diff --git a/backend/src/main/java/com/recipeapp/auth/AuthController.java b/backend/src/main/java/com/recipeapp/auth/AuthController.java
index d0f6605..93abcb1 100644
--- a/backend/src/main/java/com/recipeapp/auth/AuthController.java
+++ b/backend/src/main/java/com/recipeapp/auth/AuthController.java
@@ -7,15 +7,10 @@ import jakarta.servlet.http.HttpSession;
 import jakarta.validation.Valid;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
-import org.springframework.security.authentication.UsernamePasswordAuthenticationToken;
-import org.springframework.security.core.authority.SimpleGrantedAuthority;
-import org.springframework.security.core.context.SecurityContext;
 import org.springframework.security.core.context.SecurityContextHolder;
-import org.springframework.security.web.context.HttpSessionSecurityContextRepository;
 import org.springframework.web.bind.annotation.*;
 
 import java.security.Principal;
-import java.util.List;
 
 @RestController
 @RequestMapping("/v1/auth")
@@ -32,7 +27,7 @@ public class AuthController {
             @Valid @RequestBody SignupRequest request,
             HttpServletRequest httpRequest) {
         UserResponse user = authService.signup(request);
-        authenticateInSession(user.email(), "user", httpRequest);
+        authService.authenticateInSession(user.email(), "user", httpRequest);
         return ResponseEntity.status(HttpStatus.CREATED).body(ApiResponse.success(user));
     }
 
@@ -41,30 +36,10 @@ public class AuthController {
             @Valid @RequestBody LoginRequest request,
             HttpServletRequest httpRequest) {
         UserResponse user = authService.login(request);
-        // Session fixation protection: invalidate old session before creating new one
-        var oldSession = httpRequest.getSession(false);
-        if (oldSession != null) {
-            oldSession.invalidate();
-        }
-        authenticateInSession(user.email(), user.systemRole() != null ? user.systemRole() : "user", httpRequest);
+        authService.authenticateInSession(user.email(), user.systemRole() != null ? user.systemRole() : "user", httpRequest);
         return ResponseEntity.ok(ApiResponse.success(user));
     }
 
-    /**
-     * Creates an authenticated Spring Security context and stores it in the HTTP session
-     * so that subsequent requests from the same session are recognised as authenticated.
-     * We do this manually because we are not using Spring Security's built-in form login.
-     */
-    private void authenticateInSession(String email, String role, HttpServletRequest request) {
-        var auth = UsernamePasswordAuthenticationToken.authenticated(
-                email, null, List.of(new SimpleGrantedAuthority("ROLE_" + role.toUpperCase())));
-        SecurityContext context = SecurityContextHolder.createEmptyContext();
-        context.setAuthentication(auth);
-        SecurityContextHolder.setContext(context);
-        request.getSession(true).setAttribute(
-                HttpSessionSecurityContextRepository.SPRING_SECURITY_CONTEXT_KEY, context);
-    }
-
     @PostMapping("/logout")
     public ResponseEntity<Void> logout(HttpServletRequest httpRequest) {
         HttpSession session = httpRequest.getSession(false);
diff --git a/backend/src/main/java/com/recipeapp/auth/AuthService.java b/backend/src/main/java/com/recipeapp/auth/AuthService.java
index 1f007c6..feaee08 100644
--- a/backend/src/main/java/com/recipeapp/auth/AuthService.java
+++ b/backend/src/main/java/com/recipeapp/auth/AuthService.java
@@ -7,10 +7,18 @@ import com.recipeapp.common.ResourceNotFoundException;
 import com.recipeapp.common.ValidationException;
 import com.recipeapp.household.HouseholdMemberRepository;
 import com.recipeapp.household.entity.HouseholdMember;
+import jakarta.servlet.http.HttpServletRequest;
+import org.springframework.security.authentication.UsernamePasswordAuthenticationToken;
+import org.springframework.security.core.authority.SimpleGrantedAuthority;
+import org.springframework.security.core.context.SecurityContext;
+import org.springframework.security.core.context.SecurityContextHolder;
 import org.springframework.security.crypto.password.PasswordEncoder;
+import org.springframework.security.web.context.HttpSessionSecurityContextRepository;
 import org.springframework.stereotype.Service;
 import org.springframework.transaction.annotation.Transactional;
 
