diff --git a/.changeset/contain-csp-and-bridge-source.md b/.changeset/contain-csp-and-bridge-source.md
new file mode 100644
index 0000000..d4fdba9
--- /dev/null
+++ b/.changeset/contain-csp-and-bridge-source.md
@@ -0,0 +1,13 @@
+---
+"sideshow": patch
+---
+
+Harden surface isolation against regressions and stray frames. The viewer's
+postMessage bridge now only honors host-affecting messages (`switch-session`,
+`open-link`) from a frame the viewer actually embedded, matching the source
+check `resize`/`send-prompt` already enforced — so a stray or nested frame can't
+drive session navigation or pop an open-link dialog. New tests pin the
+load-bearing guarantee directly: a unit test asserts the board origin is never a
+`connect-src`/`script-src` source (only `img-src`/`media-src`, for asset
+embedding), and an e2e test proves on real Chromium and WebKit that script
+running inside an html part is CSP-blocked from fetching the board API.
diff --git a/e2e/isolation.spec.ts b/e2e/isolation.spec.ts
new file mode 100644
index 0000000..0e3eec7
--- /dev/null
+++ b/e2e/isolation.spec.ts
@@ -0,0 +1,36 @@
+import { expect, publish, test } from "./fixtures.ts";
+
+// The sandbox attribute (asserted across the part specs) is the *shape* of the
+// isolation; this spec asserts the *behavior* the project's core invariant
+// promises: script that runs inside an html part cannot reach the board API,
+// because the CSP connect-src omits the server origin. A regression that put the
+// board origin back into connect-src (or dropped the CSP meta tag) would keep
+// the sandbox attribute intact and pass every other test while silently opening
+// exfil — this is the test that catches it, on real Chromium and WebKit.
+//
+// The probe can't phone home (that's the point), so it self-reports the outcome
+// into its own DOM; Playwright reads that across the opaque origin.
+const PROBE = `<div id="r">running</div>
+<script>
+  // Relative URL resolves against the frame's document (the board origin), so
+  // this targets the authenticated API. connect-src must refuse it.
+  fetch('/api/surfaces')
+    .then(function (res) { document.getElementById('r').textContent = 'LEAKED status ' + res.status; })
+    .catch(function () { document.getElementById('r').textContent = 'blocked'; });
+</script>`;
+
+test("an html part's script is CSP-blocked from fetching the board API", async ({
+  page,
+  server,
+}) => {
+  await publish(server.url, { html: PROBE, title: "probe", agent: "e2e" });
+
+  await page.goto(server.url);
+  const card = page.locator(".card:not(#whatsNew)").first();
+  const probe = card.frameLocator("iframe").locator("#r");
+
+  // the fetch is refused before it leaves the frame -> the catch runs
+  await expect(probe).toHaveText("blocked", { timeout: 10_000 });
+  // and it must never have succeeded
+  await expect(probe).not.toContainText("LEAKED");
+});
diff --git a/test/surfacePage.test.ts b/test/surfacePage.test.ts
index d59019c..a904ba2 100644
--- a/test/surfacePage.test.ts
+++ b/test/surfacePage.test.ts
@@ -159,6 +159,31 @@ test("html parts keep their CDN allowlist (rich-part tightening did not leak)",
   assert.ok("connect-src" in html, "html parts still have connect-src");
 });
 
+test("the board origin is never a connect/script source — img/media only", () => {
+  // The server origin is deliberately in img-src/media-src so uploaded assets
+  // embed by URL. It must NEVER reach connect-src or script-src: that origin
+  // serves the authenticated board API and the comment->agent channel, so a
+  // contained script that could fetch it would defeat the whole sandbox. This
+  // is the exact exfil hole the existing 'self'/wildcard/`https:` checks miss —
+  // localhost:4000 is none of those, so it would slip past them.
+  for (const make of [
+    () => renderHtmlPage({ title: "t", html: "<p>x</p>", origin: ORIGIN }),
+    () => renderSandboxedPart({ body: "x", css: "", origin: ORIGIN }),
+  ]) {
+    const d = cspDirectives(make());
+    assert.ok(
+      !(d["connect-src"] ?? []).includes(ORIGIN),
+      "board origin must not be a connect source",
+    );
+    assert.ok(
+      !(d["script-src"] ?? []).includes(ORIGIN),
+      "board origin must not be a script source",
+    );
+    // it is present where it's meant to be, so this test can't pass vacuously
+    assert.ok(d["img-src"]?.includes(ORIGIN), "board origin should still embed images");
+  }
+});
+
 test("escapeHtml neutralizes markup metacharacters", () => {
   assert.equal(
     escapeHtml(`<img src=x onerror="alert(1)">`),
diff --git a/viewer/src/App.tsx b/viewer/src/App.tsx
index b373849..f23debb 100644
--- a/viewer/src/App.tsx
+++ b/viewer/src/App.tsx
@@ -255,9 +255,17 @@ async function onBridgeMessage(ev: MessageEvent) {
     key?: string;
   } | null;
   if (!d || !d.__sideshow) return;
-  // A surface iframe forwarded the session-switch shortcut because focus was
-  // inside it (see server/surfacePage.ts). Mirror the parent keydown handler.
+  // Every host-affecting message must come from a frame the viewer actually
+  // embedded — never an unexpected/nested frame. send-prompt and resize prove
+  // this implicitly (frameForSource resolves the exact html frame); the
+  // remaining types reach the host UI directly, so gate them on isOwnFrame.
+  // (frameForSource only knows html-part frames; switch-session is sent only by
+  // those, but open-link is sent by rich-part frames too, so use the broader
+  // check that recognizes any embedded iframe.)
   if (d.type === "switch-session") {
+    if (!isOwnFrame(ev.source)) return;
+    // A surface iframe forwarded the session-switch shortcut because focus was
+    // inside it (see server/surfacePage.ts). Mirror the parent keydown handler.
     void selectAdjacent(d.key === "ArrowUp" ? -1 : 1);
     return;
   }
@@ -272,11 +280,23 @@ async function onBridgeMessage(ev: MessageEvent) {
       body: JSON.stringify({ surface: src.id, text: String(d.text), author: "user" }),
     });
     toast("Sent to agent: " + d.text);
-  } else if (d.type === "open-link") {
+  } else if (d.type === "open-link" && isOwnFrame(ev.source)) {
     if (confirm(`Open external link?\n\n${d.url}`)) window.open(d.url, "_blank", "noopener");
   }
 }
 
+// True when `source` is the contentWindow of an iframe the viewer embedded
+// (html or rich part). frameForSource only tracks html-part frames; this is the
+// broader gate for messages rich-part frames also send (open-link). Identity
+// comparison works across the opaque-origin boundary even though the frame's
+// document is unreadable.
+function isOwnFrame(source: unknown): boolean {
+  for (const f of document.querySelectorAll("iframe")) {
+    if (f.contentWindow === source) return true;
+  }
+  return false;
+}
+
 function SessionItem(props: { session: SessionRow }) {
   const label = () => sessionLabel(props.session);
   return (