diff --git a/docs/security/account-lockout.md b/docs/security/account-lockout.md
new file mode 100644
index 0000000..81689f8
--- /dev/null
+++ b/docs/security/account-lockout.md
@@ -0,0 +1,22 @@
+# Account Lockout After Failed Logins
+
+Protects against brute-force attacks by locking an account after repeated failed
+login attempts. Thresholds live in `src/auth/account-lockout.constants.ts`.
+
+## Behaviour
+
+- Each failed login increments a per-account `failedAttempts` counter.
+- After `MAX_FAILED_ATTEMPTS` (5) consecutive failures the account is locked.
+- Lockout is **progressive**: each additional failure doubles the lockout
+  window (`getLockoutDurationMs`), starting at 1 minute and capped at 1 hour.
+- A successful login resets the counter.
+
+## Admin unlock
+
+- Admins can clear the counter and lockout for an account via an admin endpoint,
+  immediately restoring access.
+
+## Security alerts
+
+- When an account crosses the lockout threshold, a security alert (email) is
+  sent to the account owner so they can react to a possible attack.
diff --git a/docs/security/csrf.md b/docs/security/csrf.md
new file mode 100644
index 0000000..8714a5c
--- /dev/null
+++ b/docs/security/csrf.md
@@ -0,0 +1,21 @@
+# CSRF Protection
+
+State-changing requests are protected with the double-submit cookie pattern.
+
+## Approach
+
+1. On session start the server issues a random CSRF token
+   (`generateCsrfToken()` in `src/common/security/csrf.util.ts`) and sets it in
+   a cookie readable by the frontend.
+2. For every state-changing request (`POST`/`PATCH`/`DELETE`) the client sends
+   the same token back in the `x-csrf-token` header.
+3. A validation middleware compares the header value against the cookie value
+   using `validateCsrfToken()` (constant-time compare) and rejects mismatches
+   with `403 Forbidden`.
+
+## Notes
+
+- Safe methods (`GET`, `HEAD`, `OPTIONS`) are exempt.
+- Combine with `SameSite=Lax`/`Strict` cookies for defence in depth.
+- Because the token is compared in constant time, token bytes are not leaked via
+  response timing.
diff --git a/docs/security/jwt-strategy.md b/docs/security/jwt-strategy.md
new file mode 100644
index 0000000..c218482
--- /dev/null
+++ b/docs/security/jwt-strategy.md
@@ -0,0 +1,28 @@
+# JWT Expiration & Refresh Strategy
+
+## Tokens
+
+- **Access token** — short-lived, sent as a Bearer token on every request.
+  Default lifetime `15m`, configurable via `JWT_EXPIRES_IN`.
+- **Refresh token** — long-lived, used only to obtain a new access token.
+  Default lifetime `7d`, configurable via `JWT_REFRESH_EXPIRES_IN`.
+
+## Refresh flow
+
+1. On login the API returns an access token and a refresh token.
+2. When the access token expires the client calls `POST /auth/refresh` with the
+   refresh token.
+3. The server validates the refresh token, then issues a **new** access token
+   and a **new** refresh token (rotation), invalidating the previous refresh
+   token so a leaked token cannot be reused.
+
+## Configuration
+
+```
+JWT_SECRET=<secret>
+JWT_EXPIRES_IN=15m
+JWT_REFRESH_EXPIRES_IN=7d
+```
+
+Keeping the access token short-lived limits the blast radius of a stolen token,
+while refresh-token rotation provides reuse detection.
diff --git a/docs/wallet-verification.md b/docs/wallet-verification.md
new file mode 100644
index 0000000..cfed50c
--- /dev/null
+++ b/docs/wallet-verification.md
@@ -0,0 +1,24 @@
+# Wallet Signature Verification
+
+Proves that a user controls the Stellar wallet they are linking to their
+account, using a sign-a-challenge flow. Helpers live in
+`src/common/wallet/wallet-signature.util.ts`.
+
+## Flow
+
+1. **Challenge** — the client requests a challenge for a public key
+   (`GET /auth/wallet/challenge?publicKey=G...`). The server returns a random,
+   single-use challenge string (`generateWalletChallenge()`) and stores it
+   briefly against the session/user.
+2. **Sign** — the wallet signs the challenge with its secret key.
+3. **Verify** — the client submits the challenge and the base64 signature
+   (`POST /auth/wallet/verify`). The server calls `verifyWalletSignature()`,
+   which checks the signature against the public key.
+4. **Associate** — on success the wallet's public key is associated with the
+   authenticated user account.
+
+## Notes
+
+- Challenges are single-use and short-lived to prevent replay.
+- Verification never requires the user's secret key — only the public key,
+  challenge and signature.
diff --git a/src/auth/account-lockout.constants.ts b/src/auth/account-lockout.constants.ts
new file mode 100644
index 0000000..4e5ec44
--- /dev/null
+++ b/src/auth/account-lockout.constants.ts
@@ -0,0 +1,25 @@
+/**
+ * Configuration for progressive account lockout after repeated failed logins.
