The authentication hook (mgs_hook_authz) in mod-gnutls 0.5.10 and earlier does not validate client certificates when "GnuTLSClientVerify require" is set, which allows remote attackers to spoof clients via a crafted certificate.
https://security.gentoo.org/glsa/201709-04
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=578663