diff --git a/src/server/assets-path.ts b/src/server/assets-path.ts
new file mode 100644
index 0000000..ca16a5d
--- /dev/null
+++ b/src/server/assets-path.ts
@@ -0,0 +1,17 @@
+/** Maps a URL path to a file path relative to the assets root, or null. No dot-segments, no dotfiles. */
+export function resolveAssetPath(pathname: string): string | null {
+  let decoded: string;
+  try {
+    decoded = decodeURIComponent(pathname);
+  } catch {
+    return null;
+  }
+  if (decoded.includes("\0") || decoded.includes("\\")) return null;
+  const segments: string[] = [];
+  for (const segment of decoded.split("/")) {
+    if (segment === "") continue;
+    if (segment.startsWith(".")) return null;
+    segments.push(segment);
+  }
+  return segments.length ? segments.join("/") : "index.html";
+}
diff --git a/src/server/config.ts b/src/server/config.ts
new file mode 100644
index 0000000..37abe5b
--- /dev/null
+++ b/src/server/config.ts
@@ -0,0 +1,68 @@
+// Web-standard: parses the environment; no Bun or node APIs.
+
+export interface ServerConfig {
+  port: number;
+  hostname: string;
+  publicUrl: string | null;
+  storageDir: string | null;
+  storageMaxBytes: number;
+  trustProxy: boolean;
+  assetsDir: string;
+}
+
+export class ConfigError extends Error {
+  override name = "ConfigError";
+}
+
+export const DEFAULT_STORAGE_MAX_BYTES = 1024 ** 3;
+
+export function readConfig(env: Record<string, string | undefined>): ServerConfig {
+  const publicUrl = parsePublicUrl(env.MBSEED_PUBLIC_URL);
+  const storageDir = env.MBSEED_STORAGE_DIR?.trim() || null;
+  if (storageDir && !publicUrl) {
+    throw new ConfigError("MBSEED_STORAGE_DIR needs MBSEED_PUBLIC_URL: MangaBaka must be able to download stored covers.");
+  }
+  return {
+    port: wholeNumber(env, "MBSEED_PORT", 8787),
+    hostname: env.MBSEED_HOST?.trim() || "127.0.0.1",
+    publicUrl,
+    storageDir,
+    storageMaxBytes: wholeNumber(env, "MBSEED_STORAGE_MAX_BYTES", DEFAULT_STORAGE_MAX_BYTES),
+    trustProxy: env.MBSEED_TRUST_PROXY === "1",
+    assetsDir: env.MBSEED_ASSETS_DIR?.trim() || "dist/web",
+  };
+}
+
+function parsePublicUrl(raw: string | undefined): string | null {
+  const value = raw?.trim();
+  if (!value) return null;
+  let url: URL;
+  try {
+    url = new URL(value);
+  } catch {
+    throw new ConfigError(`MBSEED_PUBLIC_URL is not a URL: ${value}`);
+  }
+  if (!["http:", "https:"].includes(url.protocol) || url.pathname !== "/" || url.search || url.hash) {
+    throw new ConfigError(`MBSEED_PUBLIC_URL must be an origin like https://mbseed.example.org, got ${value}`);
+  }
+  return url.origin;
+}
+
+function wholeNumber(env: Record<string, string | undefined>, name: string, fallback: number): number {
+  const raw = env[name]?.trim();
+  if (!raw) return fallback;
+  if (!/^\d+$/.test(raw)) throw new ConfigError(`${name} must be a whole number, got "${raw}".`);
+  return Number(raw);
+}
+
+/**
+ * The address rate limits key on. Behind our reverse proxy (MBSEED_TRUST_PROXY=1) that is the
+ * last X-Forwarded-For entry — the one the proxy appended; earlier entries come from the client.
