diff --git a/moli-protocol-server/src/protocol_server/tests/cdp_dynamic_page/auxiliary_page_identity.rs b/moli-protocol-server/src/protocol_server/tests/cdp_dynamic_page/auxiliary_page_identity.rs
index 1a80a468f5..41c358225f 100644
--- a/moli-protocol-server/src/protocol_server/tests/cdp_dynamic_page/auxiliary_page_identity.rs
+++ b/moli-protocol-server/src/protocol_server/tests/cdp_dynamic_page/auxiliary_page_identity.rs
@@ -844,6 +844,34 @@ async fn explicit_browser_and_noopener_blank_documents_keep_their_committed_hist
     abort_test_cdp_server(server).await;
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn auxiliary_initial_domain_updates_follow_the_creator() {
+    let (fixture_addr, _fixture) = spawn_dedicated_fixture_server(
+        Router::new().fallback(get(|| async {
+            axum::response::Html("<!doctype html><p>domain</p>")
+        })),
+        "auxiliary-domain",
+    );
+    let (addr, server) = spawn_test_protocol_server().await;
+    let (mut browser, _) =
+        connect_async(format!("ws://{addr}/devtools/browser/{DEFAULT_BROWSER_ID}"))
+            .await
+            .unwrap();
+    let opener_id = create_dynamic_target(&mut browser, 1).await;
+    let mut opener = connect_dynamic_page(addr, &opener_id).await;
+    send_cdp_command(&mut opener, 1, "Page.enable", None, json!({})).await;
+    navigate_dynamic_page_and_wait_for_load(
+        &mut opener,
+        2,
+        &format!("http://{fixture_addr}/parent"),
+    )
+    .await;
+    let (_, _child) = open_auxiliary(addr, &mut opener, "").await;
+    assert_eq!(evaluate_window_name_probe(&mut opener,3,
+        "document.domain='127.0.0.1';[p.document.domain,p.document.body.ownerDocument===p.document]").await,json!(["127.0.0.1",true]));
+    abort_test_cdp_server(server).await;
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn auxiliary_blank_reload_preserves_origin_storage_and_fallback_base() {
     let (fixture_addr, _fixture) = spawn_dedicated_fixture_server(
diff --git a/moli-renderer-v8/src/native_bridge/context_host/child_frame_runtime/window.rs b/moli-renderer-v8/src/native_bridge/context_host/child_frame_runtime/window.rs
index fc6c99f6e3..79f966e160 100644
--- a/moli-renderer-v8/src/native_bridge/context_host/child_frame_runtime/window.rs
+++ b/moli-renderer-v8/src/native_bridge/context_host/child_frame_runtime/window.rs
@@ -660,7 +660,11 @@ fn contexts_can_script_access<'s>(
             });
     };
     if accessing_host_ptr != accessed_host_ptr {
-        return false;
+        return host.window_execution_context_can_access_other_host(
+            accessing_identity,
+            unsafe { &*accessed_host_ptr },
+            accessed_identity,
+        );
     }
 
     host.window_execution_context_can_access(accessing_identity, accessed_identity)
diff --git a/moli-renderer-v8/src/native_bridge/context_host/child_frames.rs b/moli-renderer-v8/src/native_bridge/context_host/child_frames.rs
index 0a7fa822ea..102ac09f7f 100644
--- a/moli-renderer-v8/src/native_bridge/context_host/child_frames.rs
+++ b/moli-renderer-v8/src/native_bridge/context_host/child_frames.rs
@@ -19,7 +19,7 @@ use moli_page_types::{
     apply_child_browsing_context_navigation_to_entry_seed as apply_child_navigation_to_seed,
     replace_child_browsing_context_navigation_in_entry_seed as replace_child_navigation_in_seed,
 };
-use std::collections::HashSet;
+use std::{cell::RefCell, collections::HashSet, rc::Rc};
 use url::Url;
 
