The remote Debian host is missing a security-related update.
Several vulnerabilities were discovered in Heimdal, an implementation of Kerberos 5 that aims to be compatible with MIT Kerberos. - CVE-2018-16860 Isaac Boukris and Andrew Bartlett discovered that Heimdal was susceptible to man-in-the-middle attacks caused by incomplete checksum validation. Details on the issue can be found in the Samba advisory at https://www.samba.org/samba/security/CVE-2018-16860.html . - CVE-2019-12098 It was discovered that failure of verification of the PA-PKINIT-KX key exchange client-side could permit to perform man-in-the-middle attack.
Upgrade the heimdal packages. For the stable distribution (stretch), these problems have been fixed in version 7.1.0+dfsg-13+deb9u3.