diff --git a/frontend/src/admin/api.js b/frontend/src/admin/api.js
index 72e3f71..fc30f34 100644
--- a/frontend/src/admin/api.js
+++ b/frontend/src/admin/api.js
@@ -1,7 +1,17 @@
-// Admin API client. Uses the rn_admin session cookie exclusively (same-origin, sent
+// Admin API client. Uses the rn_instructor session cookie exclusively (same-origin, sent
 // automatically) — completely separate credential/cookie from student sessions (see
 // platform/api/src/sessions.js). No admin data is ever cached client-side beyond the current
 // React state; nothing here reads or writes localStorage.
+//
+// rn_csrf_i is a deliberately non-HttpOnly cookie (double-submit CSRF pattern, mirrored on the
+// student side in frontend/src/platform/api.js) — read here and echoed back as a header on
+// every state-changing request.
+
+const CSRF_COOKIE = 'rn_csrf_i'
+
+function getCookie(name) {
+  return document.cookie.split('; ').find((c) => c.startsWith(`${name}=`))?.split('=')[1]
+}
 
 async function json(res) {
   if (!res.ok) {
@@ -11,23 +21,30 @@ async function json(res) {
   return res.json()
 }
 
+function withCsrf(init) {
+  return { ...init, headers: { ...init.headers, 'X-CSRF-Token': getCookie(CSRF_COOKIE) || '' } }
+}
 function post(path, body) {
-  return fetch(path, { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: body ? JSON.stringify(body) : undefined }).then(json)
+  return fetch(path, withCsrf({ method: 'POST', headers: { 'Content-Type': 'application/json' }, body: body ? JSON.stringify(body) : undefined })).then(json)
 }
 function patch(path, body) {
-  return fetch(path, { method: 'PATCH', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify(body) }).then(json)
+  return fetch(path, withCsrf({ method: 'PATCH', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify(body) })).then(json)
 }
 function del(path) {
-  return fetch(path, { method: 'DELETE' }).then(json)
+  return fetch(path, withCsrf({ method: 'DELETE' })).then(json)
 }
 function get(path) {
   return fetch(path).then(json)
 }
 
 export const adminApi = {
-  login: (password) => post('/api/admin/login', { password }),
-  logout: () => post('/api/admin/logout'),
-  whoami: () => get('/api/admin/whoami'),
+  // No CSRF token yet at login (nothing authenticated to forge into).
+  login: (email, password) => fetch('/api/auth/instructor/login', {
+    method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ email, password }),
+  }).then(json),
+  logout: () => post('/api/auth/logout'),
+  session: () => get('/api/auth/session'),
+  changePassword: (old_password, new_password) => post('/api/auth/change-password', { old_password, new_password }),
 
   dashboard: () => get('/api/admin/dashboard'),
 
@@ -47,10 +64,20 @@ export const adminApi = {
   cohortStudents: (id) => get(`/api/admin/cohorts/${id}/students`),
   cohortProgress: (id) => get(`/api/admin/cohorts/${id}/progress`),
   assignCohortCurriculum: (id, curriculum_path) => post(`/api/admin/cohorts/${id}/curriculum`, { curriculum_path }),
+  assignCohortInstructor: (id, instructor_id, role_in_cohort) => post(`/api/admin/cohorts/${id}/instructors`, { instructor_id, role_in_cohort }),
   prepareLabs: (id) => post(`/api/admin/cohorts/${id}/prepare-labs`),
   destroyLabs: (id) => post(`/api/admin/cohorts/${id}/destroy-labs`),
 
   resetEnvironment: (id) => post(`/api/admin/environments/${id}/reset`),
   destroyEnvironment: (id) => del(`/api/admin/environments/${id}`),
   environmentEvents: (id) => get(`/api/admin/environments/${id}/events`),
+
+  instructors: () => get('/api/admin/instructors'),
+  createInstructor: (fields) => post('/api/admin/instructors', fields),
+  instructor: (id) => get(`/api/admin/instructors/${id}`),
+  updateInstructor: (id, fields) => patch(`/api/admin/instructors/${id}`, fields),
+  revokeInstructorSessions: (id) => post(`/api/admin/instructors/${id}/revoke-sessions`),
+  resetInstructorPassword: (id, new_password) => post(`/api/admin/instructors/${id}/reset-password`, { new_password }),
+
+  auditEvents: () => get('/api/admin/audit-events'),
 }
diff --git a/frontend/src/platform/api.js b/frontend/src/platform/api.js
index 7748ed3..d48be89 100644
--- a/frontend/src/platform/api.js
+++ b/frontend/src/platform/api.js
@@ -1,14 +1,21 @@
-// Platform Shell API client. Phase 2B: identity is a server-controlled session cookie
-// (rn_session, HttpOnly — this code can't read it, and doesn't need to; the browser sends it
-// automatically on every same-origin request). This replaces Phase 2A's localStorage
-// student_id + X-Student-Id header, which was explicitly pilot-only and client-forgeable.
-// See docs/PHASE2B_ADMIN_AND_PLATFORM_DATA.md §3.
+// Platform Shell API client. Identity is a server-controlled session cookie (rn_session,
+// HttpOnly — this code can't read it, and doesn't need to; the browser sends it automatically
+// on every same-origin request). Roster name-picking is classroom/pilot identification, not
+// production authentication (see docs/PHASE3A_IDENTITY_AND_INSTRUCTOR_ACCESS.md §5) — what's
+// hardened here is the session that results from it: persistent (survives an API restart),
+// unforgeable, and checked on every request.
 //
 // display_name is still cached in sessionStorage purely for UI convenience (showing "Hi,
 // Alice" without an extra round trip) — it carries no authority; every actual API call is
 // authorized by the cookie alone, not by anything read from here.
+//
+// rn_csrf_s is a deliberately non-HttpOnly cookie (double-submit CSRF pattern) — this code
+// reads it and echoes it back as a header on every state-changing request, proving the request
+// came from our own origin's JS (a cross-origin attacker can trigger the cookie but can't read
+// its value or attach a matching custom header without CORS, which we don't grant elsewhere).
 
