diff --git a/src/app/config/layout.tsx b/src/app/config/layout.tsx
index 1356d78..72d9b6d 100644
--- a/src/app/config/layout.tsx
+++ b/src/app/config/layout.tsx
@@ -4,26 +4,27 @@ import Link from "next/link";
 import { usePathname, useRouter } from "next/navigation";
 import { useEffect } from "react";
 import { useAuth } from "../../frontend/components/AuthProvider";
-
-const CONFIG_TABS = [
-  { href: "/config/benutzer", label: "Benutzer" },
-  { href: "/config/konto", label: "Konto" },
-] as const;
+import {
+  getConfigRedirect,
+  getConfigTabs,
+} from "../../frontend/utils/config-access";
 
 export default function ConfigLayout({ children }: { children: React.ReactNode }) {
   const pathname = usePathname();
   const router = useRouter();
   const { status, isAdmin } = useAuth();
+  const tabs = getConfigTabs(isAdmin);
+  const redirectTarget = getConfigRedirect(pathname, isAdmin);
 
   useEffect(() => {
-    // LoginGate already guarantees a signed-in user with a set password by
-    // the time this layout renders; only the role still needs checking.
-    if (status === "signed-in" && !isAdmin) {
-      router.replace("/");
-    }
-  }, [status, isAdmin, router]);
+    if (status !== "signed-in") return;
+    if (redirectTarget) router.replace(redirectTarget);
+  }, [status, redirectTarget, router]);
 
-  if (status !== "signed-in" || !isAdmin) {
+  if (
+    status !== "signed-in" ||
+    redirectTarget !== null
+  ) {
     return null;
   }
 
@@ -34,7 +35,7 @@ export default function ConfigLayout({ children }: { children: React.ReactNode }
       </div>
 
       <nav className="config-tabs">
-        {CONFIG_TABS.map((tab) => (
+        {tabs.map((tab) => (
           <Link
             key={tab.href}
             href={tab.href}
diff --git a/src/app/config/page.tsx b/src/app/config/page.tsx
index 7897dc7..3c9d8fd 100644
--- a/src/app/config/page.tsx
+++ b/src/app/config/page.tsx
@@ -1,5 +1,3 @@
-import { redirect } from "next/navigation";
-
 export default function ConfigPage() {
-  redirect("/config/benutzer");
+  return null;
 }
diff --git a/src/db/repositories/user.repository.ts b/src/db/repositories/user.repository.ts
index 071fc33..1fd55c7 100644
--- a/src/db/repositories/user.repository.ts
+++ b/src/db/repositories/user.repository.ts
@@ -1,17 +1,24 @@
 import crypto from "node:crypto";
-import { asc, eq } from "drizzle-orm";
+import { and, asc, eq } from "drizzle-orm";
 import type { AppDatabase } from "../database-context.js";
 import { users } from "../schema/users.js";
+import {
+  LastActiveAdminStoreError,
+  UsernameAlreadyExistsStoreError,
+  type CreateUserAccount,
+  type UserStore,
+} from "../../domain/ports/user.store.js";
 import type { UserRole } from "../../shared/constants/user-roles.js";
 
-export interface CreateUserRow {
-  username: string;
-  name: string;
-  role: UserRole;
-  passwordHash: string;
+function isUniqueConstraintError(error: unknown): boolean {
+  return (
+    error instanceof Error &&
+    "code" in error &&
+    (error as { code?: unknown }).code === "SQLITE_CONSTRAINT_UNIQUE"
+  );
 }
 
-export class UserRepository {
+export class UserRepository implements UserStore {
   constructor(private readonly database: AppDatabase) {}
 
   async list() {
@@ -41,18 +48,23 @@ export class UserRepository {
     return row ?? null;
   }
 
-  async create(input: CreateUserRow) {
+  async create(input: CreateUserAccount) {
     const db = this.database;
     const id = crypto.randomUUID();
-    await db.insert(users).values({
-      id,
-      username: input.username.trim().toLowerCase(),
-      name: input.name,
-      role: input.role,
-      passwordHash: input.passwordHash,
-      active: true,
-      mustChangePassword: true,
-    });
+    try {
+      await db.insert(users).values({
+        id,
+        username: input.username.trim().toLowerCase(),
+        name: input.name,
+        role: input.role,
+        passwordHash: input.passwordHash,
+        active: true,
+        mustChangePassword: true,
+      });
+    } catch (error) {
+      if (isUniqueConstraintError(error)) throw new UsernameAlreadyExistsStoreError();
+      throw error;
+    }
     return this.findById(id);
   }
 