+ * See `docs/security/account-lockout.md`.
+ */
+
+/** Number of consecutive failed attempts before the first lockout kicks in. */
+export const MAX_FAILED_ATTEMPTS = 5;
+
+/** Base lockout duration (ms) applied at the threshold. */
+export const BASE_LOCKOUT_MS = 60_000; // 1 minute
+
+/** Upper bound on the lockout duration (ms). */
+export const MAX_LOCKOUT_MS = 60 * 60_000; // 1 hour
+
+/**
+ * Progressive lockout: each additional failed attempt beyond the threshold
+ * doubles the lockout duration, capped at {@link MAX_LOCKOUT_MS}.
+ */
+export function getLockoutDurationMs(failedAttempts: number): number {
+  if (failedAttempts < MAX_FAILED_ATTEMPTS) {
+    return 0;
+  }
+  const over = failedAttempts - MAX_FAILED_ATTEMPTS;
+  return Math.min(BASE_LOCKOUT_MS * 2 ** over, MAX_LOCKOUT_MS);
+}
diff --git a/src/auth/auth.module.ts b/src/auth/auth.module.ts
index 22fd991..97706dd 100644
--- a/src/auth/auth.module.ts
+++ b/src/auth/auth.module.ts
@@ -18,8 +18,10 @@ import { RedisModule } from '../redis/redis.module';
       useFactory: (configService: ConfigService) => ({
         secret: configService.get<string>('JWT_SECRET'),
         signOptions: {
+          // Short-lived access token. Long-lived refresh tokens are issued
+          // separately (JWT_REFRESH_EXPIRES_IN). See docs/security/jwt-strategy.md.
           expiresIn: (configService.get<string>('JWT_EXPIRES_IN') ||
-            '1d') as any,
+            '15m') as any,
         },
       }),
       inject: [ConfigService],
diff --git a/src/common/security/csrf.util.ts b/src/common/security/csrf.util.ts
new file mode 100644
index 0000000..346cf90
--- /dev/null
+++ b/src/common/security/csrf.util.ts
@@ -0,0 +1,29 @@
+import * as crypto from 'crypto';
+
+/**
+ * Minimal helpers for CSRF token generation and validation.
+ *
+ * A random token is issued to the client (e.g. in a cookie) and must be echoed
+ * back in a request header (`x-csrf-token`) for state-changing requests. The
+ * comparison is constant-time to avoid leaking token bytes via timing.
+ * See `docs/security/csrf.md`.
+ */
+
+/** Generate a new random CSRF token (hex-encoded, 32 bytes). */
+export function generateCsrfToken(): string {
+  return crypto.randomBytes(32).toString('hex');
+}
+
+/**
+ * Validate a submitted CSRF token against the expected token using a
+ * constant-time comparison.
+ */
+export function validateCsrfToken(
+  expected: string | undefined,
+  actual: string | undefined,
+): boolean {
+  if (!expected || !actual || expected.length !== actual.length) {
+    return false;
+  }
+  return crypto.timingSafeEqual(Buffer.from(expected), Buffer.from(actual));
+}
diff --git a/src/common/wallet/wallet-signature.util.ts b/src/common/wallet/wallet-signature.util.ts
new file mode 100644
index 0000000..74f91b7
--- /dev/null
+++ b/src/common/wallet/wallet-signature.util.ts
@@ -0,0 +1,36 @@
+import { Keypair } from '@stellar/stellar-sdk';
+import * as crypto from 'crypto';
+
+/**
+ * Helpers for verifying that a user controls the Stellar wallet they claim.
+ *
+ * Flow: the server issues a random challenge, the user signs it with their
+ * wallet secret key, and the server verifies the signature against the wallet's
+ * public key. See `docs/wallet-verification.md`.
+ */
+
+/** Generate a random challenge string for the user to sign. */
+export function generateWalletChallenge(): string {
+  return `stellar-aid-verify:${crypto.randomBytes(24).toString('hex')}`;
+}
+
+/**
+ * Verify that `signatureBase64` is a valid signature of `challenge` produced by
+ * the secret key corresponding to `publicKey` (a Stellar `G...` address).
+ *
+ * @returns `true` when the signature is valid, `false` otherwise (including on
+ * a malformed public key or signature).
+ */
+export function verifyWalletSignature(
+  publicKey: string,
+  challenge: string,
+  signatureBase64: string,
+): boolean {
+  try {
+    const keypair = Keypair.fromPublicKey(publicKey);
+    const signature = Buffer.from(signatureBase64, 'base64');
+    return keypair.verify(Buffer.from(challenge, 'utf8'), signature);
+  } catch {
+    return false;
+  }
+}