diff --git a/apps/gateway/src/federation/federation.module.ts b/apps/gateway/src/federation/federation.module.ts
index ebb6295..c3818ba 100644
--- a/apps/gateway/src/federation/federation.module.ts
+++ b/apps/gateway/src/federation/federation.module.ts
@@ -5,10 +5,11 @@ import { EnrollmentController } from './enrollment.controller.js';
 import { EnrollmentService } from './enrollment.service.js';
 import { FederationController } from './federation.controller.js';
 import { GrantsService } from './grants.service.js';
+import { FederationAuthGuard } from './server/index.js';
 
 @Module({
   controllers: [EnrollmentController, FederationController],
-  providers: [AdminGuard, CaService, EnrollmentService, GrantsService],
-  exports: [CaService, EnrollmentService, GrantsService],
+  providers: [AdminGuard, CaService, EnrollmentService, GrantsService, FederationAuthGuard],
+  exports: [CaService, EnrollmentService, GrantsService, FederationAuthGuard],
 })
 export class FederationModule {}
diff --git a/apps/gateway/src/federation/grants.service.ts b/apps/gateway/src/federation/grants.service.ts
index 81906fc..317a891 100644
--- a/apps/gateway/src/federation/grants.service.ts
+++ b/apps/gateway/src/federation/grants.service.ts
@@ -10,12 +10,14 @@
  */
 
 import { ConflictException, Inject, Injectable, NotFoundException } from '@nestjs/common';
-import { type Db, and, eq, federationGrants } from '@mosaicstack/db';
+import { type Db, and, eq, federationGrants, federationPeers } from '@mosaicstack/db';
 import { DB } from '../database/database.module.js';
 import { parseFederationScope } from './scope-schema.js';
 import type { CreateGrantDto, ListGrantsDto } from './grants.dto.js';
 
 export type Grant = typeof federationGrants.$inferSelect;
+export type Peer = typeof federationPeers.$inferSelect;
+export type GrantWithPeer = Grant & { peer: Peer };
 
 @Injectable()
 export class GrantsService {
@@ -60,6 +62,33 @@ export class GrantsService {
     return grant;
   }
 
+  /**
+   * Fetch a single grant by ID, joined with its associated peer row.
+   * Used by FederationAuthGuard to perform grant status + cert serial checks
+   * in a single DB round-trip.
+   *
+   * Throws NotFoundException if the grant does not exist.
+   * Throws NotFoundException if the associated peer row is missing (data integrity issue).
+   */
+  async getGrantWithPeer(id: string): Promise<GrantWithPeer> {
+    const rows = await this.db
+      .select()
+      .from(federationGrants)
+      .innerJoin(federationPeers, eq(federationGrants.peerId, federationPeers.id))
+      .where(eq(federationGrants.id, id))
+      .limit(1);
+
+    const row = rows[0];
+    if (!row) {
+      throw new NotFoundException(`Grant ${id} not found`);
+    }
+
+    return {
+      ...row.federation_grants,
+      peer: row.federation_peers,
+    };
+  }
+
   /**
    * List grants with optional filters for peerId, subjectUserId, and status.
    */
diff --git a/apps/gateway/src/federation/oid.util.ts b/apps/gateway/src/federation/oid.util.ts
new file mode 100644
index 0000000..da9cee4
--- /dev/null
+++ b/apps/gateway/src/federation/oid.util.ts
@@ -0,0 +1,116 @@
+/**
+ * Shared OID extraction helpers for Mosaic federation certificates.
+ *
+ * Custom OID registry (PRD §6, docs/federation/SETUP.md):
+ *   1.3.6.1.4.1.99999.1  — mosaic_grant_id
+ *   1.3.6.1.4.1.99999.2  — mosaic_subject_user_id
+ *
+ * The encoding convention: each extension value is an OCTET STRING wrapping
+ * an ASN.1 UTF8String TLV:
+ *   0x0C (tag) + 1-byte length + UTF-8 bytes
+ *
+ * CaService encodes values this way via encodeUtf8String(), and this module
+ * decodes them with the corresponding `.slice(2)` to skip tag + length byte.
+ *
+ * This module is intentionally pure — no NestJS, no DB, no network I/O.