@@ -60,16 +72,34 @@ export class UserRepository {
     userId: string,
     patch: { role?: UserRole; active?: boolean }
   ) {
-    const db = this.database;
-    await db
-      .update(users)
-      .set({
-        ...(patch.role !== undefined ? { role: patch.role } : {}),
-        ...(patch.active !== undefined ? { active: patch.active } : {}),
-        updatedAt: new Date(),
-      })
-      .where(eq(users.id, userId));
-    return this.findById(userId);
+    return this.database.transaction((tx) => {
+      const [target] = tx.select().from(users).where(eq(users.id, userId)).limit(1).all();
+      if (!target) return null;
+
+      const removesActiveAdmin =
+        target.role === "admin" &&
+        target.active &&
+        (patch.role === "user" || patch.active === false);
+      if (removesActiveAdmin) {
+        const activeAdmins = tx
+          .select({ id: users.id })
+          .from(users)
+          .where(and(eq(users.role, "admin"), eq(users.active, true)))
+          .limit(2)
+          .all();
+        if (activeAdmins.length <= 1) throw new LastActiveAdminStoreError();
+      }
+
+      tx.update(users)
+        .set({
+          ...(patch.role !== undefined ? { role: patch.role } : {}),
+          ...(patch.active !== undefined ? { active: patch.active } : {}),
+          updatedAt: new Date(),
+        })
+        .where(eq(users.id, userId))
+        .run();
+      return tx.select().from(users).where(eq(users.id, userId)).limit(1).all()[0] ?? null;
+    });
   }
 
   async setPassword(
diff --git a/src/db/schema/users.ts b/src/db/schema/users.ts
index 825b5e1..7105fb4 100644
--- a/src/db/schema/users.ts
+++ b/src/db/schema/users.ts
@@ -1,5 +1,6 @@
 import { sql } from "drizzle-orm";
 import { integer, sqliteTable, text, uniqueIndex } from "drizzle-orm/sqlite-core";
+import type { UserRole } from "../../shared/constants/user-roles.js";
 
 export const users = sqliteTable(
   "users",
@@ -8,7 +9,7 @@ export const users = sqliteTable(
     username: text("username").notNull(),
     passwordHash: text("password_hash").notNull(),
     name: text("name").notNull(),
-    role: text("role").notNull().default("user"),
+    role: text("role").$type<UserRole>().notNull().default("user"),
     active: integer("active", { mode: "boolean" }).notNull().default(true),
     mustChangePassword: integer("must_change_password", { mode: "boolean" })
       .notNull()
diff --git a/src/domain/ports/user.store.ts b/src/domain/ports/user.store.ts
new file mode 100644
index 0000000..9b80023
--- /dev/null
+++ b/src/domain/ports/user.store.ts
@@ -0,0 +1,40 @@
+import type { UserRole } from "../../shared/constants/user-roles.js";
+
+export interface UserAccount {
+  id: string;
+  username: string;
+  passwordHash: string;
+  name: string;
+  role: UserRole;
+  active: boolean;
+  mustChangePassword: boolean;
+  createdAt: Date;
+  updatedAt: Date;
+}
+
+export interface CreateUserAccount {
+  username: string;
+  name: string;
+  role: UserRole;
+  passwordHash: string;
+}
+
+export class UsernameAlreadyExistsStoreError extends Error {}
+export class LastActiveAdminStoreError extends Error {}
+
+export interface UserStore {
+  list(): Promise<UserAccount[]>;
+  count(): Promise<number>;
+  findById(userId: string): Promise<UserAccount | null>;
+  findByUsername(username: string): Promise<UserAccount | null>;
+  create(input: CreateUserAccount): Promise<UserAccount | null>;
+  updateRoleAndActive(
+    userId: string,
+    patch: { role?: UserRole; active?: boolean }
+  ): Promise<UserAccount | null>;
+  setPassword(
+    userId: string,
+    passwordHash: string,
+    mustChangePassword: boolean
+  ): Promise<UserAccount | null>;
+}
diff --git a/src/domain/services/user.service.ts b/src/domain/services/user.service.ts
index 1a3c27c..7ea2f0b 100644
--- a/src/domain/services/user.service.ts
+++ b/src/domain/services/user.service.ts
@@ -1,8 +1,10 @@
-import type { UserRepository } from "../../db/repositories/user.repository.js";
-import { hashPassword, verifyPassword } from "../../server/auth/password.js";
 import type { UserRole } from "../../shared/constants/user-roles.js";
-import type { AuthConfig } from "../../server/auth/auth-config.js";
 import { createLogger } from "../../shared/logging/logger.js";
+import {
+  LastActiveAdminStoreError,
+  UsernameAlreadyExistsStoreError,
+  type UserStore,
+} from "../ports/user.store.js";
 
 const logger = createLogger("user-service");
 
