ipa-kdb: Verify the correct checksum in PAC validation

This patch requires a forthcoming change in MIT libraries which allows to pass
NULL for the server_key to the krb5_pac_verify() function.

In most cases we should always only check the KDC checksum to verify the PAC
validity.

The only exception is when we are releasing a ticket to a client from another
realm. In this case the only signature we can check is the server checksum, and
we use the cross-realm key to validate in this case.

The previous code was working for normal cases because the kdc uses the same
key to create the server and the kdc checksum for a TGT, but that is not true
for evidence tickets (s4u2proxy) or cross-realm TGTs.

Fixes: https://fedorahosted.org/freeipa/ticket/2169

1 files changed, 45 insertions, 5 deletions

diff --git a/daemons/ipa-kdb/ipa_kdb_mspac.c b/daemons/ipa-kdb/ipa_kdb_mspac.c
index 7973e8c89..a22044c24 100644
--- a/daemons/ipa-kdb/ipa_kdb_mspac.c
+++ b/daemons/ipa-kdb/ipa_kdb_mspac.c
@@ -554,10 +554,27 @@ done:
     return kerr;
 }
 
+static bool is_cross_realm_krbtgt(krb5_const_principal princ)
+{
+    if ((princ->length != 2) ||
+        (princ->data[0].length != 6) ||
+        (strncasecmp(princ->data[0].data, "krbtgt", 6) != 0)) {
+        return false;
+    }
+    if (princ->data[1].length == princ->realm.length &&
+        strncasecmp(princ->data[1].data,
+                    princ->realm.data, princ->realm.length) == 0) {
+        return false;
+    }
+
+    return true;
+}
+
 static krb5_error_code ipadb_verify_pac(krb5_context context,
                                         unsigned int flags,
                                         krb5_const_principal client_princ,
-                                        krb5_db_entry *client,
+                                        krb5_db_entry *server,
+                                        krb5_db_entry *krbtgt,
                                         krb5_keyblock *server_key,
                                         krb5_keyblock *krbtgt_key,
                                         krb5_timestamp authtime,
@@ -565,6 +582,8 @@ static krb5_error_code ipadb_verify_pac(krb5_context context,
                                         krb5_pac *pac)
 {
     krb5_authdata **authdata = NULL;
+    krb5_keyblock *srv_key = NULL;
+    krb5_keyblock *priv_key = NULL;
     krb5_error_code kerr;
     krb5_ui_4 *buffer_types = NULL;
     size_t num_buffers;
@@ -598,8 +617,30 @@ static krb5_error_code ipadb_verify_pac(krb5_context context,
         goto done;
     }
 
+    /* for cross realm trusts cases we need to check the right checksum.
+     * when the PAC is signed by our realm, we can always just check it
+     * passing our realm krbtgt key as the kdc checksum key (privsvr).
+     * But when a trusted realm passes us a PAC the kdc checksum is
+     * generated with that realm krbtgt key, so we need to use the cross
+     * realm krbtgt to check the 'server' checksum instead. */
+    if (is_cross_realm_krbtgt(krbtgt->princ)) {
+        /* krbtgt from a trusted realm */
+
+        /* FIXME:
+         * We must refuse a PAC that comes signed with a cross realm TGT
+         * where the client pretends to be from our realm. It is an attempt
+         * at getting us to sign fake credentials with the help of a
+         * compromised trusted realm */
+
+        /* TODO: Here is where we need to plug our PAC Filtering, later on */
+        srv_key = krbtgt_key;
+    } else {
+        /* krbtgt from our own realm */
+        priv_key = krbtgt_key;
+    }
+
     kerr = krb5_pac_verify(context, old_pac, authtime,
-                            client_princ, krbtgt_key, NULL);
+                            client_princ, srv_key, priv_key);
     if (kerr) {
         goto done;
     }
@@ -684,9 +725,8 @@ krb5_error_code ipadb_sign_authdata(krb5_context context,
     }
 
     if (!is_as_req) {
-        kerr = ipadb_verify_pac(context, flags,
-                                ks_client_princ, client,
-                                server_key, krbtgt_key,
+        kerr = ipadb_verify_pac(context, flags, ks_client_princ,
+                                server, krbtgt, server_key, krbtgt_key,
                                 authtime, tgt_auth_data, &pac);
         if (kerr != 0) {
             goto done;