fix(webapp): block remote images in model-authored markdown

Model-authored markdown could emit a remote image whose URL the browser fetches on render — a zero-click data beacon. Constrain the shared markdown renderer with a urlTransform that strips the src of any non-local image and limits links to safe schemes.

.server-changes/agent-response-remote-images.md

@@ -0,0 +1,6 @@
+---
+area: webapp
+type: fix
+---
+
+Images in AI and agent responses are no longer loaded from arbitrary remote websites, closing a way a response could quietly signal an outside server.

apps/webapp/app/components/code/StreamdownRenderer.test.ts

@@ -0,0 +1,44 @@
+import { describe, expect, it } from "vitest";
+import { restrictModelUrls } from "./StreamdownRenderer";
+
+// streamdown calls urlTransform(url, key, node) to compute each url attribute; a
+// returned undefined removes the attribute, so no request is ever issued.
+const img = { tagName: "img" } as any;
+const link = { tagName: "a" } as any;
+
+describe("restrictModelUrls (image src)", () => {
+  it("drops a remote model-authored image (the favicon beacon)", () => {
+    expect(
+      restrictModelUrls("https://www.google.com/s2/favicons?domain=evil", "src", img)
+    ).toBeUndefined();
+  });
+
+  it("drops any absolute or protocol-relative remote image", () => {
+    expect(restrictModelUrls("http://evil.tld/pixel.gif", "src", img)).toBeUndefined();
+    expect(restrictModelUrls("//evil.tld/pixel.gif", "src", img)).toBeUndefined();
+  });
+
+  it("keeps inline and same-origin images", () => {
+    expect(restrictModelUrls("data:image/png;base64,AAAA", "src", img)).toBe(
+      "data:image/png;base64,AAAA"
+    );
+    expect(restrictModelUrls("blob:abc", "src", img)).toBe("blob:abc");
+    expect(restrictModelUrls("/local/pic.png", "src", img)).toBe("/local/pic.png");
+  });
+});
+
+describe("restrictModelUrls (link href)", () => {
+  it("keeps http(s), mailto and relative links", () => {
+    expect(restrictModelUrls("https://trigger.dev/docs", "href", link)).toBe(
+      "https://trigger.dev/docs"
+    );
+    expect(restrictModelUrls("http://example.com", "href", link)).toBe("http://example.com");
+    expect(restrictModelUrls("mailto:hi@trigger.dev", "href", link)).toBe("mailto:hi@trigger.dev");
+    expect(restrictModelUrls("/runs/123", "href", link)).toBe("/runs/123");
+  });
+
+  it("drops unsafe link schemes", () => {
+    expect(restrictModelUrls("javascript:alert(1)", "href", link)).toBeUndefined();
+    expect(restrictModelUrls("data:text/html,<script>", "href", link)).toBeUndefined();
+  });
+});

apps/webapp/app/components/code/StreamdownRenderer.tsx

@@ -1,5 +1,32 @@
 import { lazy } from "react";
-import type { CodeHighlighterPlugin } from "streamdown";
+import type { CodeHighlighterPlugin, UrlTransform } from "streamdown";
+
+const SAFE_LINK_SCHEMES = new Set(["http:", "https:", "mailto:"]);
+
+/**
+ * URL policy for model-authored markdown. A remote image is fetched the moment it
+ * renders — no click — so it is a zero-click data beacon; we drop the src of any
+ * non-local image. Links stay clickable but only for safe, human-followable schemes.
+ * streamdown removes an attribute whose transform returns undefined, so no request fires.
+ */
+export const restrictModelUrls: UrlTransform = (url, key, node) => {
+  const value = url.trim();
+  const isImage = node.tagName === "img" || key === "src" || key === "srcset";
+
+  if (isImage) {
+    // Inline images carry their own bytes; a relative path resolves to our own origin.
+    if (/^data:/i.test(value) || /^blob:/i.test(value)) return url;
+    // Absolute or protocol-relative means a remote host — strip it so nothing is fetched.
+    if (/^[a-z][a-z0-9+.-]*:/i.test(value) || value.startsWith("//")) return undefined;
+    return url;
+  }
+
+  // Links: relative and protocol-relative are fine; otherwise require a safe scheme.
+  if (value.startsWith("//")) return url;
+  const schemeMatch = /^([a-z][a-z0-9+.-]*):/i.exec(value);
+  if (!schemeMatch) return url;
+  return SAFE_LINK_SCHEMES.has(`${schemeMatch[1].toLowerCase()}:`) ? url : undefined;
+};
 
 export const StreamdownRenderer = lazy(() =>
   Promise.all([import("streamdown"), import("@streamdown/code"), import("./shikiTheme")]).then(
@@ -23,6 +50,7 @@ export const StreamdownRenderer = lazy(() =>
             isAnimating={isAnimating}
             plugins={{ code: codePlugin }}
             controls={{ code: { copy: false, download: false } }}
+            urlTransform={restrictModelUrls}
             linkSafety={{ enabled: false }}
           >
             {children}

apps/webapp/vitest.config.ts

@@ -18,6 +18,7 @@ export default defineConfig({
       "app/runEngine/concerns/**/*.test.ts",
       "app/runEngine/services/**/*.test.ts",
       "app/utils/**/*.test.ts",
+      "app/components/code/**/*.test.ts",
       "app/components/dashboard-agent/**/*.test.ts",
       "app/presenters/v3/reports/**/*.test.ts",
     ],