 const DISPLAY_CACHE_KEY = 'rootnode_student_display'
+const CSRF_COOKIE = 'rn_csrf_s'
 
 export function getCachedDisplay() {
   try { return JSON.parse(sessionStorage.getItem(DISPLAY_CACHE_KEY) || 'null') } catch { return null }
@@ -20,6 +27,10 @@ function clearCachedDisplay() {
   sessionStorage.removeItem(DISPLAY_CACHE_KEY)
 }
 
+function getCookie(name) {
+  return document.cookie.split('; ').find((c) => c.startsWith(`${name}=`))?.split('=')[1]
+}
+
 async function json(res) {
   if (!res.ok) {
     const body = await res.json().catch(() => ({}))
@@ -28,10 +39,20 @@ async function json(res) {
   return res.json()
 }
 
+function mutate(path, body) {
+  return fetch(path, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json', 'X-CSRF-Token': getCookie(CSRF_COOKIE) || '' },
+    body: body ? JSON.stringify(body) : undefined,
+  }).then(json)
+}
+
 export const api = {
   cohorts: () => fetch('/api/roster/cohorts').then(json),
   roster: (cohort) => fetch(`/api/roster?cohort=${encodeURIComponent(cohort)}`).then(json),
   login: async (student_id) => {
+    // No CSRF token yet at login (nothing authenticated to forge into) — same reasoning as the
+    // instructor login route.
     const student = await fetch('/api/login', {
       method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ student_id }),
     }).then(json)
@@ -39,7 +60,7 @@ export const api = {
     return student
   },
   logout: async () => {
-    await fetch('/api/logout', { method: 'POST' }).catch(() => {})
+    await mutate('/api/logout').catch(() => {})
     clearCachedDisplay()
   },
   whoami: () => fetch('/api/session/whoami').then(json),
@@ -47,6 +68,6 @@ export const api = {
   curriculumPath: () => fetch('/api/curriculum/path').then(json),
 