@@ -36,20 +38,22 @@ export class LastAdminError extends Error {
   }
 }
 
-function isUniqueConstraintError(error: unknown): boolean {
-  return (
-    error instanceof Error &&
-    "code" in error &&
-    (error as { code?: unknown }).code === "SQLITE_CONSTRAINT_UNIQUE"
-  );
+export interface PasswordHasher {
+  hash(password: string): Promise<string>;
+  verify(password: string, storedHash: string): Promise<boolean>;
+}
+
+export interface UserServiceOptions {
+  bootstrap: { username: string; password: string } | null;
 }
 
 export class UserService {
   private dummyHash: Promise<string> | null = null;
 
   constructor(
-    private readonly repository: UserRepository,
-    private readonly authConfig: AuthConfig
+    private readonly repository: UserStore,
+    private readonly options: UserServiceOptions,
+    private readonly passwords: PasswordHasher
   ) {}
 
   /**
@@ -65,21 +69,21 @@ export class UserService {
     await this.getDummyHash();
     const existingCount = await this.repository.count();
     if (existingCount > 0) return;
-    if (!this.authConfig.bootstrap) {
+    if (!this.options.bootstrap) {
       logger.warn(
         "no users exist and AUTH_BOOTSTRAP_USERNAME/AUTH_BOOTSTRAP_PASSWORT are not set - nobody can sign in"
       );
       return;
     }
-    const passwordHash = await hashPassword(this.authConfig.bootstrap.password);
+    const passwordHash = await this.passwords.hash(this.options.bootstrap.password);
     await this.repository.create({
-      username: this.authConfig.bootstrap.username,
+      username: this.options.bootstrap.username,
       name: "Administrator",
       role: "admin",
       passwordHash,
     });
     logger.info("bootstrap admin created", {
-      username: this.authConfig.bootstrap.username,
+      username: this.options.bootstrap.username,
     });
   }
 
@@ -89,7 +93,7 @@ export class UserService {
     // scrypt run either way, so response time doesn't reveal which usernames
     // exist.
     const passwordHash = user?.passwordHash ?? (await this.getDummyHash());
-    const passwordMatches = await verifyPassword(password, passwordHash);
+    const passwordMatches = await this.passwords.verify(password, passwordHash);
     if (!user || !user.active || !passwordMatches) {
       throw new InvalidCredentialsError();
     }
@@ -104,7 +108,7 @@ export class UserService {
   }) {
     const existing = await this.repository.findByUsername(input.username);
     if (existing) throw new UsernameTakenError();
-    const passwordHash = await hashPassword(input.initialPassword);
+    const passwordHash = await this.passwords.hash(input.initialPassword);
     try {
       return await this.repository.create({
         username: input.username,
@@ -114,29 +118,23 @@ export class UserService {
       });
     } catch (error) {
       // Two admins creating the same name at once: the unique index decides.
-      if (isUniqueConstraintError(error)) throw new UsernameTakenError();
+      if (error instanceof UsernameAlreadyExistsStoreError) throw new UsernameTakenError();
       throw error;
     }
   }
 
   async updateRoleAndActive(userId: string, patch: { role?: UserRole; active?: boolean }) {
-    const removesAdmin = patch.role === "user" || patch.active === false;
-    if (removesAdmin) {
-      const target = await this.repository.findById(userId);
-      const targetIsActiveAdmin = target?.role === "admin" && target.active;
-      if (targetIsActiveAdmin) {
-        const activeAdmins = (await this.repository.list()).filter(
-          (user) => user.role === "admin" && user.active
-        );
-        if (activeAdmins.length <= 1) throw new LastAdminError();
-      }
+    try {
+      return await this.repository.updateRoleAndActive(userId, patch);
+    } catch (error) {
+      if (error instanceof LastActiveAdminStoreError) throw new LastAdminError();
+      throw error;
     }
-    return this.repository.updateRoleAndActive(userId, patch);
   }
 
   /** Admin-triggered reset with a new start password; forces a change on next login. */
   async resetPassword(userId: string, newPassword: string) {
-    const passwordHash = await hashPassword(newPassword);
+    const passwordHash = await this.passwords.hash(newPassword);
     return this.repository.setPassword(userId, passwordHash, true);
   }
 
