diff --git a/server/src/__tests__/board-key-auth-middleware.test.ts b/server/src/__tests__/board-key-auth-middleware.test.ts
index 7413bd02ad..46af6019dd 100644
--- a/server/src/__tests__/board-key-auth-middleware.test.ts
+++ b/server/src/__tests__/board-key-auth-middleware.test.ts
@@ -15,6 +15,10 @@ import {
   resetBoardKeyAuthFailureRateLimitForTests,
 } from "../middleware/auth.js";
 import { errorHandler } from "../middleware/error-handler.js";
+import {
+  BOARD_KEY_AUTH_FAILURE_RATE_LIMIT_DEFAULTS,
+  createBoardKeyAuthFailureRateLimiter,
+} from "../security/board-key-auth-failure-rate-limit.js";
 
 const TOKEN = "pcp_board_middleware_valid_token";
 
@@ -168,6 +172,69 @@ describe("board-key authentication middleware", () => {
     expect(JSON.stringify(audits)).not.toContain(`${TOKEN}bad`);
   });
 
+  it("rate-limits rotating invalid credentials from one source before another database lookup", async () => {
+    const { db, state } = createDbState();
+    state.keyExists = false;
+    const { app } = createApp(db);
+    const sourceLimit = BOARD_KEY_AUTH_FAILURE_RATE_LIMIT_DEFAULTS.sourceLimit;
+
+    for (let index = 0; index < sourceLimit; index += 1) {
+      const response = await request(app)
+        .get("/actor")
+        .set("Authorization", `Bearer pcp_board_rotating_${index}`);
+      expect(response.status).toBe(401);
+    }
+    const lookupsBeforeThrottle = db.select.mock.calls.length;
+    const limited = await request(app)
+      .get("/actor")
+      .set("Authorization", "Bearer pcp_board_rotating_blocked");
+
+    expect(limited.status).toBe(429);
+    expect(db.select.mock.calls.length).toBe(lookupsBeforeThrottle);
+  });
+
+  it("strictly bounds credential and source failure storage during identity floods", () => {
+    const limiter = createBoardKeyAuthFailureRateLimiter({
+      windowMs: 60_000,
+      credentialLimit: 10_000,
+      sourceLimit: 10_000,
+      globalLimit: 10_000,
+      credentialMaxEntries: 7,
+      sourceMaxEntries: 5,
+    });
+
+    for (let index = 0; index < 100; index += 1) {
+      limiter.recordFailure({
+        credentialId: `credential-${index}`,
+        sourceId: `source-${index}`,
+        now: 1,
+      });
+    }
+
+    expect(limiter.snapshot()).toEqual({
+      credentialEntries: 7,
+      sourceEntries: 5,
+      globalEntries: 1,
+    });
+  });
+
+  it("opens the global circuit breaker across rotating sources and credentials", () => {
+    const limiter = createBoardKeyAuthFailureRateLimiter({
+      windowMs: 60_000,
+      credentialLimit: 100,
+      sourceLimit: 100,
+      globalLimit: 3,
+      credentialMaxEntries: 10,
+      sourceMaxEntries: 10,
+    });
+
+    for (let index = 0; index < 3; index += 1) {
+      limiter.recordFailure({ credentialId: `credential-${index}`, sourceId: `source-${index}`, now: 1 });
+    }
+
+    expect(limiter.isLimited({ credentialId: "novel-credential", sourceId: "novel-source", now: 1 })).toBe(true);
+  });
+
   it("does not resurrect last-used state when revocation wins the authentication race", async () => {
     const { db, state } = createDbState();
     state.revokeOnTouch = true;
diff --git a/server/src/middleware/auth.ts b/server/src/middleware/auth.ts
index ee02ef7407..17a1688295 100644
--- a/server/src/middleware/auth.ts
+++ b/server/src/middleware/auth.ts
@@ -57,6 +57,7 @@ function pruneCloudTenantWriteDebounce(
 import { instanceSettingsService } from "../services/instance-settings.js";
 import { ensureHumanRoleDefaultGrants } from "../services/principal-access-compatibility.js";
 import { forbidden, tooManyRequests, unauthorized, unprocessable } from "../errors.js";
+import { createBoardKeyAuthFailureRateLimiter } from "../security/board-key-auth-failure-rate-limit.js";
 
 export { isCloudManagedInstance } from "../services/cloud-instance.js";
 
@@ -64,27 +65,14 @@ function hashToken(token: string) {
   return createHash("sha256").update(token).digest("hex");
 }
 
-const BOARD_KEY_AUTH_FAILURE_WINDOW_MS = 60_000;
-const BOARD_KEY_AUTH_FAILURE_LIMIT = 10;
-const boardKeyAuthFailures = new Map<string, { count: number; resetAt: number }>();
+const boardKeyAuthFailureRateLimiter = createBoardKeyAuthFailureRateLimiter();
 
-function recordBoardKeyAuthFailure(token: string, now = Date.now()) {
-  const identity = hashToken(token);
-  const existing = boardKeyAuthFailures.get(identity);
-  const entry = !existing || existing.resetAt <= now
-    ? { count: 1, resetAt: now + BOARD_KEY_AUTH_FAILURE_WINDOW_MS }
-    : { count: existing.count + 1, resetAt: existing.resetAt };
-  boardKeyAuthFailures.set(identity, entry);
-  if (boardKeyAuthFailures.size > 10_000) {
-    for (const [key, value] of boardKeyAuthFailures) {
-      if (value.resetAt <= now) boardKeyAuthFailures.delete(key);
-    }
-  }
-  return entry.count > BOARD_KEY_AUTH_FAILURE_LIMIT;
+function boardKeyAuthFailureSource(req: Request) {
+  return req.socket.remoteAddress || req.ip || "unknown";
 }
 
