diff --git a/packages/auth/src/application/use-cases/sign-out.use-case.test.ts b/packages/auth/src/application/use-cases/sign-out.use-case.test.ts
index 8ffc8d8..125fb49 100644
--- a/packages/auth/src/application/use-cases/sign-out.use-case.test.ts
+++ b/packages/auth/src/application/use-cases/sign-out.use-case.test.ts
@@ -2,6 +2,8 @@ import { describe, it, expect } from "vitest";
 import { signOutUseCase } from "@/application/use-cases/sign-out.use-case";
 import { MockUsersRepository } from "@/infrastructure/repositories/users.repository.mock";
 import { MockAuthenticationService } from "@/infrastructure/services/authentication.service.mock";
+import { UnauthenticatedError } from "@/entities/errors/auth";
+import { userFactory } from "@/__factories__/user.factory";
 
 describe("signOutUseCase", () => {
   it("returns void on successful sign-out", async () => {
@@ -12,4 +14,22 @@ describe("signOutUseCase", () => {
     const result = await useCase({ sessionId: "session_1" });
     expect(result).toBeUndefined();
   });
+
+  it("revokes the session server-side: validateSession rejects it afterwards", async () => {
+    const users = new MockUsersRepository([]);
+    const auth = new MockAuthenticationService(users);
+    const user = userFactory.build({ username: "alice" });
+    await users.createUser(user);
+    const { session } = await auth.createSession(user);
+
+    // Sanity: session is valid before sign-out.
+    await expect(auth.validateSession(session.id)).resolves.toBeDefined();
+
+    await signOutUseCase(auth)({ sessionId: session.id });
+
+    // B5: sign-out must invalidate server-side, not just clear the cookie.
+    await expect(auth.validateSession(session.id)).rejects.toBeInstanceOf(
+      UnauthenticatedError,
+    );
+  });
 });
diff --git a/packages/auth/src/infrastructure/services/authentication.service.ts b/packages/auth/src/infrastructure/services/authentication.service.ts
index 8451824..11372a2 100644
--- a/packages/auth/src/infrastructure/services/authentication.service.ts
+++ b/packages/auth/src/infrastructure/services/authentication.service.ts
@@ -4,6 +4,7 @@ import type { IAuthenticationService } from "../../application/services/authenti
 import type { Cookie } from "../../entities/models/cookie";
 import type { Session } from "../../entities/models/session";
 import type { User } from "../../entities/models/user";
+import { InMemorySessionDenylist } from "./session-denylist";
 
 const SALT_LENGTH = 16;
 const KEY_LENGTH = 64;
@@ -15,7 +16,12 @@ const COOKIE_NAME = "payload-token";
 const SESSION_DURATION_SECONDS = 7200; // 2 hours (matches Payload default)
 
 export class AuthenticationService implements IAuthenticationService {
-  constructor(private config: SanitizedConfig) {}
+  constructor(
+    private config: SanitizedConfig,
+    // Server-side revocation (audit finding B5). In-memory: revocations are
+    // per-process — see session-denylist.ts for the limitation write-up.
+    private denylist: InMemorySessionDenylist = new InMemorySessionDenylist(),
+  ) {}
 
   generateUserId(): string {
     return crypto.randomUUID();
@@ -69,10 +75,13 @@ export class AuthenticationService implements IAuthenticationService {
     const payload = await getPayload({ config: this.config });
     const expiresAt = new Date(Date.now() + SESSION_DURATION_SECONDS * 1000);
 
-    const token = this.signToken(user.id, payload.secret);
+    // The session id doubles as the JWT `jti` so the token can be revoked
+    // server-side via the denylist (audit finding B5).
+    const sessionId = crypto.randomUUID();
+    const token = this.signToken(user.id, sessionId, payload.secret);
 
     const session: Session = {
-      id: crypto.randomUUID(),
+      id: sessionId,
       userId: user.id,
       expiresAt,
     };
@@ -97,6 +106,11 @@ export class AuthenticationService implements IAuthenticationService {
     const payload = await getPayload({ config: this.config });
     const decoded = this.verifyToken(token, payload.secret);
     if (!decoded) throw new Error("Invalid or expired session token");
+    // Server-side revocation check (audit finding B5): a signed, unexpired
+    // token is still rejected once its jti has been invalidated.
+    if (this.denylist.isRevoked(decoded.jti)) {
+      throw new Error("Session has been revoked");
+    }
 
