Prevent KDC unset status assertion failures

Assign status values if S4U2Self padata fails to decode, if an
S4U2Proxy request uses invalid KDC options, or if an S4U2Proxy request
uses an evidence ticket which does not match the canonicalized request
server principal name.  Reported by Samuel Cabrero.

If a status value is not assigned during KDC processing, default to
"UNKNOWN_REASON" rather than failing an assertion.  This change will
prevent future denial of service bugs due to similar mistakes, and
will allow us to omit assigning status values for unlikely errors such
as small memory allocation failures.

CVE-2017-11368:

In MIT krb5 1.7 and later, an authenticated attacker can cause an
assertion failure in krb5kdc by sending an invalid S4U2Self or
S4U2Proxy request.

  CVSSv3 Vector: AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H/E:H/RL:O/RC:C

(cherry picked from commit ffb35baac6981f9e8914f8f3bffd37f284b85970)

ticket: 8599
version_fixed: 1.15.2

3 files changed, 12 insertions, 5 deletions

diff --git a/src/kdc/do_as_req.c b/src/kdc/do_as_req.c
index 712ccb7..a4bf91b 100644
--- a/src/kdc/do_as_req.c
+++ b/src/kdc/do_as_req.c
@@ -365,8 +365,8 @@ finish_process_as_req(struct as_req_state *state, krb5_error_code errcode)
     did_log = 1;
 
 egress:
-    if (errcode != 0)
-        assert (state->status != 0);
+    if (errcode != 0 && state->status == NULL)
+        state->status = "UNKNOWN_REASON";
 
     au_state->status = state->status;
     au_state->reply = &state->reply;
diff --git a/src/kdc/do_tgs_req.c b/src/kdc/do_tgs_req.c
index 547a414..339259f 100644
--- a/src/kdc/do_tgs_req.c
+++ b/src/kdc/do_tgs_req.c
@@ -823,7 +823,8 @@ process_tgs_req(struct server_handle *handle, krb5_data *pkt,
     free(reply.enc_part.ciphertext.data);
 
 cleanup:
-    assert(status != NULL);
+    if (status == NULL)
+        status = "UNKNOWN_REASON";
     if (reply_key)
         krb5_free_keyblock(kdc_context, reply_key);
     if (errcode)
diff --git a/src/kdc/kdc_util.c b/src/kdc/kdc_util.c
index 29f9dbb..30c501c 100644
--- a/src/kdc/kdc_util.c
+++ b/src/kdc/kdc_util.c
@@ -1220,8 +1220,10 @@ kdc_process_for_user(kdc_realm_t *kdc_active_realm,
     req_data.data = (char *)pa_data->contents;
 
     code = decode_krb5_pa_for_user(&req_data, &for_user);
-    if (code)
+    if (code) {
+        *status = "DECODE_PA_FOR_USER";
         return code;
+    }
 
     code = verify_for_user_checksum(kdc_context, tgs_session, for_user);
     if (code) {
@@ -1320,8 +1322,10 @@ kdc_process_s4u_x509_user(krb5_context context,
     req_data.data = (char *)pa_data->contents;
 
     code = decode_krb5_pa_s4u_x509_user(&req_data, s4u_x509_user);
-    if (code)
+    if (code) {
+        *status = "DECODE_PA_S4U_X509_USER";
         return code;
+    }
 
     code = verify_s4u_x509_user_checksum(context,
                                          tgs_subkey ? tgs_subkey :
@@ -1624,6 +1628,7 @@ kdc_process_s4u2proxy_req(kdc_realm_t *kdc_active_realm,
      * that is validated previously in validate_tgs_request().
      */
     if (request->kdc_options & (NON_TGT_OPTION | KDC_OPT_ENC_TKT_IN_SKEY)) {
+        *status = "INVALID_S4U2PROXY_OPTIONS";
         return KRB5KDC_ERR_BADOPTION;
     }
 
@@ -1631,6 +1636,7 @@ kdc_process_s4u2proxy_req(kdc_realm_t *kdc_active_realm,
     if (!krb5_principal_compare(kdc_context,
                                 server->princ, /* after canon */
                                 server_princ)) {
+        *status = "EVIDENCE_TICKET_MISMATCH";
         return KRB5KDC_ERR_SERVER_NOMATCH;
     }