diff --git a/.dockerignore b/.dockerignore
index 15a9921..a2817b6 100644
--- a/.dockerignore
+++ b/.dockerignore
@@ -9,11 +9,8 @@ node_modules
 .gitignore
 .dockerignore
 
-# Documentation & screenshots (not needed at runtime)
-docs/screenshots/
-docs/superpowers/
-docs/social-preview.png
-docs/logo.svg
+# Documentation (not needed at runtime)
+docs/
 
 # Tests
 test-*.js
diff --git a/CHANGELOG.md b/CHANGELOG.md
index c5f182f..8e2c1ae 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,8 +7,21 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.5.3] - 2026-04-03
+
 ### Security
+- Fix SQLCipher PRAGMA key interpolation — encryption keys containing single quotes no longer crash on startup; key is now hex-encoded
+- Enforce minimum password length (8 characters) when admin creates new users — previously any 1-character password was accepted
+- Add length bounds on username (64 chars) and display_name (128 chars) to prevent unbounded input
+- Add input length bounds on login (username 64 chars, password 1024 chars)
+- Invalidate all other sessions when a user changes their password — previously active sessions survived password reset
 - Session and CSRF cookies now have `secure: true` by default; HTTP is only allowed when `SESSION_SECURE=false` is explicitly set in `.env` — previously cookies were sent without `Secure` flag in non-production environments
+- Document authorization model in SECURITY.md — clarify that all family members share read/write access to all data by design
+
+### Changed
+- Use multi-stage Docker build to exclude build tools (python3, make, g++) from runtime image
+- Exclude `docs/` directory from Docker image via `.dockerignore`
+- Consolidate `dotenv.config()` to single call in `server/index.js` — remove duplicate calls from `server/db.js` and `server/auth.js`
 
 ## [0.5.2] - 2026-04-01
 
diff --git a/Dockerfile b/Dockerfile
index c0f870b..afd82d3 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM node:22-slim
+FROM node:22-slim AS build
 
 # SQLCipher-Abhängigkeiten
 RUN apt-get update && apt-get install -y \
@@ -6,7 +6,6 @@ RUN apt-get update && apt-get install -y \
     make \
     g++ \
     libsqlcipher-dev \
-    gosu \
     && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app
@@ -15,7 +14,20 @@ WORKDIR /app
 COPY package*.json ./
 RUN npm ci --omit=dev
 
-# Anwendungscode
+# ---- Runtime stage ----
+FROM node:22-slim
+
+RUN apt-get update && apt-get install -y \
+    libsqlcipher0 \
+    gosu \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+
+# Node modules aus Build-Stage kopieren
+COPY --from=build /app/node_modules ./node_modules
+
+# Anwendungscode (docs/ wird via .dockerignore ausgeschlossen)
 COPY . .
 
 # Daten-Volume-Verzeichnis anlegen (Permissions werden zur Laufzeit gesetzt)
diff --git a/SECURITY.md b/SECURITY.md
index 8239fdf..d599414 100644
--- a/SECURITY.md
+++ b/SECURITY.md
@@ -36,6 +36,15 @@ Vulnerabilities that require physical access to the host or root on the server a
 - Optional SQLCipher AES-256 database encryption
 - No API endpoint accessible without session auth (except login)
 
+## Authorization Model
+
+Oikos uses a flat family authorization model:
+
+- **Admin** can create, edit, and delete all user accounts and all shared data.
+- **Member** can read and write all shared data (tasks, shopping lists, meals, calendar events, notes, contacts, budget entries) but cannot manage user accounts.
+
+There is no per-user data isolation — all family members see and can edit all data. This is intentional: Oikos is a shared family planner, not a multi-tenant application.
+
 ## Supported Versions
 
 Only the latest version on `main` receives security updates. There are no LTS branches.
diff --git a/server/auth.js b/server/auth.js
index 85a2434..68fe22c 100644
--- a/server/auth.js
+++ b/server/auth.js
@@ -6,7 +6,6 @@
 