     const userDoc = await payload.findByID({
       collection: "users" as const,
@@ -111,7 +125,7 @@ export class AuthenticationService implements IAuthenticationService {
     };
 
     const session: Session = {
-      id: token,
+      id: decoded.jti,
       userId: user.id,
       expiresAt: new Date(decoded.exp * 1000),
     };
@@ -119,9 +133,11 @@ export class AuthenticationService implements IAuthenticationService {
     return { user, session };
   }
 
-  async invalidateSession(
-    _sessionId: string,
-  ): Promise<{ blankCookie: Cookie }> {
+  async invalidateSession(sessionId: string): Promise<{ blankCookie: Cookie }> {
+    // `sessionId` is the JWT `jti` (the `session.id` returned by
+    // createSession/validateSession). Denylist it for the maximum token
+    // lifetime — beyond that, the token's own `exp` rejects it (B5).
+    this.denylist.revoke(sessionId, SESSION_DURATION_SECONDS);
     return {
       blankCookie: {
         name: COOKIE_NAME,
@@ -138,13 +154,13 @@ export class AuthenticationService implements IAuthenticationService {
   }
 
   /** Sign a HS256 JWT using Payload's instance secret. No external dependency. */
-  private signToken(userId: string, secret: string): string {
+  private signToken(userId: string, jti: string, secret: string): string {
     const header = Buffer.from(
       JSON.stringify({ alg: "HS256", typ: "JWT" }),
     ).toString("base64url");
     const exp = Math.floor(Date.now() / 1000) + SESSION_DURATION_SECONDS;
     const body = Buffer.from(
-      JSON.stringify({ id: userId, collection: "users", exp }),
+      JSON.stringify({ id: userId, collection: "users", exp, jti }),
     ).toString("base64url");
     const signature = crypto
       .createHmac("sha256", secret)
@@ -157,22 +173,38 @@ export class AuthenticationService implements IAuthenticationService {
   private verifyToken(
     token: string,
     secret: string,
-  ): { id: string; exp: number } | null {
+  ): { id: string; exp: number; jti: string } | null {
     const parts = token.split(".");
     if (parts.length !== 3) return null;
     const [header, body, signature] = parts as [string, string, string];
     const expected = crypto
       .createHmac("sha256", secret)
       .update(`${header}.${body}`)
-      .digest("base64url");
-    if (signature !== expected) return null;
+      .digest();
+    const provided = Buffer.from(signature, "base64url");
+    // Constant-time comparison, mirroring verifyPassword (audit finding B4).
+    // timingSafeEqual requires equal-length buffers; a length mismatch is
+    // already an invalid signature, and the guard leaks nothing an attacker
+    // does not know (the expected HMAC-SHA256 length is public).
+    if (provided.length !== expected.length) return null;
+    if (!crypto.timingSafeEqual(provided, expected)) return null;
     try {
       const decoded = JSON.parse(Buffer.from(body, "base64url").toString()) as {
-        id: string;
-        exp: number;
+        id?: unknown;
+        exp?: unknown;
+        jti?: unknown;
       };
+      // Fail closed: tokens without a jti cannot be revoked, so they are
+      // not accepted (audit finding B5).
+      if (
+        typeof decoded.id !== "string" ||
+        typeof decoded.exp !== "number" ||
+        typeof decoded.jti !== "string"
+      ) {
+        return null;
+      }
       if (decoded.exp < Math.floor(Date.now() / 1000)) return null;
-      return decoded;
+      return { id: decoded.id, exp: decoded.exp, jti: decoded.jti };
     } catch {
       return null;
     }
diff --git a/packages/auth/src/infrastructure/services/session-denylist.test.ts b/packages/auth/src/infrastructure/services/session-denylist.test.ts
new file mode 100644
index 0000000..8b308aa
--- /dev/null
