diff --git a/apps/api/src/routes/account.ts b/apps/api/src/routes/account.ts
index 2ae74e1..e3ba696 100644
--- a/apps/api/src/routes/account.ts
+++ b/apps/api/src/routes/account.ts
@@ -47,6 +47,12 @@ import { failure, success } from "../http"
 import { JudgeStatus } from "../judge/status"
 import { getBooleanOption } from "../services/options"
 import { getUserProfileById } from "../services/profile"
+import {
+  clientIp,
+  countAttempt,
+  lockoutRemaining,
+  type AttemptRule,
+} from "../services/throttling"
 import { localTime, weekStart } from "../time"
 import {
   isTeacherOrAbove,
@@ -57,6 +63,12 @@ import {
 
 export const accountRoutes = new Hono<AppEnv>()
 
+/**
+ * 注册限流：按 IP 每小时 100 个号，只数成功的。拦的是脚本批量造号；
+ * 机房一个班共用一个出口 IP，一节课全班现场注册也够用。
+ */
+const REGISTER_PER_IP: AttemptRule = { limit: 100, windowSeconds: 60 * 60 }
+
 accountRoutes.post("/users", async (c) => {
   const parsed = registerRequestSchema.safeParse(
     await c.req.json().catch(() => null),
@@ -72,6 +84,17 @@ accountRoutes.post("/users", async (c) => {
     )
   }
 
+  const registerKey = `register:ip:${clientIp(c)}`
+  const wait = await lockoutRemaining(registerKey, REGISTER_PER_IP)
+  if (wait !== null) {
+    return failure(
+      c,
+      429,
+      "too-many-registrations",
+      `Too many registrations, please wait ${wait} seconds`,
+    )
+  }
+
   const username = parsed.data.username.toLowerCase()
   const email = parsed.data.email.toLowerCase()
   const [duplicate] = await db
@@ -120,6 +143,7 @@ accountRoutes.post("/users", async (c) => {
       realName: null,
     })
   })
+  await countAttempt(registerKey, REGISTER_PER_IP)
   return success(c, { ok: true }, 201)
 })
 
diff --git a/apps/api/src/routes/auth.ts b/apps/api/src/routes/auth.ts
index afd3f51..a92f0ce 100644
--- a/apps/api/src/routes/auth.ts
+++ b/apps/api/src/routes/auth.ts
@@ -9,9 +9,28 @@ import { hashPassword, verifyPassword } from "../auth/password"
 import { db, schema } from "../db"
 import { failure, success } from "../http"
 import { getUserProfileById } from "../services/profile"
+import {
+  clientIp,
+  countAttempt,
+  lockoutRemaining,
+  resetAttempts,
+  type AttemptRule,
+} from "../services/throttling"
 
 export const authRoutes = new Hono<AppEnv>()
 
+/**
+ * 登录防爆破，两道都只数失败：
+ *
+ * - **按用户名**：盯着一个账号猜。学生的初始密码弱、`/classes/:name/usernames` 又能
+ *   公开列出整班用户名，这是最现实的攻击。15 分钟 10 次，登录成功清零。
+ *   代价是别人能故意输错把某个学生锁 15 分钟 —— 比被猜中密码好得多。
+ * - **按 IP**：换着用户名猜（每个号只试几次常见密码，绕开上一道）。机房一个班共用一个
+ *   出口 IP，所以放得很宽，而且成功**不清零**，否则攻击者拿自己的号登一次就能重置。
+ */
+const LOGIN_PER_USERNAME: AttemptRule = { limit: 10, windowSeconds: 15 * 60 }
+const LOGIN_PER_IP: AttemptRule = { limit: 100, windowSeconds: 15 * 60 }
+
 authRoutes.post("/auth/login", async (c) => {
   const parsed = loginRequestSchema.safeParse(
     await c.req.json().catch(() => null),
@@ -25,33 +44,46 @@ authRoutes.post("/auth/login", async (c) => {
     )
   }
 
+  const usernameKey = `login:user:${parsed.data.username.toLowerCase()}`
+  const ipKey = `login:ip:${clientIp(c)}`
+  const wait =
+    (await lockoutRemaining(usernameKey, LOGIN_PER_USERNAME)) ??
+    (await lockoutRemaining(ipKey, LOGIN_PER_IP))
+  if (wait !== null) {
+    return failure(
+      c,
+      429,
+      "too-many-login-attempts",
+      `Too many failed attempts, please wait ${wait} seconds`,
+    )
+  }
+  const loginFailed = async () => {
+    await Promise.all([
+      countAttempt(usernameKey, LOGIN_PER_USERNAME),
+      countAttempt(ipKey, LOGIN_PER_IP),
+    ])
+    return failure(
+      c,
+      401,
+      "invalid-credentials",
+      "Invalid username or password",
+    )
+  }
+
   const [user] = await db
     .select()
     .from(schema.user)
     .where(sql`lower(${schema.user.username}) = lower(${parsed.data.username})`)
     .limit(1)
 