 mod classic_scripts;
@@ -58,7 +58,7 @@ pub(super) struct ChildBrowsingContextEntry {
     performance_time_origin: ChildPerformanceTimeOrigin,
     pending_document_load_id: Option<u64>,
     classic_script_document_state: classic_scripts::ChildClassicScriptDocumentState,
-    document_domain_override: Option<String>,
+    document_domain_override: Rc<RefCell<Option<String>>>,
     credentialless: bool,
     service_worker_client_id: Option<ServiceWorkerClientId>,
     pending_service_worker_client_id: Option<ServiceWorkerClientId>,
@@ -117,15 +117,19 @@ impl ChildBrowsingContextEntry {
     }
 
     pub(super) fn document_domain_override(&self) -> Option<String> {
+        self.document_domain_override.borrow().clone()
+    }
+
+    pub(super) fn shared_document_domain_override(&self) -> Rc<RefCell<Option<String>>> {
         self.document_domain_override.clone()
     }
 
     pub(super) fn set_document_domain_override(&mut self, domain: String) {
-        self.document_domain_override = Some(domain);
+        *self.document_domain_override.borrow_mut() = Some(domain);
     }
 
     pub(super) fn clear_document_domain_override(&mut self) {
-        self.document_domain_override = None;
+        self.document_domain_override = Rc::new(RefCell::new(None));
     }
 
     pub(super) fn window_name(&self) -> &str {
diff --git a/moli-renderer-v8/src/native_bridge/context_host/child_frames/registry.rs b/moli-renderer-v8/src/native_bridge/context_host/child_frames/registry.rs
index 328fb3fc5e..23b1bc606d 100644
--- a/moli-renderer-v8/src/native_bridge/context_host/child_frames/registry.rs
+++ b/moli-renderer-v8/src/native_bridge/context_host/child_frames/registry.rs
@@ -502,7 +502,8 @@ impl JsContextHost {
                         // Document keeps its mutable origin until replacement commits.
                         document_domain_override: existing
                             .as_ref()
-                            .and_then(|entry| entry.document_domain_override()),
+                            .map(|entry| entry.document_domain_override.clone())
+                            .unwrap_or_default(),
                         credentialless,
                         service_worker_client_id: existing
                             .as_ref()
diff --git a/moli-renderer-v8/src/native_bridge/context_host/core.rs b/moli-renderer-v8/src/native_bridge/context_host/core.rs
index 751b2d3912..bdfba522a7 100644
--- a/moli-renderer-v8/src/native_bridge/context_host/core.rs
+++ b/moli-renderer-v8/src/native_bridge/context_host/core.rs
@@ -47,9 +47,11 @@ impl JsContextHost {
         &self,
         origin: &str,
         inherited_opaque_origin_matches: bool,
+        inherited_document_domain: Option<&str>,
         policy: &crate::document_runtime::DocumentPolicyContainer,
     ) -> crate::frame_owner_model::FrameDocumentLocalWindowTransition {
-        let same_origin = self.document_domain_override.is_none()
+        let same_origin = self.document_domain_override.borrow().as_deref()
+            == inherited_document_domain
             && (inherited_opaque_origin_matches
                 || moli_url::WebOrigin::from_serialized(&self.main_document_security_origin())
                     .same_origin(&moli_url::WebOrigin::from_serialized(origin)));
@@ -389,7 +391,7 @@ impl JsContextHost {
             #[cfg(test)]
             force_child_default_context_preflight_failure: false,
             child_browsing_context_document_handles: HashMap::new(),
-            document_domain_override: None,
+            document_domain_override: Rc::new(RefCell::new(None)),
             next_child_browsing_context_id: 1,
             next_child_document_load_id: 0,
             next_child_classic_script_load_id: 0,
diff --git a/moli-renderer-v8/src/native_bridge/context_host/document_domain.rs b/moli-renderer-v8/src/native_bridge/context_host/document_domain.rs
index f0db959a29..197db536b2 100644
--- a/moli-renderer-v8/src/native_bridge/context_host/document_domain.rs
+++ b/moli-renderer-v8/src/native_bridge/context_host/document_domain.rs
@@ -4,6 +4,22 @@ use std::net::IpAddr;
 use url::{Host, Url};
 