+ */
+
+import { X509Certificate } from '@peculiar/x509';
+
+// ---------------------------------------------------------------------------
+// OID constants
+// ---------------------------------------------------------------------------
+
+export const OID_MOSAIC_GRANT_ID = '1.3.6.1.4.1.99999.1';
+export const OID_MOSAIC_SUBJECT_USER_ID = '1.3.6.1.4.1.99999.2';
+
+// ---------------------------------------------------------------------------
+// Extraction result types
+// ---------------------------------------------------------------------------
+
+export interface MosaicOids {
+  grantId: string;
+  subjectUserId: string;
+}
+
+export type OidExtractionResult =
+  | { ok: true; value: MosaicOids }
+  | {
+      ok: false;
+      error: 'MISSING_GRANT_ID' | 'MISSING_SUBJECT_USER_ID' | 'PARSE_ERROR';
+      detail?: string;
+    };
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+const decoder = new TextDecoder();
+
+/**
+ * Decode an extension value encoded as ASN.1 UTF8String TLV
+ * (tag 0x0C + 1-byte length + UTF-8 bytes).
+ * Slices off the 2-byte TLV header and decodes the remainder as UTF-8.
+ */
+function decodeUtf8StringTlv(value: ArrayBuffer): string {
+  const bytes = new Uint8Array(value);
+  // Skip: tag (1 byte) + length (1 byte)
+  return decoder.decode(bytes.slice(2));
+}
+
+// ---------------------------------------------------------------------------
+// Public API
+// ---------------------------------------------------------------------------
+
+/**
+ * Extract Mosaic custom OIDs (grantId, subjectUserId) from an X.509 certificate
+ * already parsed via @peculiar/x509.
+ *
+ * Returns `{ ok: true, value: MosaicOids }` on success, or
+ * `{ ok: false, error: <code>, detail? }` on any failure — never throws.
+ */
+export function extractMosaicOids(cert: X509Certificate): OidExtractionResult {
+  try {
+    const grantIdExt = cert.getExtension(OID_MOSAIC_GRANT_ID);
+    if (!grantIdExt) {
+      return { ok: false, error: 'MISSING_GRANT_ID' };
+    }
+
+    const subjectUserIdExt = cert.getExtension(OID_MOSAIC_SUBJECT_USER_ID);
+    if (!subjectUserIdExt) {
+      return { ok: false, error: 'MISSING_SUBJECT_USER_ID' };
+    }
+
+    const grantId = decodeUtf8StringTlv(grantIdExt.value);
+    const subjectUserId = decodeUtf8StringTlv(subjectUserIdExt.value);
+
+    return {
+      ok: true,
+      value: { grantId, subjectUserId },
+    };
+  } catch (err) {
+    return {
+      ok: false,
+      error: 'PARSE_ERROR',
+      detail: err instanceof Error ? err.message : String(err),
+    };
+  }
+}
+
+/**
+ * Parse a PEM-encoded certificate and extract Mosaic OIDs.
+ * Returns an OidExtractionResult — never throws.
+ */
+export function extractMosaicOidsFromPem(certPem: string): OidExtractionResult {
+  let cert: X509Certificate;
+  try {
+    cert = new X509Certificate(certPem);
+  } catch (err) {
+    return {
+      ok: false,
+      error: 'PARSE_ERROR',
+      detail: err instanceof Error ? err.message : String(err),
+    };
+  }
+  return extractMosaicOids(cert);
+}
diff --git a/apps/gateway/src/federation/server/__tests__/federation-auth.guard.spec.ts b/apps/gateway/src/federation/server/__tests__/federation-auth.guard.spec.ts
new file mode 100644
index 0000000..a8454e9
--- /dev/null
+++ b/apps/gateway/src/federation/server/__tests__/federation-auth.guard.spec.ts
@@ -0,0 +1,438 @@
+/**
+ * Unit tests for FederationAuthGuard (FED-M3-03).
+ *
+ * Coverage:
+ *  - Missing cert (no TLS socket / no getPeerCertificate) → 401
+ *  - Cert parse failure (corrupt DER raw bytes) → 401
+ *  - Missing grantId OID → 401
+ *  - Missing subjectUserId OID → 401
+ *  - Grant not found (GrantsService throws NotFoundException) → 403
+ *  - Grant in `pending` status → 403
+ *  - Grant in `revoked` status → 403
+ *  - Grant in `expired` status → 403
+ *  - Cert serial mismatch → 403
+ *  - Happy path: active grant + matching cert serial → context attached, returns true
+ */
+
+import 'reflect-metadata';
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import type { ExecutionContext } from '@nestjs/common';
+import { NotFoundException } from '@nestjs/common';
+import { FederationAuthGuard } from '../federation-auth.guard.js';
+import { makeMosaicIssuedCert } from '../../__tests__/helpers/test-cert.js';
+import type { GrantsService, GrantWithPeer } from '../../grants.service.js';
+
+// ---------------------------------------------------------------------------
+// Test constants
+// ---------------------------------------------------------------------------
+
+const GRANT_ID = 'a1111111-1111-1111-1111-111111111111';
+const USER_ID = 'b2222222-2222-2222-2222-222222222222';
+const PEER_ID = 'c3333333-3333-3333-3333-333333333333';
+
+// Node.js TLS serialNumber is uppercase hex (no colons)
+const CERT_SERIAL_HEX = '01';
+
+const VALID_SCOPE = { resources: ['tasks'], max_rows_per_query: 100 };
+
+// ---------------------------------------------------------------------------
+// Mock builders
+// ---------------------------------------------------------------------------
+
+/**
+ * Build a minimal GrantWithPeer-shaped mock.
