diff --git a/.changeset/tame-papayas-fetch.md b/.changeset/tame-papayas-fetch.md
new file mode 100644
index 00000000000..25fc987497d
--- /dev/null
+++ b/.changeset/tame-papayas-fetch.md
@@ -0,0 +1,5 @@
+---
+'@graphql-tools/mock': patch
+---
+
+Prevent prototype polluting assignment
diff --git a/packages/mock/src/MockStore.ts b/packages/mock/src/MockStore.ts
index 8a304826bc6..828cb082807 100644
--- a/packages/mock/src/MockStore.ts
+++ b/packages/mock/src/MockStore.ts
@@ -263,6 +263,9 @@ export class MockStore implements IMockStore {
       value = deepResolveMockList(value);
     }
 
+    if (typeName === '__proto__' || typeName === 'constructor' || typeName === 'prototype') {
+      throw new Error(`Invalid typeName: ${typeName}`);
+    }
     if (this.store[typeName] === undefined) {
       this.store[typeName] = {};
     }