@@ -144,15 +142,15 @@ export class UserService {
   async changeOwnPassword(userId: string, currentPassword: string, newPassword: string) {
     const user = await this.repository.findById(userId);
     if (!user) throw new WrongPasswordError();
-    const currentMatches = await verifyPassword(currentPassword, user.passwordHash);
+    const currentMatches = await this.passwords.verify(currentPassword, user.passwordHash);
     if (!currentMatches) throw new WrongPasswordError();
     if (newPassword === currentPassword) throw new PasswordUnchangedError();
-    const passwordHash = await hashPassword(newPassword);
+    const passwordHash = await this.passwords.hash(newPassword);
     return this.repository.setPassword(userId, passwordHash, false);
   }
 
   private getDummyHash(): Promise<string> {
-    this.dummyHash ??= hashPassword("dummy-password-for-constant-time-login");
+    this.dummyHash ??= this.passwords.hash("dummy-password-for-constant-time-login");
     return this.dummyHash;
   }
 }
diff --git a/src/frontend/components/Sidebar.tsx b/src/frontend/components/Sidebar.tsx
index 5130d5d..e99b493 100644
--- a/src/frontend/components/Sidebar.tsx
+++ b/src/frontend/components/Sidebar.tsx
@@ -7,6 +7,7 @@ import { listProjects } from "../utils/api";
 import type { ProjectDto } from "../types";
 import { ThemeToggle } from "./ThemeToggle";
 import { useAuth } from "./AuthProvider";
+import { getConfigEntry } from "../utils/config-access";
 
 // Anchors on the project overview page (/projects/[id]).
 const PROJECT_SECTIONS = [
@@ -32,6 +33,7 @@ interface SidebarProps {
 export function Sidebar({ isCollapsed, onToggleCollapsed }: SidebarProps) {
   const pathname = usePathname();
   const { user, isAdmin, logout } = useAuth();
+  const configEntry = getConfigEntry(isAdmin);
   const projectMatch = pathname?.match(PROJECT_PATH_PATTERN);
   const activeProjectId = projectMatch ? projectMatch[1] : null;
   const projectSubPath = projectMatch?.[2] ?? "";
@@ -181,18 +183,16 @@ export function Sidebar({ isCollapsed, onToggleCollapsed }: SidebarProps) {
           </>
         )}
       </nav>
-      {isAdmin && (
-        <nav className="sidebar-nav sidebar-nav-bottom">
-          <Link
-            href="/config"
-            className={pathname?.startsWith("/config") ? "sidebar-link active" : "sidebar-link"}
-            title="Config"
-          >
-            <span className="sidebar-icon">⚙</span>
-            <span className="sidebar-label">Config</span>
-          </Link>
-        </nav>
-      )}
+      <nav className="sidebar-nav sidebar-nav-bottom">
+        <Link
+          href={configEntry.href}
+          className={pathname?.startsWith("/config") ? "sidebar-link active" : "sidebar-link"}
+          title={configEntry.label}
+        >
+          <span className="sidebar-icon">⚙</span>
+          <span className="sidebar-label">{configEntry.label}</span>
+        </Link>
+      </nav>
       <div className="sidebar-footer">
         {user && !isCollapsed && (
           <div className="sidebar-identity" title={`Angemeldet als ${user.name}`}>
diff --git a/src/frontend/components/cable-sizing-api.ts b/src/frontend/components/cable-sizing-api.ts
index 28530d2..73582b5 100644
--- a/src/frontend/components/cable-sizing-api.ts
+++ b/src/frontend/components/cable-sizing-api.ts
@@ -6,6 +6,10 @@ import type {
   CableSizingInput,
   CableSizingResult,
 } from "../../cable-sizing/domain/cable-sizing-calculation";
+import {
+  WRITE_REQUEST_HEADER_NAME,
+  WRITE_REQUEST_HEADER_VALUE,
+} from "../../shared/constants/request-security";
 