-  if (!user) {
-    return failure(
-      c,
-      401,
-      "invalid-credentials",
-      "Invalid username or password",
-    )
-  }
+  if (!user) return loginFailed()
   if (user.isDisabled) {
     return failure(c, 403, "account-disabled", "Your account has been disabled")
   }
 
   const password = await verifyPassword(parsed.data.password, user.password)
-  if (!password.valid) {
-    return failure(
-      c,
-      401,
-      "invalid-credentials",
-      "Invalid username or password",
-    )
-  }
+  if (!password.valid) return loginFailed()
+  await resetAttempts(usernameKey)
 
   const now = new Date().toISOString()
   const update: { lastLogin: string; password?: string } = { lastLogin: now }
diff --git a/apps/api/src/routes/contest.ts b/apps/api/src/routes/contest.ts
index 75bcd4a..f7f8048 100644
--- a/apps/api/src/routes/contest.ts
+++ b/apps/api/src/routes/contest.ts
@@ -38,6 +38,11 @@ import {
   requireContestAccess,
   type ContestEnv,
 } from "../services/contest"
+import {
+  countAttempt,
+  lockoutRemaining,
+  type AttemptRule,
+} from "../services/throttling"
 import {
   objectValue,
   publicTemplates,
@@ -149,6 +154,8 @@ contestRoutes.get("/contests/:id", optionalAuth, async (c) => {
   )
 })
 
+const CONTEST_PASSWORD_RULE: AttemptRule = { limit: 10, windowSeconds: 10 * 60 }
+
 contestRoutes.post("/contests/:id/access", requireAuth, async (c) => {
   const contest = await findAccessibleContest(
     c.get("user"),
@@ -161,7 +168,20 @@ contestRoutes.post("/contests/:id/access", requireAuth, async (c) => {
   )
   if (!parsed.success)
     return failure(c, 400, "invalid-request", "Password is required")
+  // 比赛密码往往就是几位数字，不限的话一个脚本几分钟就能扫完。
+  // 按「人 × 比赛」计失败次数：猜错的是自己，锁的也只是自己进这一场。
+  const attemptKey = `contest-password:${contest.id}:${c.get("user")!.id}`
+  const wait = await lockoutRemaining(attemptKey, CONTEST_PASSWORD_RULE)
+  if (wait !== null) {
+    return failure(
+      c,
+      429,
+      "too-many-password-attempts",
+      `Too many wrong passwords, please wait ${wait} seconds`,
+    )
+  }
   if (!checkContestPassword(parsed.data.password, contest.password)) {
+    await countAttempt(attemptKey, CONTEST_PASSWORD_RULE)
     return failure(
       c,
       403,
diff --git a/apps/api/src/services/throttling.ts b/apps/api/src/services/throttling.ts
index 8b4eb62..d59ac7d 100644
--- a/apps/api/src/services/throttling.ts
+++ b/apps/api/src/services/throttling.ts
@@ -1,3 +1,5 @@
+import type { Context } from "hono"
+
 import { redis } from "../redis"
 import { getOptions } from "./options"
 
