diff --git a/next.config.ts b/next.config.ts
index a8e3cd7..d7d902b 100644
--- a/next.config.ts
+++ b/next.config.ts
@@ -11,6 +11,38 @@ function getSupabaseHostname(): string {
   return "*.supabase.co";
 }
 
+function buildCsp(): string {
+  const supabase = `https://${getSupabaseHostname()}`;
+  const directives: Record<string, string[]> = {
+    "default-src": ["'self'"],
+    "script-src": ["'self'", "https://clerk.pawfeed.org"],
+    // Radix/shadcn primitives set inline style="" attributes for positioning —
+    // no nonce mechanism covers style attributes, so 'unsafe-inline' is required here.
+    "style-src": ["'self'", "'unsafe-inline'"],
+    "img-src": ["'self'", "data:", supabase, "https://image.mux.com", "https://img.clerk.com"],
+    "font-src": ["'self'", "data:"],
+    "media-src": ["'self'", "blob:", "https://stream.mux.com"],
+    "worker-src": ["'self'", "blob:"],
+    "connect-src": [
+      "'self'",
+      "https://clerk.pawfeed.org",
+      "https://accounts.pawfeed.org",
+      supabase,
+      "https://stream.mux.com",
+      "https://image.mux.com",
+      "https://litix.io",
+      "https://storage.googleapis.com",
+    ],
+    "frame-src": ["'self'", "https://clerk.pawfeed.org"],
+    "frame-ancestors": ["'none'"],
+    "object-src": ["'none'"],
+    "base-uri": ["'self'"],
+  };
+  return Object.entries(directives)
+    .map(([key, values]) => `${key} ${values.join(" ")}`)
+    .join("; ");
+}
+
 const nextConfig: NextConfig = {
   output: "standalone",
   allowedDevOrigins: ["192.168.1.92"],
@@ -19,6 +51,21 @@ const nextConfig: NextConfig = {
       { protocol: "https", hostname: getSupabaseHostname() },
     ],
   },
+  async headers() {
+    return [
+      {
+        source: "/(.*)",
+        headers: [
+          { key: "Content-Security-Policy", value: buildCsp() },
+          { key: "X-Content-Type-Options", value: "nosniff" },
+          { key: "X-Frame-Options", value: "DENY" },
+          { key: "Referrer-Policy", value: "strict-origin-when-cross-origin" },
+          { key: "Permissions-Policy", value: "camera=(), microphone=(), geolocation=()" },
+          { key: "Strict-Transport-Security", value: "max-age=31536000; includeSubDomains" },
+        ],
+      },
+    ];
+  },
 };
 
 export default withNextIntl(nextConfig);
diff --git a/src/__tests__/media-upload.test.ts b/src/__tests__/media-upload.test.ts
index 0f99886..39dfe4b 100644
--- a/src/__tests__/media-upload.test.ts
+++ b/src/__tests__/media-upload.test.ts
@@ -80,11 +80,13 @@ function createTestCaller(
     userId: string | null;
     prisma: import("@prisma/client").PrismaClient;
     locale: string;
+    ip: string;
   };
   const ctx: CallerCtx = {
     userId,
     prisma: mockPrisma as unknown as import("@prisma/client").PrismaClient,
     locale: "en",
+    ip: "test",
   };
   return mediaRouter.createCaller(ctx);
 }
diff --git a/src/__tests__/pet-profile.test.ts b/src/__tests__/pet-profile.test.ts
index 1ae066f..ac55b25 100644
--- a/src/__tests__/pet-profile.test.ts
+++ b/src/__tests__/pet-profile.test.ts
@@ -54,11 +54,12 @@ function createTestCaller(userId: string, mockPrisma: ReturnType<typeof createMo
   // petsRouter.createCaller accepts the same context shape as createTRPCContext returns.
   // Cast the mock to the expected PrismaClient type — mock has the same interface for
   // tested methods.
-  type CallerCtx = { userId: string | null; prisma: import("@prisma/client").PrismaClient; locale: string };
+  type CallerCtx = { userId: string | null; prisma: import("@prisma/client").PrismaClient; locale: string; ip: string };
   const ctx: CallerCtx = {
     userId,
     prisma: mockPrisma as unknown as import("@prisma/client").PrismaClient,
     locale: "en",
+    ip: "test",
   };
   return petsRouter.createCaller(ctx);
 }
diff --git a/src/app/api/auth/consume-invite/route.ts b/src/app/api/auth/consume-invite/route.ts
index 6f69ccf..ce12a23 100644
--- a/src/app/api/auth/consume-invite/route.ts
+++ b/src/app/api/auth/consume-invite/route.ts
@@ -3,6 +3,8 @@ import { NextRequest, NextResponse } from "next/server";
 import { cookies } from "next/headers";
 import { prisma } from "@/lib/prisma";
 import { generateInviteCode, INVITE_CODES_PER_USER } from "@/lib/invite-codes";
+import { checkRateLimit } from "@/lib/rate-limit";
+import { getClientIp } from "@/lib/get-client-ip";
 
 function resolveBase(request: NextRequest): string {
   const proto = request.headers.get("x-forwarded-proto") ?? "https";
@@ -17,6 +19,15 @@ export async function GET(request: NextRequest) {
   const { userId } = await auth();
   const base = resolveBase(request);
 
