diff --git a/NEWS b/NEWS
index 938aa1f72edb6..7124d1bc278ae 100644
--- a/NEWS
+++ b/NEWS
@@ -16,6 +16,10 @@ PHP                                                                        NEWS
     with a given skeleton, locale, collapse type and identity fallback.
     (BogdanUngureanu)
 
+- Fibers:
+  . Fixed bug GH-20483 (ASAN stack overflow with fiber.stack_size INI
+    small value). (David Carlier)
+
 - Opcache:
   . Fixed bug GH-20051 (apache2 shutdowns when restart is requested during
     preloading). (Arnaud, welcomycozyhom)
diff --git a/Zend/tests/fibers/gh20483.phpt b/Zend/tests/fibers/gh20483.phpt
new file mode 100644
index 0000000000000..e06cf87258ea1
--- /dev/null
+++ b/Zend/tests/fibers/gh20483.phpt
@@ -0,0 +1,16 @@
+--TEST--
+GH-20483 (ASAN stack overflow with small fiber.stack_size INI value)
+--INI--
+fiber.stack_size=1024
+--FILE--
+<?php
+$callback = function () {};
+$fiber = new Fiber($callback);
+try {
+    $fiber->start();
+} catch (Exception $e) {
+    echo $e->getMessage() . "\n";
+}
+?>
+--EXPECTF--
+Fiber stack size is too small, it needs to be at least %d bytes
diff --git a/Zend/zend_fibers.c b/Zend/zend_fibers.c
index 97b7cdcc911b7..d571a622e476b 100644
--- a/Zend/zend_fibers.c
+++ b/Zend/zend_fibers.c
@@ -207,7 +207,12 @@ static zend_fiber_stack *zend_fiber_stack_allocate(size_t size)
 {
 	void *pointer;
 	const size_t page_size = zend_fiber_get_page_size();
-	const size_t minimum_stack_size = page_size + ZEND_FIBER_GUARD_PAGES * page_size;
+	const size_t minimum_stack_size = page_size + ZEND_FIBER_GUARD_PAGES * page_size
+#ifdef __SANITIZE_ADDRESS__
+	// necessary correction due to ASAN redzones
+	* 6
+#endif
+	;
 
 	if (size < minimum_stack_size) {
 		zend_throw_exception_ex(NULL, 0, "Fiber stack size is too small, it needs to be at least %zu bytes", minimum_stack_size);
diff --git a/ext/mbstring/mbstring.c b/ext/mbstring/mbstring.c
index ceb182a0a258d..7fda240b7051a 100644
--- a/ext/mbstring/mbstring.c
+++ b/ext/mbstring/mbstring.c
@@ -5578,19 +5578,16 @@ static bool mb_check_str_encoding(zend_string *str, const mbfl_encoding *encodin
 
 static bool php_mb_check_encoding_recursive(HashTable *vars, const mbfl_encoding *encoding)
 {
-	zend_long idx;
 	zend_string *key;
 	zval *entry;
 	bool valid = true;
 
-	(void)(idx); /* Suppress spurious compiler warning that `idx` is not used */
-
 	if (GC_IS_RECURSIVE(vars)) {
 		php_error_docref(NULL, E_WARNING, "Cannot not handle circular references");
 		return false;
 	}
 	GC_TRY_PROTECT_RECURSION(vars);
-	ZEND_HASH_FOREACH_KEY_VAL(vars, idx, key, entry) {
+	ZEND_HASH_FOREACH_STR_KEY_VAL(vars, key, entry) {
 		ZVAL_DEREF(entry);
 		if (key) {
 			if (!mb_check_str_encoding(key, encoding)) {