From b6874e30aa3c833c061207f475ec933564d9b020 Mon Sep 17 00:00:00 2001
From: Nic Polumeyv <162764842+Nic-Polumeyv@users.noreply.github.com>
Date: Sat, 1 Aug 2026 16:19:31 -0400
Subject: [PATCH] validate matchers against own properties only

---
 .changeset/eleven-cases-guard.md      | 5 +++++
 packages/kit/src/utils/params.js      | 2 +-
 packages/kit/src/utils/params.spec.js | 7 +++++++
 3 files changed, 13 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/eleven-cases-guard.md

diff --git a/.changeset/eleven-cases-guard.md b/.changeset/eleven-cases-guard.md
new file mode 100644
index 000000000000..0b070264ee9a
--- /dev/null
+++ b/.changeset/eleven-cases-guard.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: don't treat `Object.prototype` members as param matchers during validation
diff --git a/packages/kit/src/utils/params.js b/packages/kit/src/utils/params.js
index 70832b288c83..c7a6d740f8fd 100644
--- a/packages/kit/src/utils/params.js
+++ b/packages/kit/src/utils/params.js
@@ -25,7 +25,7 @@ export function collect_matcher_names(routes) {
  */
 export function validate_param_matchers(params, names, file) {
 	for (const name of names) {
-		if (!(name in params)) {
+		if (!Object.hasOwn(params, name)) {
 			throw new Error(`No matcher found for parameter '${name}'${file ? ` in ${file}` : ''}`);
 		}
 	}
diff --git a/packages/kit/src/utils/params.spec.js b/packages/kit/src/utils/params.spec.js
index 5a6120b3f073..6a7c9fdf6a55 100644
--- a/packages/kit/src/utils/params.spec.js
+++ b/packages/kit/src/utils/params.spec.js
@@ -23,6 +23,13 @@ test('validate_param_matchers throws for unknown matchers', () => {
 	);
 });
 
+test('validate_param_matchers ignores inherited properties', () => {
+	assert.throws(
+		() => validate_param_matchers({}, new Set(['toString']), 'params.js'),
+		/No matcher found for parameter 'toString'/
+	);
+});
+
 test('load_and_validate_params loads and validates params', async () => {
 	const params = await load_and_validate_params({
 		routes: [