diff --git a/packages/auth/src/application/use-cases/sign-up.use-case.test.ts b/packages/auth/src/application/use-cases/sign-up.use-case.test.ts
index a3ed0c2..6f89aea 100644
--- a/packages/auth/src/application/use-cases/sign-up.use-case.test.ts
+++ b/packages/auth/src/application/use-cases/sign-up.use-case.test.ts
@@ -131,6 +131,43 @@ describe("signUpUseCase", () => {
     expect(result.clearCookie?.attributes.maxAge).toBe(0);
   });
 
+  it("drops unknown categories from the client-controlled cookie (A12)", async () => {
+    const users = new MockUsersRepository([]);
+    const auth = new MockAuthenticationService(users);
+    const bus = new RecordingEventBus();
+    const consent = new RecordingConsent();
+    const consentFactory = (_userId: string) => Promise.resolve(consent);
+    const useCase = signUpUseCase(users, auth, bus, consentFactory);
+
+    await useCase({
+      username: "ivy",
+      password: "secret_password",
+      confirmPassword: "secret_password",
+      cookieHeader: "cc_consent=analytics,evil-made-up,__proto__; session=x",
+    });
+
+    expect(consent.grants.map((g) => g.category)).toEqual(["analytics"]);
+  });
+
+  it("does not migrate consent when every cookie category is unknown (A12)", async () => {
+    const users = new MockUsersRepository([]);
+    const auth = new MockAuthenticationService(users);
+    const bus = new RecordingEventBus();
+    const consent = new RecordingConsent();
+    const consentFactory = (_userId: string) => Promise.resolve(consent);
+    const useCase = signUpUseCase(users, auth, bus, consentFactory);
+
+    const result = await useCase({
+      username: "jack",
+      password: "secret_password",
+      confirmPassword: "secret_password",
+      cookieHeader: "cc_consent=hax,not-a-category",
+    });
+
+    expect(consent.grants).toHaveLength(0);
+    expect(result.clearCookie).toBeUndefined();
+  });
+
   it("does not migrate consent when no cc_consent cookie is present", async () => {
     const users = new MockUsersRepository([]);
     const auth = new MockAuthenticationService(users);
diff --git a/packages/auth/src/application/use-cases/sign-up.use-case.ts b/packages/auth/src/application/use-cases/sign-up.use-case.ts
index 279abee..cec6dcd 100644
--- a/packages/auth/src/application/use-cases/sign-up.use-case.ts
+++ b/packages/auth/src/application/use-cases/sign-up.use-case.ts
@@ -14,6 +14,16 @@ import type { IAuthenticationService } from "../services/authentication.service.
 // Cookie name written by the anonymous consent banner (mirrors CONSENT_COOKIE_NAME in @repo/core-consent).
 const ANONYMOUS_CONSENT_COOKIE = "cc_consent";
 
+// Category allow-list (mirrors KNOWN_CONSENT_CATEGORIES in @repo/core-consent).
+// The cookie is client-controlled: unknown strings are dropped, never granted
+// (audit finding A12).
+const KNOWN_CONSENT_CATEGORIES = [
+  "necessary",
+  "functional",
+  "analytics",
+  "marketing",
+];
+
 function extractConsentFromCookieHeader(cookieHeader: string): string[] | null {
   for (const part of cookieHeader.split(";")) {
     const eqIdx = part.indexOf("=");
@@ -24,7 +34,8 @@ function extractConsentFromCookieHeader(cookieHeader: string): string[] | null {
     const cats = value
       .split(",")
       .map((c) => c.trim())
-      .filter(Boolean);
+      .filter(Boolean)
+      .filter((c) => KNOWN_CONSENT_CATEGORIES.includes(c));
     return cats.length > 0 ? cats : null;
   }
   return null;
diff --git a/packages/core-consent/src/consent-types.ts b/packages/core-consent/src/consent-types.ts
index 053dc4b..afdeec5 100644
--- a/packages/core-consent/src/consent-types.ts
+++ b/packages/core-consent/src/consent-types.ts
@@ -10,6 +10,27 @@ export type ConsentCategory =
   | "marketing"
   | (string & {});
 
