diff --git a/packages/server/src/utils/federationAttach.test.ts b/packages/server/src/utils/federationAttach.test.ts
new file mode 100644
index 00000000..a76a45d0
--- /dev/null
+++ b/packages/server/src/utils/federationAttach.test.ts
@@ -0,0 +1,64 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { signRequest } from './federationAuth.js';
+
+const PEER = { origin: 'https://orbit.test', hmacSecret: 'b'.repeat(64) };
+
+function signedResponse(bodyObj: object): Response {
+  const body = JSON.stringify(bodyObj);
+  const ts = Date.now();
+  const nonce = 'resp-nonce';
+  const sig = signRequest(body, PEER.hmacSecret, ts, nonce);
+  return new Response(body, {
+    status: 200,
+    headers: {
+      'x-federation-signature': `sha256=${sig}`,
+      'x-federation-timestamp': String(ts),
+      'x-federation-nonce': nonce,
+    },
+  });
+}
+
+afterEach(() => vi.unstubAllGlobals());
+
+describe('verifyAttachProofWithPeer', () => {
+  it('returns the verified identity for a signed valid:true response', async () => {
+    vi.stubGlobal('fetch', vi.fn(async () => signedResponse({ valid: true, homeUserId: 'h1', username: 'youruser' })));
+    const { verifyAttachProofWithPeer } = await import('./federationAttach.js');
+    const result = await verifyAttachProofWithPeer(PEER, 'a'.repeat(64));
+    expect(result).toEqual({ valid: true, homeUserId: 'h1', username: 'youruser' });
+    const call = (fetch as ReturnType<typeof vi.fn>).mock.calls[0]!;
+    expect(call[0]).toBe('https://orbit.test/api/federation/verify-attach-proof');
+    expect((call[1] as RequestInit).headers).toHaveProperty('X-Federation-Signature');
+  });
+
+  it('treats an UNSIGNED response as valid:false (never trust unauthenticated bodies)', async () => {
+    vi.stubGlobal('fetch', vi.fn(async () => new Response(JSON.stringify({ valid: true, homeUserId: 'h1', username: 'youruser' }), { status: 200 })));
+    const { verifyAttachProofWithPeer } = await import('./federationAttach.js');
+    expect(await verifyAttachProofWithPeer(PEER, 'a'.repeat(64))).toEqual({ valid: false });
+  });
+
+  it('network error → valid:false', async () => {
+    vi.stubGlobal('fetch', vi.fn(async () => { throw new Error('ECONNREFUSED'); }));
+    const { verifyAttachProofWithPeer } = await import('./federationAttach.js');
+    expect(await verifyAttachProofWithPeer(PEER, 'a'.repeat(64))).toEqual({ valid: false });
+  });
+});
+
+describe('fetchHomeProfileByHomeId', () => {
+  it('returns the profile for found:true', async () => {
+    vi.stubGlobal('fetch', vi.fn(async () => new Response(JSON.stringify({
+      found: true,
+      user: { homeUserId: 'h1', username: 'youruser', profile: { displayName: 'J', avatar: 'a.webp', avatarColor: 'coral', banner: null, bio: null } },
+    }), { status: 200 })));
+    const { fetchHomeProfileByHomeId } = await import('./federationAttach.js');
+    const result = await fetchHomeProfileByHomeId(PEER, 'h1');
+    expect(result?.username).toBe('youruser');
+    expect(result?.profile.avatar).toBe('a.webp');
+  });
+
+  it('found:false or network error → null', async () => {
+    vi.stubGlobal('fetch', vi.fn(async () => new Response(JSON.stringify({ found: false }), { status: 200 })));
+    const { fetchHomeProfileByHomeId } = await import('./federationAttach.js');
+    expect(await fetchHomeProfileByHomeId(PEER, 'h1')).toBeNull();
+  });
+});
diff --git a/packages/server/src/utils/federationAttach.ts b/packages/server/src/utils/federationAttach.ts
new file mode 100644
index 00000000..f0aff08b
--- /dev/null
+++ b/packages/server/src/utils/federationAttach.ts
@@ -0,0 +1,109 @@
+import { buildFederationHeaders, verifySignature, getOurOrigin } from './federationAuth.js';
+
+export interface PeerForAttach {
+  origin: string;
+  hmacSecret: string;
+}
+
+/**
+ * Verify a one-time attach-proof token with the detached account's home
+ * instance (re-attach spec §3.1). The response body is only trusted when its
+ * HMAC signature verifies against the shared peer secret — mirrors
+ * fetchPeerEpoch. Any failure (network, bad status, bad signature, malformed
+ * body) is treated as { valid: false }: re-attach fails closed.