 export interface CalculateCableSizingResponse {
   result: CableSizingResult;
@@ -17,7 +21,10 @@ export async function calculateCableSizing(
 ): Promise<CalculateCableSizingResponse> {
   const response = await fetch("/api/cable-sizing/calculate", {
     method: "POST",
-    headers: { "Content-Type": "application/json" },
+    headers: {
+      "Content-Type": "application/json",
+      [WRITE_REQUEST_HEADER_NAME]: WRITE_REQUEST_HEADER_VALUE,
+    },
     body: JSON.stringify(input),
     cache: "no-store",
   });
diff --git a/src/frontend/utils/api.ts b/src/frontend/utils/api.ts
index 8f5a878..5256195 100644
--- a/src/frontend/utils/api.ts
+++ b/src/frontend/utils/api.ts
@@ -1,3 +1,7 @@
+import {
+  WRITE_REQUEST_HEADER_NAME,
+  WRITE_REQUEST_HEADER_VALUE,
+} from "../../shared/constants/request-security";
 import type {
   BuildingDto,
   CircuitListDto,
@@ -122,6 +126,7 @@ async function request<T>(url: string, init?: RequestInit): Promise<T> {
     ...init,
     headers: {
       "Content-Type": "application/json",
+      [WRITE_REQUEST_HEADER_NAME]: WRITE_REQUEST_HEADER_VALUE,
       ...init?.headers,
     },
     cache: "no-store",
diff --git a/src/frontend/utils/auth-api.ts b/src/frontend/utils/auth-api.ts
index f7057f9..a6f12b3 100644
--- a/src/frontend/utils/auth-api.ts
+++ b/src/frontend/utils/auth-api.ts
@@ -1,3 +1,8 @@
+import {
+  WRITE_REQUEST_HEADER_NAME,
+  WRITE_REQUEST_HEADER_VALUE,
+} from "../../shared/constants/request-security";
+
 // Small, self-contained fetch helper for the auth/user endpoints - kept
 // separate from the large domain api.ts so this feature doesn't have to
 // touch that file's shared `request()` internals.
@@ -39,7 +44,11 @@ export class AuthApiError extends Error {
 async function request<T>(url: string, init?: RequestInit): Promise<T> {
   const response = await fetch(url, {
     ...init,
-    headers: { "Content-Type": "application/json", ...init?.headers },
+    headers: {
+      "Content-Type": "application/json",
+      [WRITE_REQUEST_HEADER_NAME]: WRITE_REQUEST_HEADER_VALUE,
+      ...init?.headers,
+    },
     cache: "no-store",
   });
   if (!response.ok) {
diff --git a/src/frontend/utils/config-access.ts b/src/frontend/utils/config-access.ts
new file mode 100644
index 0000000..86dd2a2
--- /dev/null
+++ b/src/frontend/utils/config-access.ts
@@ -0,0 +1,25 @@
+export interface ConfigNavigationEntry {
+  href: string;
+  label: string;
+}
+
+const ACCOUNT_ENTRY = { href: "/config/konto", label: "Konto" } as const;
+const USERS_ENTRY = { href: "/config/benutzer", label: "Benutzer" } as const;
+
+export function getConfigTabs(isAdmin: boolean): ConfigNavigationEntry[] {
+  return isAdmin ? [USERS_ENTRY, ACCOUNT_ENTRY] : [ACCOUNT_ENTRY];
+}
+
+export function getConfigEntry(isAdmin: boolean): ConfigNavigationEntry {
+  return isAdmin ? { href: USERS_ENTRY.href, label: "Config" } : ACCOUNT_ENTRY;
+}
+
+export function getConfigRedirect(pathname: string | null, isAdmin: boolean): string | null {
+  if (pathname === "/config") {
+    return isAdmin ? USERS_ENTRY.href : ACCOUNT_ENTRY.href;
+  }
+  if (!isAdmin && pathname?.startsWith(USERS_ENTRY.href)) {
+    return ACCOUNT_ENTRY.href;
+  }
+  return null;
+}
diff --git a/src/server/auth/auth.middleware.ts b/src/server/auth/auth.middleware.ts
index ce65bfb..bafae03 100644
--- a/src/server/auth/auth.middleware.ts
+++ b/src/server/auth/auth.middleware.ts
@@ -2,8 +2,12 @@ import type { NextFunction, Request, RequestHandler, Response } from "express";
 import { SESSION_COOKIE, type AuthConfig } from "./auth-config.js";
 import { readCookie } from "./cookie.js";
 import type { SessionStore } from "./sessions.js";
-import type { UserRepository } from "../../db/repositories/user.repository.js";
+import type { UserStore } from "../../domain/ports/user.store.js";
 import type { UserRole } from "../../shared/constants/user-roles.js";
+import {
+  WRITE_REQUEST_HEADER_NAME,
+  WRITE_REQUEST_HEADER_VALUE,
+} from "../../shared/constants/request-security.js";
 
