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 ticket: 8599 (new) target_version: 1.15-next target_version: 1.14-next tags: pullup
krb5 · Jul 24, 2017 · ffb35ba · ffb35ba
1 parent e458441
commit ffb35ba
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Showing 3 changed files with 12 additions and 5 deletions.
diff --git a/src/kdc/do_as_req.c b/src/kdc/do_as_req.c
@@ -366,8 +366,8 @@ 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
@@ -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
@@ -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 :
@@ -1624,13 +1628,15 @@ 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;
     }
 
     /* Ensure that evidence ticket server matches TGT client */
     if (!krb5_principal_compare(kdc_context,
                                 server->princ, /* after canon */
                                 server_princ)) {
+        *status = "EVIDENCE_TICKET_MISMATCH";
         return KRB5KDC_ERR_SERVER_NOMATCH;
     }