CVE-2022-2031 s4:kpasswd: Do not accept TGTs as kpasswd tickets

If TGTs can be used as kpasswd tickets, the two-minute lifetime of a
authentic kpasswd ticket may be bypassed. Furthermore, kpasswd tickets
are not supposed to be cached, but using this flaw, a stolen credentials
cache containing a TGT may be used to change that account's password,
and thus is made more valuable to an attacker.

Since all TGTs should be issued with a REQUESTER_SID PAC buffer, and
service tickets without it, we assert the absence of this buffer to
ensure we're not accepting a TGT.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15047
BUG: https://bugzilla.samba.org/show_bug.cgi?id=15049

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>

[jsutton@samba.org Fixed knownfail conflicts]

[jsutton@samba.org Fixed knownfail conflicts]

4 files changed, 48 insertions, 0 deletions

diff --git a/source4/kdc/kpasswd-helper.c b/source4/kdc/kpasswd-helper.c
index 55a2f5b3bf6..2ffdb79aea5 100644
--- a/source4/kdc/kpasswd-helper.c
+++ b/source4/kdc/kpasswd-helper.c
@@ -241,3 +241,23 @@ NTSTATUS kpasswd_samdb_set_password(TALLOC_CTX *mem_ctx,
 
 	return status;
 }
+
+krb5_error_code kpasswd_check_non_tgt(struct auth_session_info *session_info,
+				      const char **error_string)
+{
+	switch(session_info->ticket_type) {
+	case TICKET_TYPE_TGT:
+		/* TGTs are disallowed here. */
+		*error_string = "A TGT may not be used as a ticket to kpasswd";
+		return KRB5_KPASSWD_AUTHERROR;
+	case TICKET_TYPE_NON_TGT:
+		/* Non-TGTs are permitted, and expected. */
+		break;
+	default:
+		/* In case we forgot to set the type. */
+		*error_string = "Failed to ascertain that ticket to kpasswd is not a TGT";
+		return KRB5_KPASSWD_HARDERROR;
+	}
+
+	return 0;
+}
diff --git a/source4/kdc/kpasswd-helper.h b/source4/kdc/kpasswd-helper.h
index 8fad81e0a5d..94a6e2acfdd 100644
--- a/source4/kdc/kpasswd-helper.h
+++ b/source4/kdc/kpasswd-helper.h
@@ -43,4 +43,6 @@ NTSTATUS kpasswd_samdb_set_password(TALLOC_CTX *mem_ctx,
 				    enum samPwdChangeReason *reject_reason,
 				    struct samr_DomInfo1 **dominfo);
 
+krb5_error_code kpasswd_check_non_tgt(struct auth_session_info *session_info,
+				      const char **error_string);
 #endif /* _KPASSWD_HELPER_H */
diff --git a/source4/kdc/kpasswd-service-heimdal.c b/source4/kdc/kpasswd-service-heimdal.c
index a0352d1ad35..4d009b9eb24 100644
--- a/source4/kdc/kpasswd-service-heimdal.c
+++ b/source4/kdc/kpasswd-service-heimdal.c
@@ -253,6 +253,7 @@ krb5_error_code kpasswd_handle_request(struct kdc_server *kdc,
 {
 	struct auth_session_info *session_info;
 	NTSTATUS status;
+	krb5_error_code code;
 
 	status = gensec_session_info(gensec_security,
 				     mem_ctx,
@@ -264,6 +265,18 @@ krb5_error_code kpasswd_handle_request(struct kdc_server *kdc,
 		return KRB5_KPASSWD_HARDERROR;
 	}
 
+	/*
+	 * Since the kpasswd service shares its keys with the krbtgt, we might
+	 * have received a TGT rather than a kpasswd ticket. We need to check
+	 * the ticket type to ensure that TGTs cannot be misused in this manner.
+	 */
+	code = kpasswd_check_non_tgt(session_info,
+				     error_string);
+	if (code != 0) {
+		DBG_WARNING("%s\n", *error_string);
+		return code;
+	}
+
 	switch(verno) {
 	case KRB5_KPASSWD_VERS_CHANGEPW: {
 		DATA_BLOB password = data_blob_null;
diff --git a/source4/kdc/kpasswd-service-mit.c b/source4/kdc/kpasswd-service-mit.c
index de4c6f3f622..6b051567b6e 100644
--- a/source4/kdc/kpasswd-service-mit.c
+++ b/source4/kdc/kpasswd-service-mit.c
@@ -332,6 +332,7 @@ krb5_error_code kpasswd_handle_request(struct kdc_server *kdc,
 {
 	struct auth_session_info *session_info;
 	NTSTATUS status;
+	krb5_error_code code;
 
 	status = gensec_session_info(gensec_security,
 				     mem_ctx,
@@ -344,6 +345,18 @@ krb5_error_code kpasswd_handle_request(struct kdc_server *kdc,
 		return KRB5_KPASSWD_HARDERROR;
 	}
 
+	/*
+	 * Since the kpasswd service shares its keys with the krbtgt, we might
+	 * have received a TGT rather than a kpasswd ticket. We need to check
+	 * the ticket type to ensure that TGTs cannot be misused in this manner.
+	 */
+	code = kpasswd_check_non_tgt(session_info,
+				     error_string);
+	if (code != 0) {
+		DBG_WARNING("%s\n", *error_string);
+		return code;
+	}
+
 	switch(verno) {
 	case 1: {
 		DATA_BLOB password;