+ */
+function makeGrantWithPeer(overrides: Partial<GrantWithPeer> = {}): GrantWithPeer {
+  return {
+    id: GRANT_ID,
+    peerId: PEER_ID,
+    subjectUserId: USER_ID,
+    scope: VALID_SCOPE,
+    status: 'active',
+    expiresAt: null,
+    createdAt: new Date('2026-01-01T00:00:00Z'),
+    revokedAt: null,
+    revokedReason: null,
+    peer: {
+      id: PEER_ID,
+      commonName: 'test-peer',
+      displayName: 'Test Peer',
+      certPem: '',
+      certSerial: CERT_SERIAL_HEX,
+      certNotAfter: new Date(Date.now() + 86_400_000),
+      clientKeyPem: null,
+      state: 'active',
+      endpointUrl: null,
+      lastSeenAt: null,
+      createdAt: new Date('2026-01-01T00:00:00Z'),
+      revokedAt: null,
+    },
+    ...overrides,
+  };
+}
+
+/**
+ * Build a mock ExecutionContext with a pre-built TLS peer certificate.
+ *
+ * `certPem` — PEM string to present as the raw DER cert (converted to Buffer).
+ *             Pass null to simulate "no cert presented".
+ * `certSerialHex` — serialNumber string returned by the TLS socket.
+ *                   Node.js returns uppercase hex.
+ * `hasTlsSocket` — if false, raw.socket has no getPeerCertificate (plain HTTP).
+ */
+function makeContext(opts: {
+  certPem: string | null;
+  certSerialHex?: string;
+  hasTlsSocket?: boolean;
+}): {
+  ctx: ExecutionContext;
+  statusMock: ReturnType<typeof vi.fn>;
+  sendMock: ReturnType<typeof vi.fn>;
+} {
+  const { certPem, certSerialHex = CERT_SERIAL_HEX, hasTlsSocket = true } = opts;
+
+  // Build peerCert object that Node.js TLS socket.getPeerCertificate() returns
+  let peerCert: Record<string, unknown>;
+  if (certPem === null) {
+    // Simulate no cert: Node.js returns object with empty string fields
+    peerCert = { raw: null, serialNumber: '' };
+  } else {
+    // Convert PEM to DER Buffer (strip headers + base64 decode)
+    const b64 = certPem
+      .replace(/-----BEGIN CERTIFICATE-----/, '')
+      .replace(/-----END CERTIFICATE-----/, '')
+      .replace(/\s+/g, '');
+    const raw = Buffer.from(b64, 'base64');
+    peerCert = { raw, serialNumber: certSerialHex };
+  }
+
+  const getPeerCertificate = vi.fn().mockReturnValue(peerCert);
+
+  const socket = hasTlsSocket ? { getPeerCertificate } : {}; // No getPeerCertificate → non-TLS
+
+  // Fastify reply mocks
+  const sendMock = vi.fn().mockReturnValue(undefined);
+  const headerMock = vi.fn().mockReturnValue({ send: sendMock });
+  const statusMock = vi.fn().mockReturnValue({ header: headerMock });
+
+  const request = {
+    raw: {
+      socket,
+    },
+  };
+
+  const reply = {
+    status: statusMock,
+  };
+
+  const ctx = {
+    switchToHttp: () => ({
+      getRequest: () => request,
+      getResponse: () => reply,
+    }),
+  } as unknown as ExecutionContext;
+
+  return { ctx, statusMock, sendMock };
+}
+
+/**
+ * Build a mock GrantsService.
