Limit the number of filters

Chaining filters is becoming an increasingly popular primitive to exploit PHP
applications. Limiting the usage of only a few of them at the time should,
if not close entirely, make it significantly less attractive.

This should close php#10453

Author: jvoisin

ext/standard/php_fopen_wrapper.c

@@ -26,6 +26,7 @@
 #include "php_memory_streams.h"
 #include "php_fopen_wrappers.h"
 #include "SAPI.h"
+#include "zend_exceptions.h"
 
 static ssize_t php_stream_output_write(php_stream *stream, const char *buf, size_t count) /* {{{ */
 {
@@ -144,13 +145,22 @@ static const php_stream_ops php_stream_input_ops = {
 	NULL  /* set_option */
 };
 
+static const char max_stream_filters = 5;
+
 static void php_stream_apply_filter_list(php_stream *stream, char *filterlist, int read_chain, int write_chain) /* {{{ */
 {
 	char *p, *token = NULL;
 	php_stream_filter *temp_filter;
+	char nb_filters = 0;
 
 	p = php_strtok_r(filterlist, "|", &token);
 	while (p) {
+		if (nb_filters >= max_stream_filters) {
+			zend_throw_exception_ex(NULL, 0, "Unable to apply filter, maximum number (%d) reached", max_stream_filters);
+			return;
+		}
+		nb_filters++;
+
 		php_url_decode(p, strlen(p));
 		if (read_chain) {
 			if ((temp_filter = php_stream_filter_create(p, NULL, php_stream_is_persistent(stream)))) {

tests/security/bug10453.phpt

@@ -0,0 +1,20 @@
+--TEST--
+Bug #10453 (using a high amount of filters for nefarious purposes)
+--FILE--
+<?php
+$fp = fopen('php://output', 'w');
+for($i=0; $i<10; $i++)
+    stream_filter_append($fp, 'string.rot13');
+fwrite($fp, "This is a test.\n");
+
+$fp = fopen('php://filter/write=string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13|string.rot13/resource=php://output', 'w');
+fwrite($fp, "This is a test.\n");
+?>
+--EXPECTF--
+This is a test.
+
+Fatal error: Uncaught Exception: Unable to apply filter, maximum number (5) reached in %s
+Stack trace:
+#0 %s: fopen('php://filter/wr...', 'w')
+#1 {main}
+  thrown in %s