 export function resetBoardKeyAuthFailureRateLimitForTests() {
-  boardKeyAuthFailures.clear();
+  boardKeyAuthFailureRateLimiter.reset();
 }
 
 async function auditBoardKeyAuthenticationFailure(
@@ -356,10 +344,22 @@ export function actorMiddleware(db: Db, opts: ActorMiddlewareOptions): RequestHa
     }
 
     if (token.startsWith("pcp_board_")) {
+      const failureIdentity = {
+        credentialId: hashToken(token),
+        sourceId: boardKeyAuthFailureSource(req),
+      };
+      if (boardKeyAuthFailureRateLimiter.isLimited(failureIdentity)) {
+        next(tooManyRequests("Too many authentication failures"));
+        return;
+      }
       const authentication = await boardAuth.authenticateBoardApiKey(token);
       if (!authentication.ok) {
         await auditBoardKeyAuthenticationFailure(db, req, authentication);
-        next(recordBoardKeyAuthFailure(token) ? tooManyRequests("Too many authentication failures") : unauthorized());
+        next(
+          boardKeyAuthFailureRateLimiter.recordFailure(failureIdentity)
+            ? tooManyRequests("Too many authentication failures")
+            : unauthorized(),
+        );
         return;
       }
       const { key: boardKey, access, scopeConfig } = authentication;
diff --git a/server/src/security/board-key-auth-failure-rate-limit.ts b/server/src/security/board-key-auth-failure-rate-limit.ts
new file mode 100644
index 0000000000..fa73ae8b4d
--- /dev/null
+++ b/server/src/security/board-key-auth-failure-rate-limit.ts
@@ -0,0 +1,120 @@
+export type BoardKeyAuthFailureRateLimitConfig = {
+  windowMs: number;
+  credentialLimit: number;
+  sourceLimit: number;
+  globalLimit: number;
+  credentialMaxEntries: number;
+  sourceMaxEntries: number;
+};
+
+export const BOARD_KEY_AUTH_FAILURE_RATE_LIMIT_DEFAULTS = {
+  windowMs: 60_000,
+  credentialLimit: 10,
+  sourceLimit: 50,
+  globalLimit: 1_000,
+  credentialMaxEntries: 4_096,
+  sourceMaxEntries: 1_024,
+} satisfies BoardKeyAuthFailureRateLimitConfig;
+
+type FailureEntry = { count: number; resetAt: number };
+
+class BoundedFixedWindowFailures {
+  private readonly entries = new Map<string, FailureEntry>();
+
+  constructor(
+    private readonly limit: number,
+    private readonly windowMs: number,
+    private readonly maxEntries: number,
+  ) {}
+
+  isLimited(identity: string, now: number) {
+    const entry = this.entries.get(identity);
+    if (!entry) return false;
+    if (entry.resetAt <= now) {
+      this.entries.delete(identity);
+      return false;
+    }
+    return entry.count >= this.limit;
+  }
+
+  record(identity: string, now: number) {
+    const existing = this.entries.get(identity);
+    const entry = !existing || existing.resetAt <= now
+      ? { count: 1, resetAt: now + this.windowMs }
+      : { count: existing.count + 1, resetAt: existing.resetAt };
+
+    // Refresh insertion order so capacity eviction removes the least recently
+    // used identity. The store remains bounded even during unique-token floods.
+    if (existing) this.entries.delete(identity);
+    this.ensureCapacity(now);
+    this.entries.set(identity, entry);
+    return entry.count > this.limit;
+  }
+
+  clear() {
+    this.entries.clear();
+  }
+
+  get size() {
+    return this.entries.size;
+  }
+
+  private ensureCapacity(now: number) {
+    if (this.entries.size < this.maxEntries) return;
+    for (const [identity, entry] of this.entries) {
+      if (entry.resetAt <= now) this.entries.delete(identity);
+    }
+    while (this.entries.size >= this.maxEntries) {
+      const oldestIdentity = this.entries.keys().next().value;
+      if (oldestIdentity === undefined) break;
+      this.entries.delete(oldestIdentity);
+    }
+  }
+}
+
+export function createBoardKeyAuthFailureRateLimiter(
+  config: BoardKeyAuthFailureRateLimitConfig = BOARD_KEY_AUTH_FAILURE_RATE_LIMIT_DEFAULTS,
+) {
+  const credentials = new BoundedFixedWindowFailures(
+    config.credentialLimit,
+    config.windowMs,
+    config.credentialMaxEntries,
+  );
+  const sources = new BoundedFixedWindowFailures(
+    config.sourceLimit,
+    config.windowMs,
+    config.sourceMaxEntries,
+  );
+  const global = new BoundedFixedWindowFailures(config.globalLimit, config.windowMs, 1);
+
+  return {
+    isLimited(input: { credentialId: string; sourceId: string; now?: number }) {
+      const now = input.now ?? Date.now();
+      return global.isLimited("global", now)
+        || sources.isLimited(input.sourceId, now)
+        || credentials.isLimited(input.credentialId, now);
+    },
+
+    recordFailure(input: { credentialId: string; sourceId: string; now?: number }) {
+      const now = input.now ?? Date.now();
+      const globalLimited = global.record("global", now);
+      const sourceLimited = sources.record(input.sourceId, now);
+      const credentialLimited = credentials.record(input.credentialId, now);
+      return globalLimited || sourceLimited || credentialLimited;
+    },
+
+    reset() {
+      credentials.clear();
+      sources.clear();
+      global.clear();
+    },
+
+    snapshot() {
+      return {
+        credentialEntries: credentials.size,
+        sourceEntries: sources.size,
+        globalEntries: global.size,
+      };
+    },
+  };
+}