diff --git a/moli-fetch/src/blocking/mod.rs b/moli-fetch/src/blocking/mod.rs
index a6af751283..3ce85f0bc4 100644
--- a/moli-fetch/src/blocking/mod.rs
+++ b/moli-fetch/src/blocking/mod.rs
@@ -1149,6 +1149,29 @@ mod tests {
         );
     }
 
+    #[test]
+    fn opaque_request_origin_is_cross_origin_without_hiding_referrer_url() {
+        let config = FetchConfig::default();
+        let request_url = url("https://app.test/data");
+        let request = Request::new("GET", request_url.as_str(), None, Vec::new())
+            .unwrap()
+            .with_initiator_url(&url("https://app.test/sandboxed-frame"))
+            .with_request_origin(moli_url::WebOrigin::Opaque)
+            .with_browser_request_metadata(BrowserRequestMetadata::Fetch);
+
+        let headers = outgoing_request_headers_for_url(&config, &request, &request_url, None);
+
+        assert_eq!(header_value(&headers, "origin").as_deref(), Some("null"));
+        assert_eq!(
+            header_value(&headers, "sec-fetch-site").as_deref(),
+            Some("cross-site")
+        );
+        assert_eq!(
+            header_value(&headers, "referer").as_deref(),
+            Some("https://app.test/sandboxed-frame")
+        );
+    }
+
     #[test]
     fn post_navigation_without_an_initiator_serializes_opaque_origin() {
         let config = FetchConfig::default();
diff --git a/moli-fetch/src/fetch_url_list.rs b/moli-fetch/src/fetch_url_list.rs
index f046dca01a..0bb674f64b 100644
--- a/moli-fetch/src/fetch_url_list.rs
+++ b/moli-fetch/src/fetch_url_list.rs
@@ -28,7 +28,8 @@ impl<'a> FetchUrlList<'a> {
     }
 
     /// Returning to the initiating origin cannot restore basic response tainting.
-    pub fn has_cross_origin_url(self, origin: &WebOrigin) -> bool {
+    pub fn has_cross_origin_url(self, origin: impl Into<WebOrigin>) -> bool {
+        let origin = origin.into();
         self.urls().any(|url| !origin.same_origin(&url.into()))
     }
 
@@ -52,13 +53,22 @@ impl<'a> FetchUrlList<'a> {
         Ok(())
     }
 
+    pub fn has_cross_origin_url_for_origin(self, origin: &WebOrigin) -> bool {
+        self.has_cross_origin_url(origin)
+    }
+
+    pub fn serialized_origin_for_origin(self, origin: &WebOrigin) -> String {
+        self.serialized_origin(origin)
+    }
+
     pub fn has_cross_site_url(self, origin: &Url) -> bool {
         self.urls().any(|url| !same_site_urls(origin, url, true))
     }
 
     /// A first hop out of the initiating origin retains that origin. Crossing
     /// origins from an already cross-origin URL serializes the origin as null.
-    pub fn serialized_origin(self, origin: &WebOrigin) -> String {
+    pub fn serialized_origin(self, origin: impl Into<WebOrigin>) -> String {
+        let origin = origin.into();
         if self.redirects.iter().any(|redirect| {
             !same_origin(&redirect.from_url, &redirect.to_url)
                 && !origin.same_origin(&(&redirect.from_url).into())
diff --git a/moli-fetch/src/tests/cookie_context.rs b/moli-fetch/src/tests/cookie_context.rs
index eebc897b4c..41ba1ab161 100644
--- a/moli-fetch/src/tests/cookie_context.rs
+++ b/moli-fetch/src/tests/cookie_context.rs
@@ -110,6 +110,19 @@ fn request_credentials_mode_controls_cross_origin_cookie_access() {
     assert!(!omit_request.allows_credentials_for_url(&same_origin_url));
 }
 
+#[test]
+fn opaque_request_origin_disallows_same_origin_credentials_for_same_url_origin() {
+    let document_url = Url::parse("https://example.com/app/page.html").unwrap();
+    let request_url = Url::parse("https://example.com/api/data").unwrap();
+    let request = Request::new("GET", request_url.as_str(), None, vec![])
+        .unwrap()
+        .with_initiator_url(&document_url)
+        .with_request_origin(moli_url::WebOrigin::Opaque)
+        .with_credentials_mode(RequestCredentialsMode::SameOrigin);
+
+    assert!(!request.allows_credentials_for_url(&request_url));
+}
+
 #[test]
 fn explicit_same_site_override_sets_both_site_context_tracks() {
     let context = NetworkCookieRequestContext::subresource("GET")
diff --git a/moli-renderer-v8/src/network_host/response/cors.rs b/moli-renderer-v8/src/network_host/response/cors.rs
index b5f940d79f..3121ac71fe 100644
--- a/moli-renderer-v8/src/network_host/response/cors.rs
+++ b/moli-renderer-v8/src/network_host/response/cors.rs
@@ -659,6 +659,34 @@ mod tests {
         );
     }
 