+ */
+export function clientIp(req: Request, trustProxy: boolean, peer?: string): string {
+  if (trustProxy) {
+    const last = (req.headers.get("x-forwarded-for") ?? "").split(",").map((s) => s.trim()).filter(Boolean).at(-1);
+    if (last) return last;
+  }
+  return peer ?? "unknown";
+}
diff --git a/src/server/rate-limit.ts b/src/server/rate-limit.ts
new file mode 100644
index 0000000..92cd433
--- /dev/null
+++ b/src/server/rate-limit.ts
@@ -0,0 +1,23 @@
+/** Fixed-window counter per key (client IP). In memory: one server process. */
+export class RateLimiter {
+  private readonly hits = new Map<string, { start: number; count: number }>();
+
+  constructor(readonly limit: number, readonly windowMs: number, private readonly now: () => number = Date.now) {}
+
+  take(key: string): boolean {
+    const t = this.now();
+    const hit = this.hits.get(key);
+    if (!hit || t - hit.start >= this.windowMs) {
+      if (this.hits.size > 10_000) this.prune(t);
+      this.hits.set(key, { start: t, count: 1 });
+      return true;
+    }
+    if (hit.count >= this.limit) return false;
+    hit.count++;
+    return true;
+  }
+
+  private prune(t: number): void {
+    for (const [key, hit] of this.hits) if (t - hit.start >= this.windowMs) this.hits.delete(key);
+  }
+}
diff --git a/test/server-config.test.ts b/test/server-config.test.ts
new file mode 100644
index 0000000..74ee6c6
--- /dev/null
+++ b/test/server-config.test.ts
@@ -0,0 +1,69 @@
+import { describe, expect, test } from "bun:test";
+import { resolveAssetPath } from "../src/server/assets-path";
+import { ConfigError, clientIp, readConfig } from "../src/server/config";
+import { RateLimiter } from "../src/server/rate-limit";
+
+test("RateLimiter: N per window per key", () => {
+  let now = 0;
+  const limiter = new RateLimiter(2, 60_000, () => now);
+  expect([limiter.take("a"), limiter.take("a"), limiter.take("a"), limiter.take("b")]).toEqual([true, true, false, true]);
+  now = 60_000;
+  expect(limiter.take("a")).toBe(true);
+});
+
+describe("readConfig", () => {
+  test("defaults", () => {
+    expect(readConfig({})).toEqual({
+      port: 8787, hostname: "127.0.0.1", publicUrl: null, storageDir: null,
+      storageMaxBytes: 1073741824, trustProxy: false, assetsDir: "dist/web",
+    });
+  });
+  test("everything set", () => {
+    expect(readConfig({
+      MBSEED_PORT: "9000", MBSEED_HOST: "0.0.0.0", MBSEED_PUBLIC_URL: "https://mbseed.example.org/",
+      MBSEED_STORAGE_DIR: "/srv/mbseed/covers", MBSEED_STORAGE_MAX_BYTES: "5000000", MBSEED_TRUST_PROXY: "1", MBSEED_ASSETS_DIR: "/srv/mbseed/web",
+    })).toEqual({
+      port: 9000, hostname: "0.0.0.0", publicUrl: "https://mbseed.example.org", storageDir: "/srv/mbseed/covers",
+      storageMaxBytes: 5000000, trustProxy: true, assetsDir: "/srv/mbseed/web",
+    });
+  });
+  test.each([
+    [{ MBSEED_STORAGE_DIR: "/x" }, /MBSEED_PUBLIC_URL/],
+    [{ MBSEED_PUBLIC_URL: "mbseed.example.org" }, /not a URL/],
+    [{ MBSEED_PUBLIC_URL: "https://example.org/sub/path" }, /origin/],
+    [{ MBSEED_PORT: "eighty" }, /whole number/],
+    [{ MBSEED_STORAGE_MAX_BYTES: "1GB" }, /whole number/],
+  ])("%o is refused", (env, message) => {
+    expect(() => readConfig(env)).toThrow(ConfigError);
+    expect(() => readConfig(env)).toThrow(message);
+  });
+});
+
+describe("clientIp", () => {
+  const req = (xff?: string) => new Request("http://x/", { headers: xff ? { "x-forwarded-for": xff } : {} });
+  test("ignores X-Forwarded-For unless the proxy is trusted", () => {
+    expect(clientIp(req("1.2.3.4"), false, "10.0.0.1")).toBe("10.0.0.1");
+  });
+  test("trusts only the last entry, the one our proxy appended", () => {
+    // Review Focus 4: the client sent "1.2.3.4" itself; the proxy appended the real address.
+    expect(clientIp(req("1.2.3.4, 203.0.113.9"), true, "127.0.0.1")).toBe("203.0.113.9");
+    expect(clientIp(req(), true, "127.0.0.1")).toBe("127.0.0.1");
+    expect(clientIp(req(), false)).toBe("unknown");
+  });
+});
+
+describe("resolveAssetPath", () => {
+  test.each([
+    ["/", "index.html"],
+    ["/index.html", "index.html"],
+    ["/main-ab12cd.js", "main-ab12cd.js"],
+    ["/mbseed.user.js", "mbseed.user.js"],
+    ["/fonts/x.woff2", "fonts/x.woff2"],
+  ])("%p → %p", (path, file) => {
+    expect(resolveAssetPath(path)).toBe(file);
+  });
+  // Review Focus 5
+  test.each(["/..%2f.env", "/%2e%2e/package.json", "/a/../../b", "/.env", "/%00x", "/a\\..\\b", "/%E0%A4%A"])("%p is refused", (path) => {
+    expect(resolveAssetPath(path)).toBeNull();
+  });
+});