   environmentStatus: () => fetch('/api/environment/status').then(json),
-  launchEnvironment: () => fetch('/api/environment/launch', { method: 'POST' }).then(json),
-  resetEnvironment: () => fetch('/api/environment/reset', { method: 'POST' }).then(json),
+  launchEnvironment: () => mutate('/api/environment/launch'),
+  resetEnvironment: () => mutate('/api/environment/reset'),
 }
diff --git a/platform/api/src/security.js b/platform/api/src/security.js
new file mode 100644
index 0000000..4928336
--- /dev/null
+++ b/platform/api/src/security.js
@@ -0,0 +1,57 @@
+// CSRF (double-submit cookie) and login rate limiting. No new dependency — both are small
+// enough to hand-roll, and SameSite=Lax cookies already do most of the CSRF work; this is
+// defense in depth on top of that, not a replacement for it.
+//
+// CSRF: on login, a non-HttpOnly token is set as a cookie (rn_csrf_s for students, rn_csrf_i
+// for instructors) — non-HttpOnly *on purpose*, so our own frontend JS can read it via
+// document.cookie and echo it back as a header. A cross-origin attacker's page can trigger a
+// cookie-carrying request but cannot read the cookie's value (browser same-origin policy) or
+// add a custom header to it without CORS, which we don't grant to other origins — so it can
+// never produce a request where the header matches the cookie.
+
+const crypto = require('crypto');
+
+function makeCsrfToken() {
+  return crypto.randomBytes(24).toString('base64url');
+}
+
+function parseCookies(req) {
+  const header = req.headers.cookie;
+  if (!header) return {};
+  return Object.fromEntries(
+    header.split(';').map((p) => p.trim().split('=')).filter((p) => p.length === 2).map(([k, v]) => [k, decodeURIComponent(v)])
+  );
+}
+
+// Verifies X-CSRF-Token header against the given cookie for any state-changing method.
+// GET/HEAD/OPTIONS are read-only and exempt (nothing to protect).
+function checkCsrf(req, cookieName) {
+  if (['GET', 'HEAD', 'OPTIONS'].includes(req.method)) return true;
+  const cookieValue = parseCookies(req)[cookieName];
+  const header = req.headers['x-csrf-token'];
+  return !!cookieValue && !!header && cookieValue === header;
+}
+
+// ── Login rate limiting ─────────────────────────────────────────────────────────────────
+// In-memory sliding window, keyed by "ip:identifier" (e.g. ip:email). Fine for a single-process
+// Platform API — a shared store (Postgres/Redis) would be needed for multi-instance, not a
+// concern at this scale (see docs/PHASE3A... known limitations).
+const attempts = new Map();
+
+function tooManyAttempts(key, max, windowMs) {
+  const now = Date.now();
+  const arr = (attempts.get(key) || []).filter((t) => now - t < windowMs);
+  arr.push(now);
+  attempts.set(key, arr);
+  return arr.length > max;
+}
+
+setInterval(() => {
+  const cutoff = Date.now() - 60 * 60 * 1000;
+  for (const [key, arr] of attempts) {
+    const kept = arr.filter((t) => t > cutoff);
+    if (kept.length) attempts.set(key, kept); else attempts.delete(key);
+  }
+}, 15 * 60 * 1000).unref();
+
+module.exports = { makeCsrfToken, parseCookies, checkCsrf, tooManyAttempts };
diff --git a/platform/gateway/src/server.js b/platform/gateway/src/server.js
index cb19d3b..2276f49 100644
--- a/platform/gateway/src/server.js
+++ b/platform/gateway/src/server.js
@@ -23,6 +23,7 @@ const envCache = require('./env-cache');
 const PORT = Number(process.env.GATEWAY_PORT) || 4200;
 const PLATFORM_API_URL = process.env.PLATFORM_API_URL || 'http://localhost:4300';
 const STUDENT_COOKIE = 'rn_session';
+const STUDENT_CSRF_COOKIE = 'rn_csrf_s'; // Phase 3A — see relayScenarioOutcome for why the Gateway needs this too
 // Routing hint only — NOT a security check by itself (ownership is always re-verified via the
 // session, see checkOwnership/resolveOwnedRunningEnvironment). Its only job is disambiguating
 // "this unprefixed request came from inside the Lab UI" from "this came from the dashboard,"
@@ -30,7 +31,9 @@ const STUDENT_COOKIE = 'rn_session';
 // dashboard too, and naively using it hijacked EVERY unmatched dashboard request (including
 // GET /api/session/whoami) to the student's Lab container once one existed. Found by testing.
 const LAB_ACTIVE_COOKIE = 'rn_lab_active';
-const SESSION_CACHE_TTL_MS = 5000;
+// Phase 3A: shortened from 5000ms so a revoked/expired session stops working here within a
+// tighter, testable bound (see platform/test/e2e-phase3a.js "revoked mid-use" case).
+const SESSION_CACHE_TTL_MS = 2000;
 
 const app = express();
 // Deliberately no express.json() globally — see the Phase 2A note this carries forward:
@@ -120,15 +123,26 @@ async function relayScenarioOutcome({ env, req, res, scenarioPath, extractOutcom
   const passed = extractOutcome(body);
   if (passed !== null) {
     const validationCount = Array.isArray(body.checks) ? body.checks.length : undefined;
-    const token = parseCookies(req)[STUDENT_COOKIE];
+    const cookies = parseCookies(req);
+    const token = cookies[STUDENT_COOKIE];
+    const csrf = cookies[STUDENT_CSRF_COOKIE] || '';
+    // Phase 3A: /api/progress/record is CSRF-protected like every other student mutation. The
+    // double-submit check needs BOTH the rn_csrf_s cookie AND a matching X-CSRF-Token header on
+    // the request Platform API actually sees — so both are forwarded here, not just the header
+    // (an earlier version of this fix sent only the header, which still 403'd).
     fetch(`${PLATFORM_API_URL}/api/progress/record`, {
       method: 'POST',
-      headers: { 'Content-Type': 'application/json', Cookie: `${STUDENT_COOKIE}=${token}` },
+      headers: {
+        'Content-Type': 'application/json',
+        Cookie: `${STUDENT_COOKIE}=${token}; ${STUDENT_CSRF_COOKIE}=${csrf}`,
+        'X-CSRF-Token': csrf,
+      },
       body: JSON.stringify({
         scenario_id: req.params.scenarioId, status: passed ? 'completed' : 'in_progress',
         environment_id: env.environment_id, validation_count: validationCount,
       }),
-    }).catch((e) => console.error('[progress relay] failed:', e.message));
+    }).then((r) => { if (!r.ok) console.error('[progress relay] Platform API rejected the update:', r.status); })
+      .catch((e) => console.error('[progress relay] failed:', e.message));
   }
 }