+    #[test]
+    fn opaque_request_origin_requires_null_cors_opt_in_for_same_url_origin() {
+        let response_url = url("https://example.test/data");
+        let response = header_response(response_url.clone(), Vec::new());
+
+        assert!(
+            validate_cors_response_chain(
+                &WebOrigin::Opaque,
+                &response,
+                RequestCredentialsMode::SameOrigin,
+            )
+            .is_err()
+        );
+
+        let allowed_response = header_response(
+            response_url,
+            vec![("Access-Control-Allow-Origin".to_owned(), "null".to_owned())],
+        );
+        assert!(
+            validate_cors_response_chain(
+                &WebOrigin::Opaque,
+                &allowed_response,
+                RequestCredentialsMode::SameOrigin,
+            )
+            .is_ok()
+        );
+    }
+
     #[test]
     fn validate_cors_preflight_response_checks_method_and_headers() {
         let request_headers = vec![
diff --git a/moli-renderer-v8/src/script_vm/subresource_fetch.rs b/moli-renderer-v8/src/script_vm/subresource_fetch.rs
index e0a4070f9e..5ca2fb9a1e 100644
--- a/moli-renderer-v8/src/script_vm/subresource_fetch.rs
+++ b/moli-renderer-v8/src/script_vm/subresource_fetch.rs
@@ -5857,6 +5857,7 @@ impl ScriptVm {
                             trace_fields,
                             record_started,
                         );
+                        let request_origin = streaming.pending.request_origin();
                         if let PendingSubresourceContinuation::Xhr(xhr) =
                             streaming.pending.continuation
                             && let Some(response_body) = xhr_delivery_body
@@ -5881,7 +5882,7 @@ impl ScriptVm {
                             {
                                 observable_head.headers =
                                     crate::network_host::filter_cors_exposed_response_headers(
-                                        &streaming.pending.request_origin,
+                                        &request_origin,
                                         &observable_head,
                                         streaming.pending.credentials_mode,
                                     );
diff --git a/moli-renderer-v8/src/script_vm/tests/extracted/realms_and_teardown.rs b/moli-renderer-v8/src/script_vm/tests/extracted/realms_and_teardown.rs
index ac11200d32..17e27bb41f 100644
--- a/moli-renderer-v8/src/script_vm/tests/extracted/realms_and_teardown.rs
+++ b/moli-renderer-v8/src/script_vm/tests/extracted/realms_and_teardown.rs
@@ -71,6 +71,21 @@ async fn opaque_child_isolated_world_projects_only_its_own_document() {
             .child_browsing_context_has_opaque_origin(child_handle),
         "sandbox without allow-same-origin must create an opaque child origin"
     );
+    let child_request_origin = {
+        let host = vm._context_host.borrow();
+        let owner = crate::native_bridge::OwnerDispatchScope::Child(child_handle);
+        let loader = host
+            .document_resource_loader_for_dispatch_scope(owner)
+            .expect("opaque child resource loader should exist");
+        host.subresource_request_environment(&loader, owner)
+            .expect("opaque child request environment should exist")
+            .request_origin
+    };
+    assert_eq!(
+        child_request_origin,
+        moli_url::WebOrigin::Opaque,
+        "sandboxed child subresource requests must use an opaque client origin"
+    );
     assert_eq!(
         vm.eval("document.getElementById('opaque-isolated-frame').contentDocument === null")
             .expect("top opaque contentDocument visibility should evaluate"),
diff --git a/moli-renderer-v8/src/types.rs b/moli-renderer-v8/src/types.rs
index c8210e668f..0fa9708082 100644
--- a/moli-renderer-v8/src/types.rs
+++ b/moli-renderer-v8/src/types.rs
@@ -521,6 +521,10 @@ pub(super) struct PendingSubresourceFetchState {
 }
 
 impl PendingSubresourceFetchState {
+    pub(super) fn request_origin(&self) -> moli_url::WebOrigin {
+        self.request_origin.clone()
+    }
+
     pub(super) fn detach_keepalive_window_fetch(&mut self) -> bool {
         let PendingSubresourceExecutionContext::WindowFetch(context) = &self.execution_context
         else {
diff --git a/moli-url/src/origin.rs b/moli-url/src/origin.rs
index b8ab25a8ee..2b748e4876 100644
--- a/moli-url/src/origin.rs
+++ b/moli-url/src/origin.rs
@@ -17,7 +17,7 @@ impl TupleOrigin {
     }
 }
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Eq, PartialEq)]
 pub enum WebOrigin {
     Tuple(TupleOrigin),
     Opaque,
@@ -45,6 +45,12 @@ impl WebOrigin {
         }
     }
 
+    pub fn from_ascii_serialization(serialized: &str) -> Self {
+        Url::parse(serialized)
+            .ok()
+            .map_or(Self::Opaque, |url| Self::from_url(&url))
+    }
+
     pub fn is_opaque(&self) -> bool {
         matches!(self, Self::Opaque)
     }
@@ -69,6 +75,10 @@ impl WebOrigin {
             _ => false,
         }
     }
+
+    pub fn same_origin_url(&self, url: &Url) -> bool {
+        self.same_origin(&Self::from_url(url))
+    }
 }
 
 impl From<&Url> for WebOrigin {
@@ -206,6 +216,27 @@ mod tests {
         ));
     }
 
+    #[test]
+    fn web_origin_compares_directly_with_urls() {
+        let origin = WebOrigin::from_url(&url("https://example.test/document"));
+
+        assert!(origin.same_origin_url(&url("https://example.test/resource")));
+        assert!(!origin.same_origin_url(&url("https://other.test/resource")));
+        assert!(!WebOrigin::Opaque.same_origin_url(&url("https://example.test/resource")));
+    }
+
+    #[test]
+    fn web_origin_rehydrates_tuple_serializations_and_keeps_null_opaque() {
+        assert_eq!(
+            WebOrigin::from_ascii_serialization("https://example.test:8443"),
+            WebOrigin::from_url(&url("https://example.test:8443/path"))
+        );
+        assert_eq!(
+            WebOrigin::from_ascii_serialization("null"),
+            WebOrigin::Opaque
+        );
+    }
+
     #[test]
     fn blob_url_path_fallback_only_accepts_http_https_and_file_schemes() {
         let blob = url("blob:https://example.test/object-1");