From 7d86c93ddc22df2424b2a666f1bf2a78e8ca4dbd Mon Sep 17 00:00:00 2001
From: David Wendt <dcrkid@yahoo.com>
Date: Tue, 8 Jun 2021 17:22:11 -0400
Subject: [PATCH] Check to make sure that overrides only apply to
 already-installed properties.

---
 core/src/avm2/object.rs | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/core/src/avm2/object.rs b/core/src/avm2/object.rs
index c0ab82d8f..7d51f8039 100644
--- a/core/src/avm2/object.rs
+++ b/core/src/avm2/object.rs
@@ -460,6 +460,15 @@ pub trait TObject<'gc>: 'gc + Collect + Debug + Into<Object<'gc>> + Clone + Copy
         let receiver = (*self).into();
         let scope = self.get_scope();
         let trait_name = trait_entry.name().clone();
+
+        if trait_entry.is_override() && !self.has_own_property(&trait_name)? {
+            return Err(format!(
+                "Attempted to override property {:?}, which is not already defined",
+                trait_name
+            )
+            .into());
+        }
+
         avm_debug!(
             activation.avm2(),
             "Installing trait {:?} of kind {:?}",