diff --git a/NEWS b/NEWS
index df90db4bfd0..8c62fb8ebf4 100644
--- a/NEWS
+++ b/NEWS
@@ -8,6 +8,8 @@ PHP                                                                        NEWS
   . Fixed bug GH-20695 (Assertion failure in normalize_value() when parsing
     malformed INI input via parse_ini_string()). (ndossche)
   . Fixed bug GH-20714 (Uncatchable exception thrown in generator). (ilutov)
+  . Fixed bug GH-20352 (UAF in php_output_handler_free via re-entrant
+    ob_start() during error deactivation). (ndossche)
 
 - Bz2:
   . Fixed bug GH-20620 (bzcompress overflow on large source size).
diff --git a/main/output.c b/main/output.c
index ef6be672d1c..4e542318139 100644
--- a/main/output.c
+++ b/main/output.c
@@ -187,8 +187,12 @@ PHPAPI void php_output_deactivate(void)
 		/* release all output handlers */
 		if (OG(handlers).elements) {
 			while ((handler = zend_stack_top(&OG(handlers)))) {
-				php_output_handler_free(handler);
 				zend_stack_del_top(&OG(handlers));
+				/* It's possible to start a new output handler and mark it as active,
+				 * however this loop will destroy all active handlers. */
+				OG(active) = NULL;
+				ZEND_ASSERT(OG(running) == NULL && "output is deactivated therefore running should stay NULL");
+				php_output_handler_free(handler);
 			}
 		}
 		zend_stack_destroy(&OG(handlers));
@@ -718,10 +722,11 @@ PHPAPI void php_output_handler_dtor(php_output_handler *handler)
  * Destroy and free an output handler */
 PHPAPI void php_output_handler_free(php_output_handler **h)
 {
-	if (*h) {
-		php_output_handler_dtor(*h);
-		efree(*h);
+	php_output_handler *handler = *h;
+	if (handler) {
 		*h = NULL;
+		php_output_handler_dtor(handler);
+		efree(handler);
 	}
 }
 /* }}} */
diff --git a/tests/output/gh20352.phpt b/tests/output/gh20352.phpt
new file mode 100644
index 00000000000..16be0b920e8
--- /dev/null
+++ b/tests/output/gh20352.phpt
@@ -0,0 +1,24 @@
+--TEST--
+GH-20352 (UAF in php_output_handler_free via re-entrant ob_start() during error deactivation)
+--FILE--
+<?php
+class Test {
+    public function __destruct() {
+        // Spray output stack
+        for ($i = 0; $i < 1000; $i++)
+            ob_start(static function() {});
+    }
+
+    public function __invoke($x) {
+        // Trigger php_output_deactivate() through forbidden operation
+        ob_start('foo');
+        return $x;
+    }
+}
+
+ob_start(new Test, 1);
+
+echo "trigger bug";
+?>
+--EXPECTF--
+Fatal error: ob_start(): Cannot use output buffering in output buffering display handlers in %s on line %d