+ */
+function makeGrantsService(
+  overrides: Partial<Pick<GrantsService, 'getGrantWithPeer'>> = {},
+): GrantsService {
+  return {
+    getGrantWithPeer: vi.fn().mockResolvedValue(makeGrantWithPeer()),
+    ...overrides,
+  } as unknown as GrantsService;
+}
+
+// ---------------------------------------------------------------------------
+// Test suite
+// ---------------------------------------------------------------------------
+
+describe('FederationAuthGuard', () => {
+  let certPem: string;
+
+  beforeEach(async () => {
+    // Generate a real Mosaic-issued cert with the standard OIDs
+    certPem = await makeMosaicIssuedCert({ grantId: GRANT_ID, subjectUserId: USER_ID });
+  });
+
+  // ── 401: No TLS socket ────────────────────────────────────────────────────
+
+  it('returns 401 when there is no TLS socket (plain HTTP connection)', async () => {
+    const { ctx, statusMock } = makeContext({
+      certPem: certPem,
+      hasTlsSocket: false,
+    });
+
+    const guard = new FederationAuthGuard(makeGrantsService());
+    const result = await guard.canActivate(ctx);
+
+    expect(result).toBe(false);
+    expect(statusMock).toHaveBeenCalledWith(401);
+  });
+
+  // ── 401: Cert not presented ───────────────────────────────────────────────
+
+  it('returns 401 when the peer did not present a certificate', async () => {
+    const { ctx, statusMock } = makeContext({ certPem: null });
+
+    const guard = new FederationAuthGuard(makeGrantsService());
+    const result = await guard.canActivate(ctx);
+
+    expect(result).toBe(false);
+    expect(statusMock).toHaveBeenCalledWith(401);
+  });
+
+  // ── 401: Cert parse failure ───────────────────────────────────────────────
+
+  it('returns 401 when the certificate DER bytes are corrupt', async () => {
+    // Build context with a cert that has garbage DER bytes
+    const corruptPem = '-----BEGIN CERTIFICATE-----\naW52YWxpZA==\n-----END CERTIFICATE-----';
+    const { ctx, statusMock } = makeContext({ certPem: corruptPem });
+
+    const guard = new FederationAuthGuard(makeGrantsService());
+    const result = await guard.canActivate(ctx);
+
+    expect(result).toBe(false);
+    expect(statusMock).toHaveBeenCalledWith(401);
+  });
+
+  // ── 401: Missing grantId OID ─────────────────────────────────────────────
+
+  it('returns 401 when the cert is missing the grantId OID', async () => {
+    // makeSelfSignedCert produces a cert without any Mosaic OIDs
+    const { makeSelfSignedCert } = await import('../../__tests__/helpers/test-cert.js');
+    const plainCert = await makeSelfSignedCert();
+    const { ctx, statusMock } = makeContext({ certPem: plainCert });
+
+    const guard = new FederationAuthGuard(makeGrantsService());
+    const result = await guard.canActivate(ctx);
+
+    expect(result).toBe(false);
+    expect(statusMock).toHaveBeenCalledWith(401);
+  });
+
+  // ── 401: Missing subjectUserId OID ───────────────────────────────────────
+
+  it('returns 401 when the cert has grantId OID but is missing subjectUserId OID', async () => {
+    // Build a cert with only the grantId OID by importing cert generator internals
+    const { webcrypto } = await import('node:crypto');
+    const {
+      X509CertificateGenerator,
+      Extension,
+      KeyUsagesExtension,
+      KeyUsageFlags,
+      BasicConstraintsExtension,
+      cryptoProvider,
+    } = await import('@peculiar/x509');
+
+    cryptoProvider.set(webcrypto as unknown as Parameters<typeof cryptoProvider.set>[0]);
+
+    const alg = { name: 'ECDSA', namedCurve: 'P-256', hash: 'SHA-256' } as const;
+    const keys = await webcrypto.subtle.generateKey(alg, false, ['sign', 'verify']);
+    const now = new Date();
+    const tomorrow = new Date(now.getTime() + 86_400_000);
+
+    // Encode grantId only — missing subjectUserId extension
+    const utf8 = new TextEncoder().encode(GRANT_ID);
+    const encoded = new Uint8Array(2 + utf8.length);
+    encoded[0] = 0x0c;
+    encoded[1] = utf8.length;
+    encoded.set(utf8, 2);
+
+    const cert = await X509CertificateGenerator.createSelfSigned({
+      serialNumber: '01',
+      name: 'CN=partial-oid-test',
+      notBefore: now,
+      notAfter: tomorrow,
+      signingAlgorithm: alg,
+      keys,
+      extensions: [
+        new BasicConstraintsExtension(false),
+        new KeyUsagesExtension(KeyUsageFlags.digitalSignature),
+        new Extension('1.3.6.1.4.1.99999.1', false, encoded), // grantId only