 impl JsContextHost {
+    pub(crate) fn inherit_document_domain_override(&mut self, domain: Rc<RefCell<Option<String>>>) {
+        self.document_domain_override = domain;
+    }
+
+    pub(crate) fn inherited_document_domain(
+        &self,
+        child: Option<DomHandle>,
+    ) -> Rc<RefCell<Option<String>>> {
+        match child.and_then(|handle| self.child_browsing_context_security_origin_owner(handle)) {
+            Some(ChildSecurityOriginOwner::Child(owner)) => {
+                self.child_browsing_contexts[&owner].shared_document_domain_override()
+            }
+            Some(ChildSecurityOriginOwner::Main) | None => self.document_domain_override.clone(),
+        }
+    }
+
     pub(crate) fn document_domain_value_for_document_handle(
         &self,
         document_handle: DomHandle,
@@ -11,6 +27,7 @@ impl JsContextHost {
         if document_handle == self.document_handle() {
             return self
                 .document_domain_override
+                .borrow()
                 .clone()
                 .unwrap_or_else(|| self.main_document_domain_host().unwrap_or_default());
         }
@@ -48,7 +65,7 @@ impl JsContextHost {
             if !document_domain_is_allowed_for_host(&current_host, &domain) {
                 return false;
             }
-            self.document_domain_override = Some(domain);
+            *self.document_domain_override.borrow_mut() = Some(domain);
             return true;
         }
         let Some(child_handle) =
@@ -68,7 +85,7 @@ impl JsContextHost {
         handle: DomHandle,
     ) -> Option<String> {
         match self.child_browsing_context_security_origin_owner(handle)? {
-            ChildSecurityOriginOwner::Main => self.document_domain_override.clone(),
+            ChildSecurityOriginOwner::Main => self.document_domain_override.borrow().clone(),
             ChildSecurityOriginOwner::Child(owner) => self
                 .child_browsing_contexts
                 .get(&owner)
@@ -115,7 +132,9 @@ impl JsContextHost {
             return false;
         };
         match origin_owner {
-            ChildSecurityOriginOwner::Main => self.document_domain_override = Some(domain),
+            ChildSecurityOriginOwner::Main => {
+                *self.document_domain_override.borrow_mut() = Some(domain);
+            }
             ChildSecurityOriginOwner::Child(owner) => {
                 let Some(entry) = self.child_browsing_contexts.get_mut(&owner) else {
                     return false;
diff --git a/moli-renderer-v8/src/native_bridge/context_host/mod.rs b/moli-renderer-v8/src/native_bridge/context_host/mod.rs
index 732c2f80de..0e81db50d0 100644
--- a/moli-renderer-v8/src/native_bridge/context_host/mod.rs
+++ b/moli-renderer-v8/src/native_bridge/context_host/mod.rs
@@ -867,7 +867,7 @@ pub(crate) struct JsContextHost {
     #[cfg(test)]
     force_child_default_context_preflight_failure: bool,
     child_browsing_context_document_handles: HashMap<DomHandle, DomHandle>,
-    document_domain_override: Option<String>,
+    document_domain_override: Rc<RefCell<Option<String>>>,
     next_child_browsing_context_id: u64,
     next_child_document_load_id: u64,
     next_child_classic_script_load_id: u64,
diff --git a/moli-renderer-v8/src/native_bridge/context_host/popups/renderer_owned.rs b/moli-renderer-v8/src/native_bridge/context_host/popups/renderer_owned.rs
index 5bdc403798..38c0128aee 100644
--- a/moli-renderer-v8/src/native_bridge/context_host/popups/renderer_owned.rs
+++ b/moli-renderer-v8/src/native_bridge/context_host/popups/renderer_owned.rs
@@ -271,6 +271,12 @@ impl JsContextHost {
         };
         let (pending, window_proxy) =
             allocator.stage_in_scope(scope, &environment, &self.bridge.bindings, init)?;
+        if !policy.sandbox.forces_opaque_origin {
+            let target = crate::util::context_host_ptr_from_window_object(scope, window_proxy)
+                .expect("a related Window retains its native host");
+            unsafe { &mut *target }.document_domain_override =
+                self.inherited_document_domain(creator_child_handle);
+        }
         Ok(OpenedRendererWindow {
             window_proxy,
             window: Some(window),
diff --git a/moli-renderer-v8/src/native_bridge/context_host/window_execution_context/settings.rs b/moli-renderer-v8/src/native_bridge/context_host/window_execution_context/settings.rs
index ba2280913f..2f641b81d8 100644
--- a/moli-renderer-v8/src/native_bridge/context_host/window_execution_context/settings.rs
+++ b/moli-renderer-v8/src/native_bridge/context_host/window_execution_context/settings.rs
@@ -105,7 +105,7 @@ impl WindowEnvironmentSettings {
         } = &mut origin
         {
             if self.domain_owner_document == host.document_handle() {
-                *document_domain = host.document_domain_override.clone();
+                *document_domain = host.document_domain_override.borrow().clone();
             } else if let Some(handle) =
                 host.child_browsing_context_handle_for_stored_document(self.domain_owner_document)
             {
diff --git a/moli-renderer-v8/src/native_bridge/context_host/window_security_tokens.rs b/moli-renderer-v8/src/native_bridge/context_host/window_security_tokens.rs
index ed88d682c3..f57634414f 100644
--- a/moli-renderer-v8/src/native_bridge/context_host/window_security_tokens.rs
+++ b/moli-renderer-v8/src/native_bridge/context_host/window_security_tokens.rs
@@ -18,7 +18,7 @@ impl JsContextHost {
             return None;
         }
         let origin = self.main_document_security_origin();
-        if self.document_domain_override.is_some() {
+        if self.document_domain_override.borrow().is_some() {
             return None;
         }
         window_security_token_key(origin)
@@ -47,7 +47,7 @@ impl JsContextHost {
         }
         window_isolated_world_security_token_key(
             self.main_document_security_origin(),
-            self.document_domain_override.is_some(),
+            self.document_domain_override.borrow().is_some(),
         )
     }
 