 'use strict';
 
-require('dotenv').config();
 const express = require('express');
 const bcrypt = require('bcrypt');
 const session = require('express-session');
@@ -164,6 +163,10 @@ router.post('/login', loginLimiter, async (req, res) => {
       return res.status(400).json({ error: 'Benutzername und Passwort erforderlich.', code: 400 });
     }
 
+    if (username.length > 64 || password.length > 1024) {
+      return res.status(400).json({ error: 'Eingabe zu lang.', code: 400 });
+    }
+
     const user = db.get().prepare('SELECT * FROM users WHERE username = ?').get(username);
 
     if (!user) {
@@ -279,6 +282,18 @@ router.post('/users', requireAuth, requireAdmin, csrfMiddleware, async (req, res
       return res.status(400).json({ error: 'Benutzername, Anzeigename und Passwort erforderlich.', code: 400 });
     }
 
+    if (password.length < 8) {
+      return res.status(400).json({ error: 'Passwort muss mindestens 8 Zeichen haben.', code: 400 });
+    }
+
+    if (username.length > 64) {
+      return res.status(400).json({ error: 'Benutzername darf maximal 64 Zeichen lang sein.', code: 400 });
+    }
+
+    if (display_name.length > 128) {
+      return res.status(400).json({ error: 'Anzeigename darf maximal 128 Zeichen lang sein.', code: 400 });
+    }
+
     if (!['admin', 'member'].includes(role)) {
       return res.status(400).json({ error: 'Ungültige Rolle.', code: 400 });
     }
@@ -330,6 +345,19 @@ router.patch('/me/password', requireAuth, csrfMiddleware, async (req, res) => {
     const hash = await bcrypt.hash(new_password, 12);
     db.get().prepare('UPDATE users SET password_hash = ? WHERE id = ?').run(hash, req.session.userId);
 
+    // Alle anderen Sessions dieses Users invalidieren (aktuelle behalten)
+    const currentSid = req.sessionID;
+    const allSessions = db.get().prepare('SELECT sid, sess FROM sessions').all();
+    for (const row of allSessions) {
+      if (row.sid === currentSid) continue;
+      try {
+        const sess = JSON.parse(row.sess);
+        if (sess.userId === req.session.userId) {
+          db.get().prepare('DELETE FROM sessions WHERE sid = ?').run(row.sid);
+        }
+      } catch { /* ignore malformed session */ }
+    }
+
     res.json({ ok: true });
   } catch (err) {
     console.error('[Auth] Passwort-Ändern-Fehler:', err);
diff --git a/server/db.js b/server/db.js
index 4be645f..c146d95 100644
--- a/server/db.js
+++ b/server/db.js
@@ -1,7 +1,7 @@
 /**
  * Modul: Datenbank (Database)
  * Zweck: SQLite/SQLCipher Verbindung, Schema-Migration (versioniert) und Query-Helfer
- * Abhängigkeiten: better-sqlite3, dotenv
+ * Abhängigkeiten: better-sqlite3
  *
  * SQLCipher-Hinweis:
  *   Verschlüsselung funktioniert nur wenn better-sqlite3 gegen SQLCipher kompiliert wurde.
@@ -11,7 +11,6 @@
 
 'use strict';
 
-require('dotenv').config();
 const Database = require('better-sqlite3');
 const path = require('path');
 
@@ -34,7 +33,7 @@ function init() {
 
   if (DB_KEY) {
     // Nur wirksam wenn Binary gegen SQLCipher kompiliert ist (Docker)
-    db.pragma(`key='${DB_KEY}'`);
+    db.pragma(`key=x'${Buffer.from(DB_KEY, 'utf8').toString('hex')}'`);
     // Sicherstellen dass die Datenbank tatsächlich entschlüsselbar ist
     try {
       db.prepare('SELECT count(*) FROM sqlite_master').get();