+      ],
+    });
+
+    const { ctx, statusMock } = makeContext({ certPem: cert.toString('pem') });
+
+    const guard = new FederationAuthGuard(makeGrantsService());
+    const result = await guard.canActivate(ctx);
+
+    expect(result).toBe(false);
+    expect(statusMock).toHaveBeenCalledWith(401);
+  });
+
+  // ── 403: Grant not found ─────────────────────────────────────────────────
+
+  it('returns 403 when the grantId from the cert does not exist in DB', async () => {
+    const grantsService = makeGrantsService({
+      getGrantWithPeer: vi
+        .fn()
+        .mockRejectedValue(new NotFoundException(`Grant ${GRANT_ID} not found`)),
+    });
+
+    const { ctx, statusMock } = makeContext({ certPem });
+
+    const guard = new FederationAuthGuard(grantsService);
+    const result = await guard.canActivate(ctx);
+
+    expect(result).toBe(false);
+    expect(statusMock).toHaveBeenCalledWith(403);
+  });
+
+  // ── 403: Grant in `pending` status ───────────────────────────────────────
+
+  it('returns 403 when the grant is in pending status', async () => {
+    const grantsService = makeGrantsService({
+      getGrantWithPeer: vi.fn().mockResolvedValue(makeGrantWithPeer({ status: 'pending' })),
+    });
+
+    const { ctx, statusMock } = makeContext({ certPem });
+
+    const guard = new FederationAuthGuard(grantsService);
+    const result = await guard.canActivate(ctx);
+
+    expect(result).toBe(false);
+    expect(statusMock).toHaveBeenCalledWith(403);
+  });
+
+  // ── 403: Grant in `revoked` status ───────────────────────────────────────
+
+  it('returns 403 when the grant is in revoked status', async () => {
+    const grantsService = makeGrantsService({
+      getGrantWithPeer: vi
+        .fn()
+        .mockResolvedValue(makeGrantWithPeer({ status: 'revoked', revokedAt: new Date() })),
+    });
+
+    const { ctx, statusMock } = makeContext({ certPem });
+
+    const guard = new FederationAuthGuard(grantsService);
+    const result = await guard.canActivate(ctx);
+
+    expect(result).toBe(false);
+    expect(statusMock).toHaveBeenCalledWith(403);
+  });
+
+  // ── 403: Grant in `expired` status ───────────────────────────────────────
+
+  it('returns 403 when the grant is in expired status', async () => {
+    const grantsService = makeGrantsService({
+      getGrantWithPeer: vi.fn().mockResolvedValue(makeGrantWithPeer({ status: 'expired' })),
+    });
+
+    const { ctx, statusMock } = makeContext({ certPem });
+
+    const guard = new FederationAuthGuard(grantsService);
+    const result = await guard.canActivate(ctx);
+
+    expect(result).toBe(false);
+    expect(statusMock).toHaveBeenCalledWith(403);
+  });
+
+  // ── 403: Cert serial mismatch ─────────────────────────────────────────────
+
+  it('returns 403 when the cert serial does not match the registered peer cert serial', async () => {
+    // Return a grant whose peer has a different stored serial
+    const grantsService = makeGrantsService({
+      getGrantWithPeer: vi.fn().mockResolvedValue(
+        makeGrantWithPeer({
+          peer: {
+            id: PEER_ID,
+            commonName: 'test-peer',
+            displayName: 'Test Peer',
+            certPem: '',
+            certSerial: 'DEADBEEF', // different from CERT_SERIAL_HEX='01'
+            certNotAfter: new Date(Date.now() + 86_400_000),
+            clientKeyPem: null,
+            state: 'active',
+            endpointUrl: null,
+            lastSeenAt: null,
+            createdAt: new Date('2026-01-01T00:00:00Z'),
+            revokedAt: null,
+          },
+        }),
+      ),
+    });
+
+    // Context presents cert with serial '01' but DB has 'DEADBEEF'
+    const { ctx, statusMock } = makeContext({ certPem, certSerialHex: '01' });
+
+    const guard = new FederationAuthGuard(grantsService);
+    const result = await guard.canActivate(ctx);
+
+    expect(result).toBe(false);
+    expect(statusMock).toHaveBeenCalledWith(403);
+  });
+
+  // ── Happy path ────────────────────────────────────────────────────────────
+
+  it('returns true and attaches federationContext on happy path', async () => {
+    const grant = makeGrantWithPeer({
+      status: 'active',
+      peer: {
+        id: PEER_ID,
+        commonName: 'test-peer',
+        displayName: 'Test Peer',
+        certPem: '',
+        certSerial: CERT_SERIAL_HEX,
