Assists Samba to address CVE-2020-25719
This may be contary to RFC4120 3.3.3 at
https://datatracker.ietf.org/doc/html/rfc4120/#section-3.3.3
(clearer in the GSS mechanism here:
https://datatracker.ietf.org/doc/html/draft-swift-win2k-krb-user2user-03 )
as server-name is decribed as optional, however Windows AD and Samba
both require that the server-name exist and be a valid SPN matching
the provided TGT.
The lookup of SPN -> entry ensures that the SPN the client thought it
was connecting to was held by the target server. it could be the
typical user principal, or a service principal, but needs to be checked
for the client not to be fooled into connecting to the wrong service.
Samba BUG: https://bugzilla.samba.org/show_bug.cgi?id=14873
Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(Similar to Samba commit fd50fecbe99ae4fc63843c796d0a516731a1fe6a)
4112f6fc79