diff --git a/NEWS b/NEWS
index fe3239269e787..da14c247ee0ec 100644
--- a/NEWS
+++ b/NEWS
@@ -49,6 +49,9 @@ PHP                                                                        NEWS
 - Reflection:
   . Fixed bug GH-16601 (Memory leak in Reflection constructors). (nielsdos)
 
+- Session:
+  . Fixed bug GH-16590 (UAF in session_encode()). (nielsdos)
+
 - SPL:
   . Fixed bug GH-16588 (UAF in Observer->serialize). (nielsdos)
   . Fix GH-16477 (Segmentation fault when calling __debugInfo() after failed
diff --git a/UPGRADING.INTERNALS b/UPGRADING.INTERNALS
index 36096ebdd415e..a186dbf77febc 100644
--- a/UPGRADING.INTERNALS
+++ b/UPGRADING.INTERNALS
@@ -391,6 +391,8 @@ PHP 8.4 INTERNALS UPGRADE NOTES
      needing to create a zend_string.
    - The ext/session/php_session.h doesn't transitively include the
      ext/hash/php_hash.h header anymore.
+   - It is no longer allowed to return out of the PS_ENCODE_LOOP macro.
+     Instead, you should break out of the loop instead.
 
  i. ext/xml
    - Made the expat compatibility wrapper XML_GetCurrentByteIndex return a long
diff --git a/ext/session/php_session.h b/ext/session/php_session.h
index 65bf7de204491..dfa7632e5a447 100644
--- a/ext/session/php_session.h
+++ b/ext/session/php_session.h
@@ -291,8 +291,13 @@ PHPAPI zend_result php_session_reset_id(void);
 	zend_ulong num_key;													\
 	zval *struc;
 
+/* Do not use a return statement in `code` because that may leak memory.
+ * Break out of the loop instead. */
 #define PS_ENCODE_LOOP(code) do {									\
-	HashTable *_ht = Z_ARRVAL_P(Z_REFVAL(PS(http_session_vars)));	\
+	zval _zv;														\
+	/* protect against user interference */							\
+	ZVAL_COPY(&_zv, Z_REFVAL(PS(http_session_vars)));				\
+	HashTable *_ht = Z_ARRVAL(_zv);									\
 	ZEND_HASH_FOREACH_KEY(_ht, num_key, key) {						\
 		if (key == NULL) {											\
 			php_error_docref(NULL, E_WARNING,						\
@@ -303,6 +308,7 @@ PHPAPI zend_result php_session_reset_id(void);
 			code;		 											\
 		} 															\
 	} ZEND_HASH_FOREACH_END();										\
+	zval_ptr_dtor(&_zv);											\
 } while(0)
 
 PHPAPI ZEND_EXTERN_MODULE_GLOBALS(ps)
diff --git a/ext/session/session.c b/ext/session/session.c
index b23f1fce3584b..86e1cfa7c43d8 100644
--- a/ext/session/session.c
+++ b/ext/session/session.c
@@ -1056,6 +1056,7 @@ PS_SERIALIZER_ENCODE_FUNC(php) /* {{{ */
 {
 	smart_str buf = {0};
 	php_serialize_data_t var_hash;
+	bool fail = false;
 	PS_ENCODE_VARS;
 
 	PHP_VAR_SERIALIZE_INIT(var_hash);
@@ -1065,12 +1066,17 @@ PS_SERIALIZER_ENCODE_FUNC(php) /* {{{ */
 		if (memchr(ZSTR_VAL(key), PS_DELIMITER, ZSTR_LEN(key))) {
 			PHP_VAR_SERIALIZE_DESTROY(var_hash);
 			smart_str_free(&buf);
-			return NULL;
+			fail = true;
+			break;
 		}
 		smart_str_appendc(&buf, PS_DELIMITER);
 		php_var_serialize(&buf, struc, &var_hash);
 	);
 
+	if (fail) {
+		return NULL;
+	}
+
 	smart_str_0(&buf);
 
 	PHP_VAR_SERIALIZE_DESTROY(var_hash);
diff --git a/ext/session/tests/gh16590.phpt b/ext/session/tests/gh16590.phpt
new file mode 100644
index 0000000000000..781f6a78e093e
--- /dev/null
+++ b/ext/session/tests/gh16590.phpt
@@ -0,0 +1,36 @@
+--TEST--
+GH-16590 (UAF in session_encode())
+--EXTENSIONS--
+session
+--SKIPIF--
+<?php include('skipif.inc'); ?>
+--INI--
+session.use_cookies=0
+session.cache_limiter=
+session.serialize_handler=php
+session.save_handler=files
+--FILE--
+<?php
+
+class C {
+    function __serialize() {
+        $_SESSION = [];
+        return [];
+    }
+}
+
+session_start();
+
+$_SESSION['Lz'] = new C;
+for ($i = 0; $i < 2; $i++) {
+    $_SESSION[$i] = $i;
+}
+
+var_dump(session_encode());
+
+?>
+--EXPECTF--
+Warning: session_encode(): Skipping numeric key 0 in %s on line %d
+
+Warning: session_encode(): Skipping numeric key 1 in %s on line %d
+string(15) "Lz|O:1:"C":0:{}"