+  const { success } = await checkRateLimit({
+    key: `auth.consume-invite:${getClientIp(request)}`,
+    limit: 20,
+    windowSeconds: 60,
+  });
+  if (!success) {
+    return NextResponse.redirect(new URL("/", base));
+  }
+
   if (!userId) {
     return NextResponse.redirect(new URL("/log-in", base));
   }
diff --git a/src/app/api/auth/set-invite/route.ts b/src/app/api/auth/set-invite/route.ts
index fb11bde..b80d8e1 100644
--- a/src/app/api/auth/set-invite/route.ts
+++ b/src/app/api/auth/set-invite/route.ts
@@ -1,5 +1,7 @@
 import { NextRequest, NextResponse } from "next/server";
 import { prisma } from "@/lib/prisma";
+import { checkRateLimit } from "@/lib/rate-limit";
+import { getClientIp } from "@/lib/get-client-ip";
 
 function resolveBase(request: NextRequest): string {
   const proto = request.headers.get("x-forwarded-proto") ?? "https";
@@ -11,9 +13,19 @@ function resolveBase(request: NextRequest): string {
 }
 
 export async function GET(request: NextRequest) {
-  const code = request.nextUrl.searchParams.get("code")?.trim().toUpperCase();
   const base = resolveBase(request);
 
+  const { success } = await checkRateLimit({
+    key: `auth.set-invite:${getClientIp(request)}`,
+    limit: 20,
+    windowSeconds: 60,
+  });
+  if (!success) {
+    return NextResponse.redirect(new URL("/join?error=rate-limited", base));
+  }
+
+  const code = request.nextUrl.searchParams.get("code")?.trim().toUpperCase();
+
   if (!code) {
     return NextResponse.redirect(new URL("/join?error=missing", base));
   }
diff --git a/src/lib/get-client-ip.ts b/src/lib/get-client-ip.ts
new file mode 100644
index 0000000..d795cd6
--- /dev/null
+++ b/src/lib/get-client-ip.ts
@@ -0,0 +1,10 @@
+import "server-only";
+import type { NextRequest } from "next/server";
+
+export function getClientIp(request: NextRequest): string {
+  return (
+    request.headers.get("cf-connecting-ip") ??
+    request.headers.get("x-forwarded-for")?.split(",")[0]?.trim() ??
+    "unknown"
+  );
+}
diff --git a/src/lib/rate-limit.ts b/src/lib/rate-limit.ts
new file mode 100644
index 0000000..10b1dd3
--- /dev/null
+++ b/src/lib/rate-limit.ts
@@ -0,0 +1,32 @@
+import "server-only";
+import { redis } from "@/lib/redis";
+
+interface RateLimitOptions {
+  /** Unique key identifying the caller + the action being limited, e.g. "posts.create:user_123". */
+  key: string;
+  /** Max requests allowed within the window. */
+  limit: number;
+  windowSeconds: number;
+}
+
+interface RateLimitResult {
+  success: boolean;
+}
+
+/**
+ * Fixed-window rate limiter backed by Redis INCR/EXPIRE.
+ * Fails open (allows the request) if Redis is unreachable — a rate-limit
+ * outage must not take the whole app down with it.
+ */
+export async function checkRateLimit({ key, limit, windowSeconds }: RateLimitOptions): Promise<RateLimitResult> {
+  try {
+    const redisKey = `ratelimit:${key}`;
+    const count = await redis.incr(redisKey);
+    if (count === 1) {
+      await redis.expire(redisKey, windowSeconds);
+    }
+    return { success: count <= limit };
+  } catch {
+    return { success: true };
+  }
+}
diff --git a/src/trpc/init.ts b/src/trpc/init.ts
index e00d826..45afa65 100644
--- a/src/trpc/init.ts
+++ b/src/trpc/init.ts
@@ -1,8 +1,17 @@
 import { initTRPC, TRPCError } from "@trpc/server";
 import { auth } from "@clerk/nextjs/server";
-import { cookies } from "next/headers";
+import { cookies, headers } from "next/headers";
 import { prisma } from "@/lib/prisma";
 import { LOCALES, defaultLocale } from "@/i18n/request";
+import { checkRateLimit } from "@/lib/rate-limit";
+
+function resolveClientIp(headerList: Awaited<ReturnType<typeof headers>>): string {
+  return (
+    headerList.get("cf-connecting-ip") ??
+    headerList.get("x-forwarded-for")?.split(",")[0]?.trim() ??
+    "unknown"
+  );
+}
 