@@ -166,3 +168,50 @@ export async function consumeToken(
   if (Number(result[0]) === 1) return { allowed: true }
   return { allowed: false, wait: Number(result[1]) || 0 }
 }
+
+/**
+ * 计数锁定：固定窗口内计满 `limit` 次就锁到窗口结束。**什么时候计数由调用方决定**：
+ *
+ * - 登录、比赛密码只在**失败**时计数 —— 正常用户几乎不会连续输错，暴力猜解全是失败，
+ *   只数失败的话机房一个班同时登录也不会误伤；
+ * - 注册在**成功**时计数 —— 限的是批量造号。
+ *
+ * 和上面的令牌桶是两回事：令牌桶限的是资源消耗（判题沙箱、AI），带匀速回填；
+ * 这里是「窗口内满多少次就停」。用固定窗口而不是滑动窗口：一个 INCR + EXPIRE NX 就够，
+ * 锁定时长就是 key 的剩余 TTL，精度对防爆破足够了。
+ */
+export type AttemptRule = { limit: number; windowSeconds: number }
+
+/** 已锁定时返回还要等几秒，否则 null。只读，不计数。 */
+export async function lockoutRemaining(key: string, rule: AttemptRule) {
+  const [count, ttl] = await Promise.all([
+    redis.get(`attempts:${key}`),
+    redis.ttl(`attempts:${key}`),
+  ])
+  if (Number(count) < rule.limit) return null
+  // ttl 为 -1（没有过期时间）理论上不会出现，EXPIRE NX 总会补上；兜底按整窗算
+  return ttl > 0 ? ttl : rule.windowSeconds
+}
+
+export async function countAttempt(key: string, rule: AttemptRule) {
+  await redis
+    .multi()
+    .incr(`attempts:${key}`)
+    // NX：只在窗口里第一次计数时设过期，之后不续期 —— 否则攻击者持续猜就永远锁着，
+    // 真正的主人过了窗口也登不进来
+    .expire(`attempts:${key}`, rule.windowSeconds, "NX")
+    .exec()
+}
+
+export async function resetAttempts(key: string) {
+  await redis.del(`attempts:${key}`)
+}
+
+/**
+ * 客户端 IP。线上 api 端口不对外发布，只有 Caddy 连得到它，Caddy 用 `{client_ip}`
+ * **覆盖**写入 X-Real-IP（它只信任内网来源的 X-Forwarded-For，见 docker/Caddyfile），
+ * 所以这个头是可信的。本机 dev 走 Vite 代理没有这个头，统一落到一个桶里，无所谓。
+ */
+export function clientIp(c: Context) {
+  return c.req.header("x-real-ip") || "unknown"
+}
diff --git a/apps/web/src/oj/contest/detail.vue b/apps/web/src/oj/contest/detail.vue
index 15ae6de..bbfbc38 100644
--- a/apps/web/src/oj/contest/detail.vue
+++ b/apps/web/src/oj/contest/detail.vue
@@ -17,8 +17,13 @@ const { isDesktop } = useBreakpoints()
 const password = ref("")
 
 async function check() {
-  await contestStore.checkPassword(props.contestID, password.value)
-  if (!contestStore.access) {
+  const error = await contestStore.checkPassword(
+    props.contestID,
+    password.value,
+  )
+  if (error === "too-many-password-attempts") {
+    message.error("密码错误次数太多，请 10 分钟后再试")
+  } else if (!contestStore.access) {
     message.error("密码错误")
   }
 }
diff --git a/apps/web/src/oj/store/contest.ts b/apps/web/src/oj/store/contest.ts
index 058a184..e01fa29 100644
--- a/apps/web/src/oj/store/contest.ts
+++ b/apps/web/src/oj/store/contest.ts
@@ -87,6 +87,7 @@ export const useContestStore = defineStore("contest", () => {
     timer = 0
   }
 
+  /** 返回失败时的错误码（成功为 null），调用方据此给提示 */
   async function checkPassword(contestID: string, password: string) {
     try {
       const res = await checkContestPassword(contestID, password)
@@ -94,8 +95,10 @@ export const useContestStore = defineStore("contest", () => {
       if (res) {
         _getProblems(contestID)
       }
-    } catch (err) {
+      return null
+    } catch (err: any) {
       toggleAccess(false)
+      return (err?.error as string | undefined) ?? "unknown"
     }
   }
 
diff --git a/apps/web/src/shared/components/Login.vue b/apps/web/src/shared/components/Login.vue
index 7d42ad7..6d5223c 100644
--- a/apps/web/src/shared/components/Login.vue
+++ b/apps/web/src/shared/components/Login.vue
@@ -130,6 +130,8 @@ function submit() {
         authStore.setLoginError("此账号已被封禁")
       } else if (err.error === "invalid-credentials") {
         authStore.setLoginError("用户名或密码不正确")
+      } else if (err.error === "too-many-login-attempts") {
+        authStore.setLoginError("密码错误次数太多，请 15 分钟后再试")
       } else {
         authStore.setLoginError("无法登录")
       }
diff --git a/apps/web/src/shared/components/Signup.vue b/apps/web/src/shared/components/Signup.vue
index ab76b2b..4ef25b0 100644
--- a/apps/web/src/shared/components/Signup.vue
+++ b/apps/web/src/shared/components/Signup.vue
@@ -52,6 +52,8 @@ function submit() {
           authStore.setSignupError("用户名已存在")
         } else if (err.error === "email-exists") {
           authStore.setSignupError("邮箱已存在")
+        } else if (err.error === "too-many-registrations") {
+          authStore.setSignupError("注册太频繁，请稍后再试")
         } else {
           authStore.setSignupError("无法注册")
         }