@@ -107,6 +107,39 @@ impl JsContextHost {
             }
         }
         self.refresh_child_window_access_surfaces_after_origin_mutation(scope);
+        if let Some(environment) = self.page_script_environment() {
+            for related in environment.related_document_hosts() {
+                let related_ptr = related.as_ptr();
+                if std::ptr::eq(related_ptr, self) {
+                    continue;
+                }
+                let related_host = unsafe { &*related_ptr };
+                if !std::rc::Rc::ptr_eq(
+                    &self.document_domain_override,
+                    &related_host.document_domain_override,
+                ) {
+                    continue;
+                }
+                let dispatch_scope = OwnerDispatchScope::Top;
+                let Some(owner) =
+                    related_host.current_window_execution_context_owner(dispatch_scope)
+                else {
+                    continue;
+                };
+                if let Some((_, context)) =
+                    related_host.window_execution_context(scope, owner, dispatch_scope)
+                    && set_window_security_token(
+                        scope,
+                        context,
+                        related_host
+                            .main_default_world_security_token_key()
+                            .as_deref(),
+                    )
+                {
+                    updated += 1;
+                }
+            }
+        }
         updated
     }
 
@@ -179,6 +212,39 @@ impl JsContextHost {
         )
     }
 
+    pub(crate) fn window_execution_context_can_access_other_host(
+        &self,
+        accessing: WindowExecutionContextIdentity,
+        target_host: &JsContextHost,
+        accessed: WindowExecutionContextIdentity,
+    ) -> bool {
+        if !self.window_execution_context_identity_is_current(accessing)
+            || !target_host.window_execution_context_identity_is_current(accessed)
+        {
+            return false;
+        }
+        if accessing.grants_universal_access() {
+            return true;
+        }
+        let Some(origin) = self.window_access_origin(accessing) else {
+            return false;
+        };
+        let Some(target) =
+            target_host.window_access_origin_for_dispatch_scope(accessed.dispatch_scope())
+        else {
+            return false;
+        };
+        // Opaque owner identifiers are local to each host. Shared opaque
+        // origins are authorized by their V8 security token before this path.
+        matches!(
+            (&origin, &target),
+            (
+                WindowAccessOrigin::Tuple { .. },
+                WindowAccessOrigin::Tuple { .. }
+            )
+        ) && origin.can_access(&target)
+    }
+
     /// Checks Window access before the target realm is entered or materialized.
     ///
     /// WebIDL operations such as a borrowed `fetch()` must authorize the
@@ -262,7 +328,7 @@ impl JsContextHost {
         }
         WindowAccessOrigin::from_serialized_origin(
             serialized_origin,
-            self.document_domain_override.clone(),
+            self.document_domain_override.borrow().clone(),
         )
     }
 
diff --git a/moli-renderer-v8/src/script_vm.rs b/moli-renderer-v8/src/script_vm.rs
index 129f5796a4..0ef7a5b5a9 100644
--- a/moli-renderer-v8/src/script_vm.rs
+++ b/moli-renderer-v8/src/script_vm.rs
@@ -1953,6 +1953,7 @@ impl ScriptVm {
 
 pub(crate) struct ScriptVmInitialDocumentEnvironment {
