1From d775c95af7606a51bf79547a94fa52ddd1cb7f49 Mon Sep 17 00:00:00 2001
2From: Greg Hudson <ghudson@mit.edu>
3Date: Tue, 3 Aug 2021 01:15:27 -0400
4Subject: [PATCH] Fix KDC null deref on TGS inner body null server
5
6After the KDC decodes a FAST inner body, it does not check for a null
7server.  Prior to commit 39548a5b17bbda9eeb63625a201cfd19b9de1c5b this
8would typically result in an error from krb5_unparse_name(), but with
9the addition of get_local_tgt() it results in a null dereference.  Add
10a null check.
11
12Reported by Joseph Sutton of Catalyst.
13
14CVE-2021-37750:
15
16In MIT krb5 releases 1.14 and later, an authenticated attacker can
17cause a null dereference in the KDC by sending a FAST TGS request with
18no server field.
19
20ticket: 9008 (new)
21tags: pullup
22target_version: 1.19-next
23target_version: 1.18-next
24
25[Retrieved from:
26https://github.com/krb5/krb5/commit/d775c95af7606a51bf79547a94fa52ddd1cb7f49]
27Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
28---
29 src/kdc/do_tgs_req.c | 5 +++++
30 1 file changed, 5 insertions(+)
31
32diff --git a/src/kdc/do_tgs_req.c b/src/kdc/do_tgs_req.c
33index 582e497cc9..32dc65fa8e 100644
34--- a/src/kdc/do_tgs_req.c
35+++ b/src/kdc/do_tgs_req.c
36@@ -204,6 +204,11 @@ process_tgs_req(krb5_kdc_req *request, krb5_data *pkt,
37         status = "FIND_FAST";
38         goto cleanup;
39     }
40+    if (sprinc == NULL) {
41+        status = "NULL_SERVER";
42+        errcode = KRB5KDC_ERR_S_PRINCIPAL_UNKNOWN;
43+        goto cleanup;
44+    }
45
46     errcode = get_local_tgt(kdc_context, &sprinc->realm, header_server,
47                             &local_tgt, &local_tgt_storage, &local_tgt_key);
48