 export interface AuthUser {
   id: string;
@@ -30,7 +34,7 @@ export function sessionIdFromRequest(req: {
 export async function identityFromRequest(
   req: { headers: { cookie?: string | undefined } },
   sessions: SessionStore,
-  users: UserRepository,
+  users: Pick<UserStore, "findById">,
   slideExpiry = true
 ): Promise<AuthUser | undefined> {
   const session = sessions.get(sessionIdFromRequest(req), slideExpiry);
@@ -74,10 +78,11 @@ const SAFE_METHODS = new Set(["GET", "HEAD", "OPTIONS"]);
  * CSRF defense for cookie-authenticated writes.
  *
  * SameSite=Lax alone is not enough here: every other service on the same
- * host (other ports of the same IP) counts as "same-site" and would get the
- * cookie attached. Browsers send Sec-Fetch-Site on every request, so a write
- * is only accepted from the app's own origin. Clients without the header
- * (curl, scripts) carry no ambient browser cookie and are not a CSRF vector.
+ * host (other ports of the same IP) counts as "same-site" and may receive the
+ * cookie. Fetch Metadata is also not guaranteed on plain HTTP in the LAN.
+ * Every write therefore needs an application header that HTML forms cannot
+ * set; a cross-origin fetch would require a CORS preflight, which the API does
+ * not permit. Sec-Fetch-Site remains an additional browser-side check.
  */
 export function requireSameOriginForWrites(): RequestHandler {
   return (req: Request, res: Response, next: NextFunction) => {
@@ -85,6 +90,10 @@ export function requireSameOriginForWrites(): RequestHandler {
       next();
       return;
     }
+    if (req.get(WRITE_REQUEST_HEADER_NAME) !== WRITE_REQUEST_HEADER_VALUE) {
+      res.status(403).json({ error: "Schreibzugriff ohne gültigen Anwendungsnachweis abgelehnt." });
+      return;
+    }
     const fetchSite = req.headers["sec-fetch-site"];
     if (fetchSite === undefined || fetchSite === "same-origin" || fetchSite === "none") {
       next();
@@ -98,7 +107,7 @@ export function requireSameOriginForWrites(): RequestHandler {
 export function createAuthGuard(
   authConfig: AuthConfig,
   sessions: SessionStore,
-  users: UserRepository,
+  users: Pick<UserStore, "findById">,
   openPaths: readonly string[] = ["/auth/login", "/auth/status", "/auth/logout"]
 ): RequestHandler {
   return (req: Request, res: Response, next: NextFunction) => {
diff --git a/src/server/auth/password.ts b/src/server/auth/password.ts
index 96814e1..97132af 100644
--- a/src/server/auth/password.ts
+++ b/src/server/auth/password.ts
@@ -4,6 +4,12 @@ const SCRYPT_N = 16384;
 const SCRYPT_R = 8;
 const SCRYPT_P = 1;
 const KEY_LENGTH = 64;
+const SALT_LENGTH = 16;
+const MAX_SCRYPT_MEMORY = 64 * 1024 * 1024;
+const MIN_SCRYPT_N = 1 << 14;
+const MAX_SCRYPT_N = 1 << 16;
+const MAX_SCRYPT_R = 16;
+const MAX_SCRYPT_P = 8;
 
 // A hand-rolled wrapper instead of util.promisify(scrypt): promisify only
 // resolves to scrypt's 3-argument overload, so the 4-argument form used for
@@ -30,8 +36,13 @@ function scryptAsync(
 // Format: scrypt$N$r$p$saltHex$hashHex - the parameters travel with the
 // hash so they can be tuned later without invalidating existing rows.
 export async function hashPassword(password: string): Promise<string> {
-  const salt = randomBytes(16);
-  const derivedKey = await scryptAsync(password, salt, KEY_LENGTH);
+  const salt = randomBytes(SALT_LENGTH);
+  const derivedKey = await scryptAsync(password, salt, KEY_LENGTH, {
+    N: SCRYPT_N,
+    r: SCRYPT_R,
+    p: SCRYPT_P,
+    maxmem: MAX_SCRYPT_MEMORY,
+  });
   return [
     "scrypt",
     SCRYPT_N,
@@ -52,12 +63,33 @@ export async function verifyPassword(
   const n = Number(nRaw);
   const r = Number(rRaw);
   const p = Number(pRaw);
-  if (!Number.isFinite(n) || !Number.isFinite(r) || !Number.isFinite(p)) {
+  if (
+    !Number.isSafeInteger(n) ||
+    n < MIN_SCRYPT_N ||
+    n > MAX_SCRYPT_N ||
+    (n & (n - 1)) !== 0 ||
+    !Number.isSafeInteger(r) ||
+    r < 1 ||
+    r > MAX_SCRYPT_R ||
+    !Number.isSafeInteger(p) ||
+    p < 1 ||
+    p > MAX_SCRYPT_P ||
+    !/^[0-9a-f]{32}$/i.test(saltHex) ||
+    !/^[0-9a-f]{128}$/i.test(hashHex)
+  ) {
     return false;
   }
   const salt = Buffer.from(saltHex, "hex");
   const expected = Buffer.from(hashHex, "hex");
-  const derivedKey = await scryptAsync(password, salt, expected.length, { N: n, r, p });
-  if (derivedKey.length !== expected.length) return false;
-  return timingSafeEqual(derivedKey, expected);
+  try {
+    const derivedKey = await scryptAsync(password, salt, expected.length, {
+      N: n,
+      r,
+      p,
+      maxmem: MAX_SCRYPT_MEMORY,
+    });
+    return timingSafeEqual(derivedKey, expected);
+  } catch {
+    return false;
+  }
 }
diff --git a/src/server/composition/auth.ts b/src/server/composition/auth.ts
index 1b5ce09..07b822b 100644
--- a/src/server/composition/auth.ts
+++ b/src/server/composition/auth.ts
@@ -3,9 +3,14 @@ import { LoginThrottle } from "../auth/login-throttle.js";
 import { SessionStore } from "../auth/sessions.js";
 import { UserService } from "../../domain/services/user.service.js";
 import { userRepository } from "./application-repositories.js";
+import { hashPassword, verifyPassword } from "../auth/password.js";
 