+        certNotAfter: new Date(Date.now() + 86_400_000),
+        clientKeyPem: null,
+        state: 'active',
+        endpointUrl: null,
+        lastSeenAt: null,
+        createdAt: new Date('2026-01-01T00:00:00Z'),
+        revokedAt: null,
+      },
+    });
+
+    const grantsService = makeGrantsService({
+      getGrantWithPeer: vi.fn().mockResolvedValue(grant),
+    });
+
+    // Build context manually to capture what gets set on request.federationContext
+    const b64 = certPem
+      .replace(/-----BEGIN CERTIFICATE-----/, '')
+      .replace(/-----END CERTIFICATE-----/, '')
+      .replace(/\s+/g, '');
+    const raw = Buffer.from(b64, 'base64');
+    const peerCert = { raw, serialNumber: CERT_SERIAL_HEX };
+
+    const sendMock = vi.fn().mockReturnValue(undefined);
+    const headerMock = vi.fn().mockReturnValue({ send: sendMock });
+    const statusMock = vi.fn().mockReturnValue({ header: headerMock });
+
+    const request: Record<string, unknown> = {
+      raw: {
+        socket: { getPeerCertificate: vi.fn().mockReturnValue(peerCert) },
+      },
+    };
+
+    const reply = { status: statusMock };
+
+    const ctx = {
+      switchToHttp: () => ({
+        getRequest: () => request,
+        getResponse: () => reply,
+      }),
+    } as unknown as ExecutionContext;
+
+    const guard = new FederationAuthGuard(grantsService);
+    const result = await guard.canActivate(ctx);
+
+    expect(result).toBe(true);
+    expect(statusMock).not.toHaveBeenCalled();
+
+    // Verify the context was attached correctly
+    expect(request['federationContext']).toEqual({
+      grantId: GRANT_ID,
+      subjectUserId: USER_ID,
+      peerId: PEER_ID,
+      scope: VALID_SCOPE,
+    });
+  });
+});
diff --git a/apps/gateway/src/federation/server/federation-auth.guard.ts b/apps/gateway/src/federation/server/federation-auth.guard.ts
new file mode 100644
index 0000000..feb04fe
--- /dev/null
+++ b/apps/gateway/src/federation/server/federation-auth.guard.ts
@@ -0,0 +1,211 @@
+/**
+ * FederationAuthGuard — NestJS CanActivate guard for inbound federation requests.
+ *
+ * Validates the mTLS client certificate presented by a peer gateway, extracts
+ * custom OIDs to identify the grant + subject user, loads the grant from DB,
+ * asserts it is active, and verifies the cert serial against the registered peer
+ * cert serial as a defense-in-depth measure.
+ *
+ * On success, attaches `request.federationContext` for downstream verb controllers.
+ * On failure, responds with the federation wire-format error envelope (not raw
+ * NestJS exception JSON) to match the federation protocol contract.
+ *
+ * ## Cert-serial check decision
+ * The guard validates that the inbound client cert's serial number matches the
+ * `certSerial` stored on the associated `federation_peers` row. This is a
+ * defense-in-depth measure: even if the mTLS handshake is compromised at the
+ * transport layer (e.g. misconfigured TLS terminator that forwards arbitrary
+ * client certs), an attacker cannot replay a cert with a different serial than
+ * what was registered during enrollment. This check is NOT loosened because:
+ *  1. It is O(1) — no additional DB round-trip (peerId is on the grant row,
+ *     so we join to federationPeers in the same query).
+ *  2. Cert renewal MUST update the stored serial — enforced by M6 scheduler.
+ *  3. The OID-only path (without serial check) would allow any cert from the
+ *     same CA bearing the same grantId OID to succeed after cert compromise.
+ *
+ * ## FastifyRequest typing path
+ * NestJS + Fastify wraps the raw Node.js IncomingMessage in a FastifyRequest.
+ * The underlying TLS socket is accessed via `request.raw.socket`, which is a
+ * `tls.TLSSocket` when the server is listening on HTTPS. In development/test
+ * the gateway may run over plain HTTP, in which case `getPeerCertificate` is
+ * not available. The guard safely handles both cases by checking for the
+ * method's existence before calling it.
+ *
+ * Note: The guard reads the peer certificate from the *already-completed*
+ * TLS handshake via `socket.getPeerCertificate(detailed=true)`. This relies
+ * on the server being configured with `requestCert: true` at the TLS level
+ * so Fastify/Node.js requests the client cert during the handshake.
+ * The guard does NOT verify the cert chain itself — that is handled by the
+ * TLS layer (Node.js `rejectUnauthorized: true` with the CA cert pinned).
