s4:auth/kerberos: don't mix s4u2self creds with machine account creds

It's important that we don't store the tgt for the machine account
in the same krb5_ccache as the ticket for the impersonated principal.

We may pass it to some krb5/gssapi functions and they may use them
in the wrong way, which would grant machine account privileges to
the client.

metze

1 files changed, 76 insertions, 24 deletions

diff --git a/source4/auth/kerberos/kerberos.c b/source4/auth/kerberos/kerberos.c
index 5e38d878be..5feb3e6221 100644
--- a/source4/auth/kerberos/kerberos.c
+++ b/source4/auth/kerberos/kerberos.c
@@ -84,7 +84,7 @@
   The target_service defaults to the krbtgt if NULL, but could be kpasswd/realm or the local service (if we are doing s4u2self)
 
 */
- krb5_error_code kerberos_kinit_password_cc(krb5_context ctx, krb5_ccache cc, 
+ krb5_error_code kerberos_kinit_password_cc(krb5_context ctx, krb5_ccache store_cc,
 					    krb5_principal init_principal,
 					    const char *init_password,
 					    krb5_principal impersonate_principal,
@@ -93,14 +93,15 @@
 					    time_t *expire_time, time_t *kdc_time)
 {
 	krb5_error_code code = 0;
-	krb5_creds init_creds;
 	krb5_get_creds_opt options;
+	krb5_principal store_principal;
+	krb5_creds store_creds;
 	const char *self_service = target_service;
 
 	/* If we are not impersonating, then get this ticket for the
 	 * target service, otherwise a krbtgt, and get the next ticket
 	 * for the target */
-	if ((code = krb5_get_init_creds_password(ctx, &init_creds,
+	if ((code = krb5_get_init_creds_password(ctx, &store_creds,
 						 init_principal,
 						 init_password,
 						 NULL, NULL,
@@ -110,32 +111,44 @@
 		return code;
 	}
 
-	if ((code = krb5_cc_initialize(ctx, cc, init_principal))) {
-		krb5_free_cred_contents(ctx, &init_creds);
-		return code;
-	}
-	
-	if ((code = krb5_cc_store_cred(ctx, cc, &init_creds))) {
-		krb5_free_cred_contents(ctx, &init_creds);
-		return code;
-	}
-	
-	if (expire_time) {
-		*expire_time = (time_t) init_creds.times.endtime;
-	}
-
-	if (kdc_time) {
-		*kdc_time = (time_t) init_creds.times.starttime;
-	}
-
-	krb5_free_cred_contents(ctx, &init_creds);
+	store_principal = init_principal;
 
 	if (code == 0 && impersonate_principal) {
+		krb5_ccache tmp_cc;
 		krb5_creds *s4u2self_creds;
 		krb5_principal self_princ;
 		const char *self_realm;
 
 		/*
+		 * As we do not want to expose our TGT in the
+		 * krb5_ccache, which is also holds the impersonated creds.
+		 *
+		 * Some low level krb5/gssapi function might use the TGT
+		 * identity and let the client act as our machine account.
+		 *
+		 * We need to avoid that and use a temporary krb5_ccache
+		 * in order to pass our TGT to the krb5_get_creds() function.
+		 */
+		if ((code = krb5_cc_new_unique(ctx, NULL, NULL, &tmp_cc))) {
+			krb5_free_cred_contents(ctx, &store_creds);
+			return code;
+		}
+
+		if ((code = krb5_cc_initialize(ctx, tmp_cc, store_creds.client))) {
+			krb5_cc_destroy(ctx, tmp_cc);
+			krb5_free_cred_contents(ctx, &store_creds);
+			return code;
+		}
+
+		if ((code = krb5_cc_store_cred(ctx, tmp_cc, &store_creds))) {
+			krb5_cc_destroy(ctx, tmp_cc);
+			krb5_free_cred_contents(ctx, &store_creds);
+			return code;
+		}
+
+		krb5_free_cred_contents(ctx, &store_creds);
+
+		/*
 		 * For S4U2Self we need our own service principal,
 		 * which belongs to our own realm (available on
 		 * our client principal.
@@ -143,40 +156,79 @@
 		self_realm = krb5_principal_get_realm(ctx, init_principal);
 
 		if ((code = krb5_parse_name(ctx, self_service, &self_princ))) {
+			krb5_cc_destroy(ctx, tmp_cc);
 			return code;
 		}
 
 		if ((code = krb5_principal_set_realm(ctx, self_princ, self_realm))) {
 			krb5_free_principal(ctx, self_princ);
+			krb5_cc_destroy(ctx, tmp_cc);
 			return code;
 		}
 
 		if ((code = krb5_get_creds_opt_alloc(ctx, &options))) {
 			krb5_free_principal(ctx, self_princ);
+			krb5_cc_destroy(ctx, tmp_cc);
 			return code;
 		}
 
 		if ((code = krb5_get_creds_opt_set_impersonate(ctx, options, impersonate_principal))) {
 			krb5_get_creds_opt_free(ctx, options);
 			krb5_free_principal(ctx, self_princ);
+			krb5_cc_destroy(ctx, tmp_cc);
 			return code;
 		}
 
-		if ((code = krb5_get_creds(ctx, options, cc, self_princ, &s4u2self_creds))) {
+		if ((code = krb5_get_creds(ctx, options, tmp_cc, self_princ, &s4u2self_creds))) {
 			krb5_get_creds_opt_free(ctx, options);
 			krb5_free_principal(ctx, self_princ);
+			krb5_cc_destroy(ctx, tmp_cc);
 			return code;
 		}
 
 		krb5_get_creds_opt_free(ctx, options);
 		krb5_free_principal(ctx, self_princ);
+		krb5_cc_destroy(ctx, tmp_cc);
 
-		code = krb5_cc_store_cred(ctx, cc, s4u2self_creds);
+		/*
+		 * Now make sure we store the impersonated principal
+		 * and creds instead of the TGT related stuff
+		 * in the krb5_ccache of the caller.
+		 */
+		if ((code = krb5_copy_creds_contents(ctx, s4u2self_creds, &store_creds))) {
+			krb5_free_creds(ctx, s4u2self_creds);
+			return code;
+		}
 		krb5_free_creds(ctx, s4u2self_creds);
 
+		/*
+		 * It's important to store the principal the KDC
+		 * returned, as otherwise the caller would not find
+		 * the S4U2Self ticket in the krb5_ccache lookup.
+		 */
+		store_principal = store_creds.client;
+	}
+
+	if ((code = krb5_cc_initialize(ctx, store_cc, store_principal))) {
+		krb5_free_cred_contents(ctx, &store_creds);
 		return code;
 	}
 
+	if ((code = krb5_cc_store_cred(ctx, store_cc, &store_creds))) {
+		krb5_free_cred_contents(ctx, &store_creds);
+		return code;
+	}
+
+	if (expire_time) {
+		*expire_time = (time_t) store_creds.times.endtime;
+	}
+
+	if (kdc_time) {
+		*kdc_time = (time_t) store_creds.times.starttime;
+	}
+
+	krb5_free_cred_contents(ctx, &store_creds);
+
 	return 0;
 }