 export const authConfig = readAuthConfig();
 export const sessionStore = new SessionStore(authConfig.sessionTtlMs, authConfig.sessionMaxAgeMs);
 sessionStore.startSweeping();
 export const loginThrottle = new LoginThrottle();
-export const userService = new UserService(userRepository, authConfig);
+export const userService = new UserService(
+  userRepository,
+  { bootstrap: authConfig.bootstrap },
+  { hash: hashPassword, verify: verifyPassword }
+);
diff --git a/src/shared/constants/request-security.ts b/src/shared/constants/request-security.ts
new file mode 100644
index 0000000..f89ce10
--- /dev/null
+++ b/src/shared/constants/request-security.ts
@@ -0,0 +1,2 @@
+export const WRITE_REQUEST_HEADER_NAME = "X-Leistungsbilanz-Request";
+export const WRITE_REQUEST_HEADER_VALUE = "1";
diff --git a/tests/authentication.test.ts b/tests/authentication.test.ts
index e634716..e5c431d 100644
--- a/tests/authentication.test.ts
+++ b/tests/authentication.test.ts
@@ -21,6 +21,10 @@ import { requireSameOriginForWrites } from "../src/server/auth/auth.middleware.j
 import type { AuthConfig } from "../src/server/auth/auth-config.js";
 import { createUserSchema } from "../src/shared/validation/user.schemas.js";
 import { loginSchema } from "../src/shared/validation/auth.schemas.js";
+import {
+  WRITE_REQUEST_HEADER_NAME,
+  WRITE_REQUEST_HEADER_VALUE,
+} from "../src/shared/constants/request-security.js";
 
 const HOUR = 60 * 60 * 1000;
 
@@ -34,7 +38,14 @@ function createUserService() {
     cookieSecure: false,
     bootstrap: { username: "admin", password: "bootstrap-password-123" },
   };
-  return { repository, service: new UserService(repository, config) };
+  return {
+    repository,
+    service: new UserService(
+      repository,
+      { bootstrap: config.bootstrap },
+      { hash: hashPassword, verify: verifyPassword }
+    ),
+  };
 }
 
 describe("password hashing", () => {
@@ -52,6 +63,17 @@ describe("password hashing", () => {
   it("rejects malformed stored hashes instead of throwing", async () => {
     assert.equal(await verifyPassword("x", "not-a-hash"), false);
     assert.equal(await verifyPassword("x", "scrypt$a$b$c$00$00"), false);
+    assert.equal(
+      await verifyPassword(
+        "x",
+        `scrypt$1073741824$8$1$${"00".repeat(16)}$${"00".repeat(64)}`
+      ),
+      false
+    );
+    assert.equal(
+      await verifyPassword("x", `scrypt$16384$8$1$00$${"00".repeat(64)}`),
+      false
+    );
   });
 });
 
@@ -135,12 +157,20 @@ describe("cookie parsing", () => {
 });
 
 describe("same-origin guard for writes", () => {
-  function run(method: string, fetchSite?: string) {
+  function run(method: string, fetchSite?: string, includeApplicationHeader = true) {
     let status: number | undefined;
     let passed = false;
     const req = {
       method,
-      headers: fetchSite ? { "sec-fetch-site": fetchSite } : {},
+      headers: {
+        ...(fetchSite ? { "sec-fetch-site": fetchSite } : {}),
+        ...(includeApplicationHeader
+          ? { [WRITE_REQUEST_HEADER_NAME.toLowerCase()]: WRITE_REQUEST_HEADER_VALUE }
+          : {}),
+      },
+      get(name: string) {
+        return this.headers[name.toLowerCase() as keyof typeof this.headers];
+      },
     } as unknown as Request;
     const res = {
       status(code: number) {
@@ -158,12 +188,13 @@ describe("same-origin guard for writes", () => {
   }
 
   it("lets reads and same-origin writes through", () => {
-    assert.equal(run("GET", "cross-site").passed, true);
+    assert.equal(run("GET", "cross-site", false).passed, true);
     assert.equal(run("POST", "same-origin").passed, true);
-    assert.equal(run("POST").passed, true, "non-browser clients carry no ambient cookie");
+    assert.equal(run("POST").passed, true, "scripts can send the explicit application header");
   });
 