+/**
+ * The known consent categories (audit finding A12). Untrusted inputs — e.g.
+ * the anonymous banner cookie migrated at sign-up — MUST be validated against
+ * this list before being granted; the open ConsentCategory union is for
+ * first-party code registering custom categories deliberately, not for
+ * client-controlled strings.
+ */
+export const KNOWN_CONSENT_CATEGORIES = [
+  "necessary",
+  "functional",
+  "analytics",
+  "marketing",
+] as const;
+
+/** Type guard for the allow-list above. */
+export function isKnownConsentCategory(
+  value: string,
+): value is (typeof KNOWN_CONSENT_CATEGORIES)[number] {
+  return (KNOWN_CONSENT_CATEGORIES as readonly string[]).includes(value);
+}
+
 /** Whether a subject has granted or denied consent for a category. */
 export type ConsentState = "granted" | "denied" | "pending";
 
diff --git a/packages/core-consent/src/index.ts b/packages/core-consent/src/index.ts
index de08935..f7dc110 100644
--- a/packages/core-consent/src/index.ts
+++ b/packages/core-consent/src/index.ts
@@ -4,6 +4,10 @@ export type {
   UserConsentState,
   ConsentGrantMeta,
 } from "./consent-types";
+export {
+  KNOWN_CONSENT_CATEGORIES,
+  isKnownConsentCategory,
+} from "./consent-types";
 export type { IConsent } from "./consent.interface";
 export type { ConsentChecked } from "./with-consent";
 export { withConsent } from "./with-consent";
diff --git a/packages/core-consent/src/migration.test.ts b/packages/core-consent/src/migration.test.ts
index 2236061..42d7897 100644
--- a/packages/core-consent/src/migration.test.ts
+++ b/packages/core-consent/src/migration.test.ts
@@ -43,6 +43,21 @@ describe("extractAnonymousConsent", () => {
   });
 });
 
+describe("extractAnonymousConsent — category allow-list (A12)", () => {
+  it("drops unknown categories from the client-controlled cookie", () => {
+    const result = extractAnonymousConsent(
+      `${CONSENT_COOKIE_NAME}=necessary,evil-injection,analytics`,
+    );
+    expect(result).toEqual(["necessary", "analytics"]);
+  });
+
+  it("returns null when every category is unknown", () => {
+    expect(
+      extractAnonymousConsent(`${CONSENT_COOKIE_NAME}=hax,__proto__`),
+    ).toBeNull();
+  });
+});
+
 describe("migrateAnonymousConsent", () => {
   it("calls IConsent.grant with method signup-migration for each category", async () => {
     const consent = new RecordingConsent();
@@ -105,3 +120,17 @@ describe("migrateAnonymousConsent", () => {
     expect(consent.isGranted("marketing")).toBe(true);
   });
 });
+
+describe("migrateAnonymousConsent — category allow-list (A12)", () => {
+  it("never grants unknown categories even when passed directly", async () => {
+    const consent = new RecordingConsent();
+    await migrateAnonymousConsent({
+      consent,
+      cookieState: ["analytics", "totally-made-up", "marketing"],
+    });
+    expect(consent.grants.map((g) => g.category)).toEqual([
+      "analytics",
+      "marketing",
+    ]);
+  });
+});
diff --git a/packages/core-consent/src/migration.ts b/packages/core-consent/src/migration.ts
index b3f13d5..f4205f7 100644
--- a/packages/core-consent/src/migration.ts
+++ b/packages/core-consent/src/migration.ts
@@ -1,4 +1,8 @@
-import type { ConsentCategory, ConsentGrantMeta } from "./consent-types";
+import {
+  isKnownConsentCategory,
+  type ConsentCategory,
+  type ConsentGrantMeta,
+} from "./consent-types";
 import type { IConsent } from "./consent.interface";
 
 /** Cookie name written by the anonymous consent banner. */
@@ -11,6 +15,10 @@ export const CONSENT_COOKIE_NAME = "cc_consent";
  *
  * Expected cookie value format: comma-separated category names,
  * e.g. "necessary,analytics,marketing".
+ *
+ * The cookie is client-controlled, so values are validated against
+ * KNOWN_CONSENT_CATEGORIES (audit finding A12) — unknown strings are
+ * dropped rather than granted.
  */
 export function extractAnonymousConsent(
   cookieHeader: string,
@@ -21,7 +29,8 @@ export function extractAnonymousConsent(
   const categories = raw
     .split(",")
     .map((c) => c.trim())
-    .filter(Boolean) as ConsentCategory[];
+    .filter(Boolean)
+    .filter(isKnownConsentCategory) as ConsentCategory[];
   return categories.length > 0 ? categories : null;
 }
 
@@ -42,7 +51,9 @@ export async function migrateAnonymousConsent(opts: {
   const meta: ConsentGrantMeta = { method: "signup-migration" };
   if (bannerVersion !== undefined) meta.bannerVersion = bannerVersion;
   if (policyVersion !== undefined) meta.policyVersion = policyVersion;
-  for (const category of cookieState) {
+  // Defense in depth (A12): even a caller that bypassed
+  // extractAnonymousConsent cannot grant unknown categories.
+  for (const category of cookieState.filter(isKnownConsentCategory)) {
     await consent.grant(category, meta);
   }
 }