+ */
+export async function verifyAttachProofWithPeer(
+  peer: PeerForAttach,
+  token: string,
+): Promise<{ valid: true; homeUserId: string; username: string } | { valid: false }> {
+  const body = JSON.stringify({ token });
+  const headers = buildFederationHeaders(body, peer.hmacSecret, getOurOrigin());
+
+  let res: Response;
+  try {
+    res = await fetch(`${peer.origin}/api/federation/verify-attach-proof`, {
+      method: 'POST',
+      headers,
+      body,
+      signal: AbortSignal.timeout(10_000),
+    });
+  } catch {
+    return { valid: false };
+  }
+  if (!res.ok) return { valid: false };
+
+  let text: string;
+  try {
+    text = await res.text();
+  } catch {
+    return { valid: false };
+  }
+
+  // Verify the response signature with the SAME secret and arg order the peer's
+  // handler signed it with (buildFederationHeaders). A mismatch means we must
+  // not trust the body — never trust an unauthenticated body (spec §2).
+  const sig = (res.headers.get('x-federation-signature') ?? '').replace(/^sha256=/, '');
+  const ts = Number(res.headers.get('x-federation-timestamp'));
+  const nonce = res.headers.get('x-federation-nonce');
+  if (!sig || !Number.isFinite(ts) || !verifySignature(text, sig, peer.hmacSecret, ts, nonce)) {
+    return { valid: false };
+  }
+
+  try {
+    const parsed = JSON.parse(text) as { valid?: boolean; homeUserId?: string; username?: string };
+    if (parsed.valid === true && typeof parsed.homeUserId === 'string' && typeof parsed.username === 'string') {
+      return { valid: true, homeUserId: parsed.homeUserId, username: parsed.username };
+    }
+  } catch {
+    // fall through
+  }
+  return { valid: false };
+}
+
+/**
+ * Fetch the home instance's current profile for a native user via the
+ * existing POST /api/federation/users/by-home-id endpoint. Best-effort:
+ * null on any failure — re-attach proceeds without an initial profile
+ * (the next profile_update relay fills it).
+ */
+export async function fetchHomeProfileByHomeId(
+  peer: PeerForAttach,
+  homeUserId: string,
+): Promise<{ username: string; profile: { displayName: string | null; avatar: string | null; avatarColor: string | null; banner: string | null; bio: string | null } } | null> {
+  const body = JSON.stringify({ homeUserId });
+  const headers = buildFederationHeaders(body, peer.hmacSecret, getOurOrigin());
+
+  let res: Response;
+  try {
+    res = await fetch(`${peer.origin}/api/federation/users/by-home-id`, {
+      method: 'POST',
+      headers,
+      body,
+      signal: AbortSignal.timeout(10_000),
+    });
+  } catch {
+    return null;
+  }
+  if (!res.ok) return null;
+
+  try {
+    const parsed = await res.json() as {
+      found?: boolean;
+      user?: { username?: string; profile?: { displayName?: string | null; avatar?: string | null; avatarColor?: string | null; banner?: string | null; bio?: string | null } };
+    };
+    if (!parsed.found || !parsed.user || typeof parsed.user.username !== 'string' || !parsed.user.profile) return null;
+    const p = parsed.user.profile;
+    return {
+      username: parsed.user.username,
+      profile: {
+        displayName: p.displayName ?? null,
+        avatar: p.avatar ?? null,
+        avatarColor: p.avatarColor ?? null,
+        banner: p.banner ?? null,
+        bio: p.bio ?? null,
+      },
+    };
+  } catch {
+    return null;
+  }
+}