     security_token: Option<v8::Global<v8::Value>>,
+    document_domain_override: Rc<RefCell<Option<String>>>,
     origin: String,
     pub(crate) policy_container: crate::document_runtime::DocumentPolicyContainer,
     fallback_base_url: Option<url::Url>,
@@ -2052,6 +2053,9 @@ impl ScriptVmPageRealmBootstrap {
         let (page_context_cancel_tx, page_context_cancel_rx) =
             renderer_page_context_cancel_channel();
 
+        let inherited_document_domain = initial_environment
+            .as_ref()
+            .map(|environment| environment.document_domain_override.clone());
         let inherited_security_token =
             initial_environment.and_then(|environment| environment.security_token);
         let RendererDocumentIsolateBootstrap {
@@ -2106,6 +2110,12 @@ impl ScriptVmPageRealmBootstrap {
             top_level_storage_key,
             reserved_service_worker_client_id,
         )));
+        if let Some(domain) = inherited_document_domain {
+            context_host
+                .borrow_mut()
+                .inherit_document_domain_override(domain);
+        }
+
         context_host
             .borrow_mut()
             .bind_deferred_context_host_release_queue(
diff --git a/moli-renderer-v8/src/script_vm/document_environment.rs b/moli-renderer-v8/src/script_vm/document_environment.rs
index 3dabdf28ee..9b82d25189 100644
--- a/moli-renderer-v8/src/script_vm/document_environment.rs
+++ b/moli-renderer-v8/src/script_vm/document_environment.rs
@@ -71,8 +71,23 @@ impl ScriptVmInitialDocumentEnvironment {
             .ok_or_else(|| anyhow!("navigation initiator has no live creation context"))?;
         let security_token = (!policy_container.sandbox.forces_opaque_origin)
             .then(|| v8::Global::new(scope, context.get_security_token(scope)));
+        let document_domain_override = (!policy_container.sandbox.forces_opaque_origin)
+            .then(|| crate::util::context_host_ptr_from_context_slot(context))
+            .flatten()
+            .map(|host_ptr| {
+                let host = unsafe { &*host_ptr };
+                let child = host
+                    .window_execution_context_identity_for_access_check(context)
+                    .and_then(|identity| match identity.dispatch_scope() {
+                        crate::native_bridge::OwnerDispatchScope::Child(handle) => Some(handle),
+                        _ => None,
+                    });
+                host.inherited_document_domain(child)
+            })
+            .unwrap_or_default();
         Ok(Self {
             security_token,
+            document_domain_override,
             origin,
             policy_container,
             fallback_base_url: Some(base_url),
@@ -435,6 +450,7 @@ impl ScriptVm {
             let token = scope.get_current_context().get_security_token(scope);
             Ok(ScriptVmInitialDocumentEnvironment {
                 security_token: Some(v8::Global::new(scope, token)),
+                document_domain_override: host.inherited_document_domain(None),
                 origin,
                 policy_container,
                 fallback_base_url,
diff --git a/moli-renderer-v8/src/script_vm/document_isolate.rs b/moli-renderer-v8/src/script_vm/document_isolate.rs
index 27e360b77e..060879e278 100644
--- a/moli-renderer-v8/src/script_vm/document_isolate.rs
+++ b/moli-renderer-v8/src/script_vm/document_isolate.rs
@@ -495,6 +495,17 @@ impl RendererPageScriptEnvironment {
             .find_map(|inner| matches(&inner).map(|()| Self { inner }))
     }
 