+ */
+
+import { type CanActivate, type ExecutionContext, Injectable, Logger } from '@nestjs/common';
+import type { FastifyReply, FastifyRequest } from 'fastify';
+import * as tls from 'node:tls';
+import { X509Certificate } from '@peculiar/x509';
+import { FederationForbiddenError, FederationUnauthorizedError } from '@mosaicstack/types';
+import { extractMosaicOids } from '../oid.util.js';
+import { GrantsService } from '../grants.service.js';
+import type { FederationContext } from './federation-context.js';
+import './federation-context.js'; // side-effect import: applies FastifyRequest module augmentation
+
+// ---------------------------------------------------------------------------
+// Internal helpers
+// ---------------------------------------------------------------------------
+
+/**
+ * Send a federation wire-format error response directly on the Fastify reply.
+ * Returns false — callers return this value from canActivate.
+ */
+function sendFederationError(
+  reply: FastifyReply,
+  error: FederationUnauthorizedError | FederationForbiddenError,
+): boolean {
+  const statusCode = error.code === 'unauthorized' ? 401 : 403;
+  void reply.status(statusCode).header('content-type', 'application/json').send(error.toEnvelope());
+  return false;
+}
+
+// ---------------------------------------------------------------------------
+// Guard
+// ---------------------------------------------------------------------------
+
+@Injectable()
+export class FederationAuthGuard implements CanActivate {
+  private readonly logger = new Logger(FederationAuthGuard.name);
+
+  constructor(private readonly grantsService: GrantsService) {}
+
+  async canActivate(context: ExecutionContext): Promise<boolean> {
+    const http = context.switchToHttp();
+    const request = http.getRequest<FastifyRequest>();
+    const reply = http.getResponse<FastifyReply>();
+
+    // ── Step 1: Extract peer certificate from TLS socket ────────────────────
+    const rawSocket = request.raw.socket;
+
+    // Check TLS socket: getPeerCertificate is only available on TLS connections.
+    if (
+      !rawSocket ||
+      typeof (rawSocket as Partial<tls.TLSSocket>).getPeerCertificate !== 'function'
+    ) {
+      this.logger.warn('No TLS socket — client cert unavailable (non-mTLS connection)');
+      return sendFederationError(
+        reply,
+        new FederationUnauthorizedError('Client certificate required'),
+      );
+    }
+
+    const tlsSocket = rawSocket as tls.TLSSocket;
+    const peerCert = tlsSocket.getPeerCertificate(true);
+
+    // Node.js returns an object with empty string fields when no cert was presented.
+    if (!peerCert || !peerCert.raw) {
+      this.logger.warn('Peer certificate not presented (mTLS handshake did not supply cert)');
+      return sendFederationError(
+        reply,
+        new FederationUnauthorizedError('Client certificate required'),
+      );
+    }
+
+    // ── Step 2: Parse the DER-encoded certificate via @peculiar/x509 ────────
+    let cert: X509Certificate;
+    try {
+      // peerCert.raw is a Buffer containing the DER-encoded cert
+      cert = new X509Certificate(peerCert.raw);
+    } catch (err) {
+      this.logger.warn(
+        `Failed to parse peer certificate: ${err instanceof Error ? err.message : String(err)}`,
+      );
+      return sendFederationError(
+        reply,
+        new FederationUnauthorizedError('Client certificate could not be parsed'),
+      );
+    }
+
+    // ── Step 3: Extract Mosaic custom OIDs ──────────────────────────────────
+    const oidResult = extractMosaicOids(cert);
+
+    if (!oidResult.ok) {
+      const message =
+        oidResult.error === 'MISSING_GRANT_ID'
+          ? 'Client certificate is missing required OID: mosaic_grant_id (1.3.6.1.4.1.99999.1)'
+          : oidResult.error === 'MISSING_SUBJECT_USER_ID'
+            ? 'Client certificate is missing required OID: mosaic_subject_user_id (1.3.6.1.4.1.99999.2)'
+            : `Client certificate OID extraction failed: ${oidResult.detail ?? 'unknown error'}`;
+      this.logger.warn(`OID extraction failure [${oidResult.error}]: ${message}`);
+      return sendFederationError(reply, new FederationUnauthorizedError(message));
+    }
+
+    const { grantId, subjectUserId } = oidResult.value;
+
+    // ── Step 4: Load grant from DB ───────────────────────────────────────────
+    let grant: Awaited<ReturnType<GrantsService['getGrantWithPeer']>>;
+    try {
+      grant = await this.grantsService.getGrantWithPeer(grantId);
+    } catch {
+      // getGrantWithPeer throws NotFoundException when not found
+      this.logger.warn(`Grant not found: ${grantId}`);
+      return sendFederationError(reply, new FederationForbiddenError(`Grant ${grantId} not found`));
+    }
+
+    // ── Step 5: Assert grant is active ──────────────────────────────────────
+    if (grant.status !== 'active') {
+      this.logger.warn(`Grant ${grantId} is not active — status=${grant.status}`);
+      return sendFederationError(
+        reply,
+        new FederationForbiddenError(`Grant ${grantId} is not active (status: ${grant.status})`),
+      );
+    }
+
+    // ── Step 6: Defense-in-depth — cert serial must match registered peer ───
+    // The serial number from Node.js TLS is upper-case hex without colons.
