diff --git a/modules/lab/test/runs.test.ts b/modules/lab/test/runs.test.ts
index 196a9c45..6be1063b 100644
--- a/modules/lab/test/runs.test.ts
+++ b/modules/lab/test/runs.test.ts
@@ -370,3 +370,14 @@ test("the public root is read from the binding, and only an https root at the np
   assert.match(publicRootOf(undefined).problem ?? "", /no npm-package-registry binding/);
   rmSync(dir, { recursive: true, force: true });
 });
+
+// A forge setting that is not http(s) is never a trusted origin (review of #280): a `file:` tarball's origin
+// is "null", as is a `file:` forge's, and the two must not match.
+test("a forge setting that is not http(s) trusts no origin of its own", async () => {
+  const got = await npmRegistry("file:///srv/forge", aRegistry(() => packument("file:///api/packages/novox/npm/x.tgz")).get);
+  assert.match(got.problem ?? "", /neither the forge's public root nor its own address/);
+  const root = "https://git.example.org/api/packages/novox/npm/";
+  const viaRoot = await npmRegistry("ftp://novox.internal", aRegistry(() =>
+    packument("ftp://novox.internal/api/packages/novox/npm/x.tgz")).get, root);
+  assert.match(viaRoot.problem ?? "", /neither/);
+});
diff --git a/modules/lab/tools/runs.ts b/modules/lab/tools/runs.ts
index c048fa60..e3eb1b9d 100644
--- a/modules/lab/tools/runs.ts
+++ b/modules/lab/tools/runs.ts
@@ -126,7 +126,9 @@ export async function npmRegistry(forge: string, get: typeof fetch = fetch, publ
   } catch {
     return fallback(`the forge ${JSON.stringify(forge)} is not a URL`);
   }
-  const trusted = [own.origin];
+  // The forge setting's own origin is trusted only as an http(s) one: any other scheme's origin is
+  // "null", or names no registry npm would fetch from.
+  const trusted = own.protocol === "https:" || own.protocol === "http:" ? [own.origin] : [];
   if (publicRoot) {
     try {
       trusted.unshift(new URL(publicRoot).origin);