Fix KDC null deref on TGS inner body null server

After the KDC decodes a FAST inner body, it does not check for a null
server.  Prior to commit 39548a5b17bbda9eeb63625a201cfd19b9de1c5b this
would typically result in an error from krb5_unparse_name(), but with
the addition of get_local_tgt() it results in a null dereference.  Add
a null check.

Reported by Joseph Sutton of Catalyst.

CVE-2021-37750:

In MIT krb5 releases 1.14 and later, an authenticated attacker can
cause a null dereference in the KDC by sending a FAST TGS request with
no server field.

(cherry picked from commit d775c95af7606a51bf79547a94fa52ddd1cb7f49)

ticket: 9008
version_fixed: 1.19.3

1 files changed, 5 insertions, 0 deletions

diff --git a/src/kdc/do_tgs_req.c b/src/kdc/do_tgs_req.c
index 6d244ff..39a504c 100644
--- a/src/kdc/do_tgs_req.c
+++ b/src/kdc/do_tgs_req.c
@@ -207,6 +207,11 @@ process_tgs_req(krb5_kdc_req *request, krb5_data *pkt,
         status = "FIND_FAST";
         goto cleanup;
     }
+    if (sprinc == NULL) {
+        status = "NULL_SERVER";
+        errcode = KRB5KDC_ERR_S_PRINCIPAL_UNKNOWN;
+        goto cleanup;
+    }
 
     errcode = get_local_tgt(kdc_context, &sprinc->realm, header_server,
                             &local_tgt, &local_tgt_storage, &local_tgt_key);