In the client side of Heimdal before 7.6.0, failure to verify anonymous PKINIT PA-PKINIT-KX key exchange permits a man-in-the-middle attack. This issue is in krb5_init_creds_step in lib/krb5/init_creds_pw.c.
https://www.debian.org/security/2019/dsa-4455
https://seclists.org/bugtraq/2019/Jun/1
https://github.com/heimdal/heimdal/releases/tag/heimdal-7.6.0
https://github.com/heimdal/heimdal/compare/3e58559...bbafe72
https://github.com/heimdal/heimdal/commit/2f7f3d9960aa6ea21358bdf3687cee5149aa35cf
http://www.h5l.org/pipermail/heimdal-announce/2019-May/000009.html
http://lists.opensuse.org/opensuse-security-announce/2019-08/msg00026.html
http://lists.opensuse.org/opensuse-security-announce/2019-07/msg00003.html
http://lists.opensuse.org/opensuse-security-announce/2019-07/msg00002.html