+    // The @peculiar/x509 serialNumber is decimal. We compare using the native
+    // Node.js crypto cert serial which is uppercase hex, matching DB storage.
+    // Both are derived from the peerCert.serialNumber Node.js provides.
+    const inboundSerial: string = peerCert.serialNumber ?? '';
+
+    if (!grant.peer.certSerial) {
+      // Peer row exists but has no stored serial — something is wrong with enrollment
+      this.logger.error(`Peer ${grant.peerId} has no stored certSerial — enrollment incomplete`);
+      return sendFederationError(
+        reply,
+        new FederationForbiddenError(
+          'Peer registration incomplete — no certificate serial on record',
+        ),
+      );
+    }
+
+    // Normalize both to uppercase for comparison (Node.js serialNumber is
+    // already uppercase hex; DB value was stored from extractSerial() which
+    // returns crypto.X509Certificate.serialNumber — also uppercase hex).
+    if (inboundSerial.toUpperCase() !== grant.peer.certSerial.toUpperCase()) {
+      this.logger.warn(
+        `Cert serial mismatch for grant ${grantId}: ` +
+          `inbound=${inboundSerial} registered=${grant.peer.certSerial}`,
+      );
+      return sendFederationError(
+        reply,
+        new FederationForbiddenError(
+          'Client certificate serial does not match registered peer certificate',
+        ),
+      );
+    }
+
+    // ── Step 7: Attach FederationContext to request ──────────────────────────
+    const federationContext: FederationContext = {
+      grantId,
+      subjectUserId,
+      peerId: grant.peerId,
+      scope: grant.scope as Record<string, unknown>,
+    };
+
+    request.federationContext = federationContext;
+
+    this.logger.debug(
+      `Federation auth OK — grantId=${grantId} peerId=${grant.peerId} subjectUserId=${subjectUserId}`,
+    );
+
+    return true;
+  }
+}
diff --git a/apps/gateway/src/federation/server/federation-context.ts b/apps/gateway/src/federation/server/federation-context.ts
new file mode 100644
index 0000000..0dcb56f
--- /dev/null
+++ b/apps/gateway/src/federation/server/federation-context.ts
@@ -0,0 +1,39 @@
+/**
+ * FederationContext — attached to inbound federation requests after successful
+ * mTLS + grant validation by FederationAuthGuard.
+ *
+ * Downstream verb controllers access this via `request.federationContext`.
+ */
+
+/**
+ * Augment FastifyRequest so TypeScript knows about the federation context
+ * property that FederationAuthGuard attaches on success.
+ */
+declare module 'fastify' {
+  interface FastifyRequest {
+    federationContext?: FederationContext;
+  }
+}
+
+/**
+ * Typed context object attached to the request by FederationAuthGuard.
+ * Carries all data extracted from the mTLS cert + grant DB row needed
+ * by downstream federation verb handlers.
+ */
+export interface FederationContext {
+  /** The federation grant ID extracted from OID 1.3.6.1.4.1.99999.1 */
+  grantId: string;
+
+  /** The local subject user whose data is accessible under this grant */
+  subjectUserId: string;
+
+  /** The peer gateway ID (from the grant's peerId FK) */
+  peerId: string;
+
+  /**
+   * Grant scope — determines which resources the peer may query.
+   * Typed as Record<string, unknown> because the full scope schema lives in
+   * scope-schema.ts; downstream handlers should narrow via parseFederationScope.
+   */
+  scope: Record<string, unknown>;
+}
diff --git a/apps/gateway/src/federation/server/index.ts b/apps/gateway/src/federation/server/index.ts
new file mode 100644
index 0000000..d369e00
--- /dev/null
+++ b/apps/gateway/src/federation/server/index.ts
@@ -0,0 +1,13 @@
+/**
+ * Federation server-side barrel — inbound request handling.
+ *
+ * Exports the mTLS auth guard and the FederationContext interface
+ * for use by verb controllers (M3-05/06/07).
+ *
+ * Usage:
+ *   import { FederationAuthGuard } from './server/index.js';
+ *   @UseGuards(FederationAuthGuard)
+ */
+
+export { FederationAuthGuard } from './federation-auth.guard.js';
+export type { FederationContext } from './federation-context.js';