-  it("rejects writes from other sites and other ports of the same host", () => {
+  it("rejects writes without the application header or from another origin", () => {
+    assert.deepEqual(run("POST", undefined, false), { passed: false, status: 403 });
     assert.deepEqual(run("POST", "cross-site"), { passed: false, status: 403 });
     assert.deepEqual(run("PATCH", "same-site"), { passed: false, status: 403 });
     assert.deepEqual(run("DELETE", "same-site"), { passed: false, status: 403 });
@@ -239,6 +270,30 @@ describe("user service", () => {
     await assert.rejects(service.updateRoleAndActive(second.id, { active: false }), LastAdminError);
   });
 
+  it("preserves one active admin across concurrent updates", async () => {
+    const { repository, service } = createUserService();
+    await service.ensureBootstrapAdmin();
+    const second = await service.createUser({
+      username: "zweiter-admin",
+      name: "Zweiter Admin",
+      role: "admin",
+      initialPassword: "start-passwort",
+    });
+    assert.ok(second);
+    const [first] = (await repository.list()).filter((user) => user.username === "admin");
+
+    const results = await Promise.allSettled([
+      service.updateRoleAndActive(first.id, { role: "user" }),
+      service.updateRoleAndActive(second.id, { role: "user" }),
+    ]);
+    assert.equal(results.filter((result) => result.status === "fulfilled").length, 1);
+    assert.equal(results.filter((result) => result.status === "rejected").length, 1);
+    const activeAdmins = (await repository.list()).filter(
+      (user) => user.role === "admin" && user.active
+    );
+    assert.equal(activeAdmins.length, 1);
+  });
+
   it("requires the current password and a different new one", async () => {
     const { service } = createUserService();
     const user = await service.createUser({
diff --git a/tests/config-access.test.ts b/tests/config-access.test.ts
new file mode 100644
index 0000000..3fdc83d
--- /dev/null
+++ b/tests/config-access.test.ts
@@ -0,0 +1,26 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import {
+  getConfigEntry,
+  getConfigRedirect,
+  getConfigTabs,
+} from "../src/frontend/utils/config-access.js";
+
+describe("config access", () => {
+  it("offers account settings to every signed-in user", () => {
+    assert.deepEqual(getConfigEntry(false), { href: "/config/konto", label: "Konto" });
+    assert.deepEqual(getConfigTabs(false), [{ href: "/config/konto", label: "Konto" }]);
+    assert.equal(getConfigRedirect("/config", false), "/config/konto");
+    assert.equal(getConfigRedirect("/config/konto", false), null);
+  });
+
+  it("keeps user administration admin-only", () => {
+    assert.equal(getConfigRedirect("/config/benutzer", false), "/config/konto");
+    assert.deepEqual(getConfigTabs(true), [
+      { href: "/config/benutzer", label: "Benutzer" },
+      { href: "/config/konto", label: "Konto" },
+    ]);
+    assert.deepEqual(getConfigEntry(true), { href: "/config/benutzer", label: "Config" });
+    assert.equal(getConfigRedirect("/config/benutzer", true), null);
+  });
+});
diff --git a/tests/write-request-headers.test.ts b/tests/write-request-headers.test.ts
new file mode 100644
index 0000000..c78a277
--- /dev/null
+++ b/tests/write-request-headers.test.ts
@@ -0,0 +1,40 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import { authApi } from "../src/frontend/utils/auth-api.js";
+import { createProject } from "../src/frontend/utils/api.js";
+import { calculateCableSizing } from "../src/frontend/components/cable-sizing-api.js";
+import type { CableSizingInput } from "../src/cable-sizing/domain/cable-sizing-calculation.js";
+import {
+  WRITE_REQUEST_HEADER_NAME,
+  WRITE_REQUEST_HEADER_VALUE,
+} from "../src/shared/constants/request-security.js";
+
+describe("write request clients", () => {
+  it("send the application header from every frontend API client", async () => {
+    const originalFetch = globalThis.fetch;
+    const requests: RequestInit[] = [];
+    globalThis.fetch = async (_input, init) => {
+      requests.push(init ?? {});
+      return new Response(JSON.stringify({}), {
+        status: 200,
+        headers: { "Content-Type": "application/json" },
+      });
+    };
+
+    try {
+      await authApi.login("admin", "start-passwort");
+      await createProject("Testprojekt");
+      await calculateCableSizing({} as CableSizingInput);
+    } finally {
+      globalThis.fetch = originalFetch;
+    }
+
+    assert.equal(requests.length, 3);
+    for (const request of requests) {
+      assert.equal(
+        new Headers(request.headers).get(WRITE_REQUEST_HEADER_NAME),
+        WRITE_REQUEST_HEADER_VALUE
+      );
+    }
+  });
+});