+    pub(crate) fn related_document_hosts(&self) -> Vec<Rc<RefCell<JsContextHost>>> {
+        self.inner
+            .renderer_document_isolate
+            .related_pages
+            .borrow()
+            .values()
+            .filter_map(Weak::upgrade)
+            .filter_map(|page| page.document_host.borrow().upgrade())
+            .collect()
+    }
+
     pub(crate) fn window_identity(
         &self,
     ) -> Option<(
diff --git a/moli-renderer-v8/src/script_vm/initial_window_document.rs b/moli-renderer-v8/src/script_vm/initial_window_document.rs
index 48a587456b..bc7fdcc2f3 100644
--- a/moli-renderer-v8/src/script_vm/initial_window_document.rs
+++ b/moli-renderer-v8/src/script_vm/initial_window_document.rs
@@ -25,12 +25,17 @@ impl ScriptVm {
         } else {
             false
         };
+        let inherited_domain =
+            inherited.map(|environment| environment.document_domain_override.borrow());
         Ok(self
             ._context_host
             .borrow()
             .main_document_local_window_transition_for_commit(
                 origin,
                 inherited_opaque_origin_matches,
+                inherited_domain
+                    .as_ref()
+                    .and_then(|domain| domain.as_deref()),
                 policy,
             ))
     }
@@ -111,6 +116,11 @@ impl ScriptVm {
                 vm.page_runtime_wake_tx = page_task_tx.page_runtime_wake_sender();
                 {
                     let mut host = vm._context_host.borrow_mut();
+                    if let Some(environment) = inherited.as_ref() {
+                        host.inherit_document_domain_override(
+                            environment.document_domain_override.clone(),
+                        );
+                    }
                     host.rebind_initial_main_document_senders(
                         resource_completion,
                         page_task_tx.top_level_navigation_handoff_sender(),
diff --git a/moli-renderer-v8/src/script_vm/related_page.rs b/moli-renderer-v8/src/script_vm/related_page.rs
index 6321e33719..ddae6de294 100644
--- a/moli-renderer-v8/src/script_vm/related_page.rs
+++ b/moli-renderer-v8/src/script_vm/related_page.rs
@@ -38,6 +38,7 @@ impl ScriptVmDefaultWorldBootstrap {
             None,
             Some(ScriptVmInitialDocumentEnvironment {
                 security_token: None,
+                document_domain_override: Rc::default(),
                 fallback_base_url: None,
                 storage_key: None,
                 origin: inherited_origin.clone(),