 /**
  * tRPC context — created per request in the API route handler.
@@ -16,9 +25,11 @@ import { LOCALES, defaultLocale } from "@/i18n/request";
 export const createTRPCContext = async () => {
   const { userId } = await auth();
   const cookieStore = await cookies();
+  const headerList = await headers();
   const raw = cookieStore.get("NEXT_LOCALE")?.value ?? "";
   const locale = (LOCALES as readonly string[]).includes(raw) ? raw : defaultLocale;
-  return { userId, prisma, locale };
+  const ip = resolveClientIp(headerList);
+  return { userId, prisma, locale, ip };
 };
 
 type Context = Awaited<ReturnType<typeof createTRPCContext>>;
@@ -43,3 +54,26 @@ export const protectedProcedure = t.procedure.use(({ ctx, next }) => {
     },
   });
 });
+
+/**
+ * Rate-limit middleware factory — Redis-backed fixed window, keyed by
+ * caller identity (authenticated userId, or IP for public procedures).
+ * `scope` namespaces the Redis key per call site (e.g. "posts.create").
+ */
+export function rateLimited(scope: string, limit: number, windowSeconds: number) {
+  return t.middleware(async ({ ctx, next }) => {
+    const identifier = ctx.userId ?? ctx.ip;
+    const { success } = await checkRateLimit({
+      key: `${scope}:${identifier}`,
+      limit,
+      windowSeconds,
+    });
+    if (!success) {
+      throw new TRPCError({
+        code: "TOO_MANY_REQUESTS",
+        message: "Too many requests — please slow down.",
+      });
+    }
+    return next();
+  });
+}
diff --git a/src/trpc/routers/comments.ts b/src/trpc/routers/comments.ts
index d88fcb1..519fa14 100644
--- a/src/trpc/routers/comments.ts
+++ b/src/trpc/routers/comments.ts
@@ -1,13 +1,14 @@
 import "server-only";
 import { z } from "zod";
 import { TRPCError } from "@trpc/server";
-import { router, protectedProcedure } from "@/trpc/init";
+import { router, protectedProcedure, rateLimited } from "@/trpc/init";
 import { createMentionNotifications } from "@/lib/mention-helpers";
 import { assertPetOwnership } from "@/lib/assert-pet-ownership";
 import { notify } from "@/lib/notify";
 
 export const commentsRouter = router({
   create: protectedProcedure
+    .use(rateLimited("comments.create", 20, 300))
     .input(
       z.object({
         postId: z.string().min(1),
diff --git a/src/trpc/routers/explore.ts b/src/trpc/routers/explore.ts
index ccf294d..9114981 100644
--- a/src/trpc/routers/explore.ts
+++ b/src/trpc/routers/explore.ts
@@ -1,6 +1,6 @@
 import "server-only";
 import { z } from "zod";
-import { router, publicProcedure } from "@/trpc/init";
+import { router, publicProcedure, rateLimited } from "@/trpc/init";
 import { pickLocalizedName } from "@/lib/localized-name";
 
 const TRENDING_POSTS_WINDOW_MS = 48 * 60 * 60 * 1000; // 48 h
@@ -8,6 +8,7 @@ const TRENDING_HASHTAGS_WINDOW_MS = 7 * 24 * 60 * 60 * 1000; // 7 days
 
 export const exploreRouter = router({
   listPets: publicProcedure
+    .use(rateLimited("explore.listPets", 60, 60))
     .input(
       z.object({
         speciesId: z.string().optional(),
@@ -50,7 +51,9 @@ export const exploreRouter = router({
    * getTrending — trending posts (48 h window, ranked by reaction count)
    * and trending hashtags (7 day window, ranked by post count).
    */
-  getTrending: publicProcedure.query(async ({ ctx }) => {
+  getTrending: publicProcedure
+    .use(rateLimited("explore.getTrending", 30, 60))
+    .query(async ({ ctx }) => {
     const now = Date.now();
     const since48h = new Date(now - TRENDING_POSTS_WINDOW_MS);
     const since7d = new Date(now - TRENDING_HASHTAGS_WINDOW_MS);
diff --git a/src/trpc/routers/posts.ts b/src/trpc/routers/posts.ts
index 5be56e3..5fe055b 100644
--- a/src/trpc/routers/posts.ts
+++ b/src/trpc/routers/posts.ts
@@ -1,7 +1,7 @@
 import "server-only";
 import { z } from "zod";
 import { TRPCError } from "@trpc/server";
-import { router, protectedProcedure } from "@/trpc/init";
+import { router, protectedProcedure, rateLimited } from "@/trpc/init";
 import {
   createSignedUploadUrl,
   ALLOWED_CONTENT_TYPES,
@@ -90,6 +90,7 @@ export const postsRouter = router({
    * Fan-out: fanOutPost adds this post to every follower's Redis sorted set.
    */
   create: protectedProcedure
+    .use(rateLimited("posts.create", 10, 600))
     .input(
       z.object({
         petId: z.string().min(1),