fact that the HAVE_GSSAPI define was meanwhile substituted by USE_HTTP_NEGOTIATE. This issue was introduced in RHEL 6.7 and affects RHEL 6 curl only.
https://bugzilla.redhat.com/show_bug.cgi?id=1422464