+++ b/packages/auth/src/infrastructure/services/session-denylist.test.ts
@@ -0,0 +1,44 @@
+import { describe, it, expect } from "vitest";
+import { InMemorySessionDenylist } from "@/infrastructure/services/session-denylist";
+
+describe("InMemorySessionDenylist", () => {
+  it("reports a revoked jti as revoked", () => {
+    const denylist = new InMemorySessionDenylist();
+    denylist.revoke("jti-1", 60);
+    expect(denylist.isRevoked("jti-1")).toBe(true);
+  });
+
+  it("does not report unknown jtis as revoked", () => {
+    const denylist = new InMemorySessionDenylist();
+    expect(denylist.isRevoked("never-seen")).toBe(false);
+  });
+
+  it("prunes entries after their ttl elapses", () => {
+    let now = 1_000_000;
+    const denylist = new InMemorySessionDenylist(() => now);
+    denylist.revoke("jti-1", 60);
+    expect(denylist.isRevoked("jti-1")).toBe(true);
+
+    now += 60_000; // exactly at expiry — entry is prunable
+    expect(denylist.isRevoked("jti-1")).toBe(false);
+  });
+
+  it("keeps entries alive until the ttl elapses", () => {
+    let now = 1_000_000;
+    const denylist = new InMemorySessionDenylist(() => now);
+    denylist.revoke("jti-1", 60);
+    now += 59_999;
+    expect(denylist.isRevoked("jti-1")).toBe(true);
+  });
+
+  it("prunes expired entries on revoke, not just on reads", () => {
+    let now = 1_000_000;
+    const denylist = new InMemorySessionDenylist(() => now);
+    denylist.revoke("old", 1);
+    now += 5_000;
+    denylist.revoke("new", 60);
+    // Reach into nothing — observable via isRevoked semantics only.
+    expect(denylist.isRevoked("old")).toBe(false);
+    expect(denylist.isRevoked("new")).toBe(true);
+  });
+});
diff --git a/packages/auth/src/infrastructure/services/session-denylist.ts b/packages/auth/src/infrastructure/services/session-denylist.ts
new file mode 100644
index 0000000..6f40655
--- /dev/null
+++ b/packages/auth/src/infrastructure/services/session-denylist.ts
@@ -0,0 +1,46 @@
+/**
+ * In-memory JWT `jti` denylist backing server-side session revocation
+ * (audit finding B5).
+ *
+ * `AuthenticationService.createSession` mints a session id and embeds it in
+ * the JWT as `jti`; `invalidateSession(jti)` records it here and
+ * `validateSession` rejects any token whose `jti` is denylisted. Entries
+ * expire with the token they revoke (max session lifetime), so the map is
+ * self-pruning and cannot grow past the number of sign-outs per lifetime
+ * window.
+ *
+ * SINGLE-PROCESS LIMITATION: this denylist lives in process memory. It is
+ * correct for a single server process (the template's deployment shape) but
+ * revocations are NOT shared across processes/instances and do not survive
+ * restarts — a restarted process accepts a signed, unexpired token again.
+ * Multi-instance deployments must swap this for a shared store (Redis, DB)
+ * behind the same two methods.
+ */
+export class InMemorySessionDenylist {
+  /** jti -> epoch-ms after which the entry may be pruned. */
+  private readonly revoked = new Map<string, number>();
+
+  constructor(private readonly clock: () => number = () => Date.now()) {}
+
+  /**
+   * Record a revoked `jti`. `ttlSeconds` should be the maximum remaining
+   * token lifetime — after that, the token's own `exp` rejects it anyway.
+   */
+  revoke(jti: string, ttlSeconds: number): void {
+    this.prune();
+    this.revoked.set(jti, this.clock() + ttlSeconds * 1000);
+  }
+
+  isRevoked(jti: string): boolean {
+    this.prune();
+    return this.revoked.has(jti);
+  }
+
+  /** Expiry-based pruning — runs on every access; the map stays small. */
+  private prune(): void {
+    const now = this.clock();
+    for (const [jti, expiresAt] of this.revoked) {
+      if (expiresAt <= now) this.revoked.delete(jti);
+    }
+  }
+}