+import java.util.List;
+
 @Service
 public class AuthService {
 
@@ -82,6 +90,24 @@ public class AuthService {
         return UserResponse.basic(user.getId(), user.getEmail(), user.getDisplayName());
     }
 
+    /**
+     * Establishes an authenticated Spring Security session for the given user.
+     * Invalidates any existing session first (session fixation protection).
+     */
+    public void authenticateInSession(String email, String role, HttpServletRequest request) {
+        var oldSession = request.getSession(false);
+        if (oldSession != null) {
+            oldSession.invalidate();
+        }
+        var auth = UsernamePasswordAuthenticationToken.authenticated(
+                email, null, List.of(new SimpleGrantedAuthority("ROLE_" + role.toUpperCase())));
+        SecurityContext context = SecurityContextHolder.createEmptyContext();
+        context.setAuthentication(auth);
+        SecurityContextHolder.setContext(context);
+        request.getSession(true).setAttribute(
+                HttpSessionSecurityContextRepository.SPRING_SECURITY_CONTEXT_KEY, context);
+    }
+
     private UserResponse toUserResponse(UserAccount user) {
         return householdMemberRepository.findByUserEmailIgnoreCase(user.getEmail())
                 .map(member -> UserResponse.withHousehold(
diff --git a/backend/src/main/java/com/recipeapp/household/HouseholdController.java b/backend/src/main/java/com/recipeapp/household/HouseholdController.java
index d009393..c3eb59d 100644
--- a/backend/src/main/java/com/recipeapp/household/HouseholdController.java
+++ b/backend/src/main/java/com/recipeapp/household/HouseholdController.java
@@ -1,17 +1,12 @@
 package com.recipeapp.household;
 
-import com.recipeapp.auth.entity.UserAccount;
+import com.recipeapp.auth.AuthService;
 import com.recipeapp.common.ApiResponse;
 import com.recipeapp.household.dto.*;
 import jakarta.servlet.http.HttpServletRequest;
 import jakarta.validation.Valid;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
-import org.springframework.security.authentication.UsernamePasswordAuthenticationToken;
-import org.springframework.security.core.authority.SimpleGrantedAuthority;
-import org.springframework.security.core.context.SecurityContext;
-import org.springframework.security.core.context.SecurityContextHolder;
-import org.springframework.security.web.context.HttpSessionSecurityContextRepository;
 import org.springframework.web.bind.annotation.*;
 
 import java.security.Principal;
@@ -24,9 +19,11 @@ import java.util.UUID;
 public class HouseholdController {
 
     private final HouseholdService householdService;
+    private final AuthService authService;
 
-    public HouseholdController(HouseholdService householdService) {
+    public HouseholdController(HouseholdService householdService, AuthService authService) {
         this.householdService = householdService;
+        this.authService = authService;
     }
 
     @PostMapping("/households")
@@ -91,21 +88,7 @@ public class HouseholdController {
             HttpServletRequest httpRequest) {
         AcceptInviteResponse response = householdService.acceptInvite(
                 code, request.name(), request.email(), request.password());
-        authenticateInSession(request.email(), httpRequest);
+        authService.authenticateInSession(request.email(), "user", httpRequest);
         return ResponseEntity.ok(ApiResponse.success(response));
     }
-
-    private void authenticateInSession(String email, HttpServletRequest request) {
-        var oldSession = request.getSession(false);
-        if (oldSession != null) {
-            oldSession.invalidate();
-        }
-        var auth = UsernamePasswordAuthenticationToken.authenticated(
-                email, null, List.of(new SimpleGrantedAuthority("ROLE_USER")));
-        SecurityContext context = SecurityContextHolder.createEmptyContext();
-        context.setAuthentication(auth);
-        SecurityContextHolder.setContext(context);
-        request.getSession(true).setAttribute(
-                HttpSessionSecurityContextRepository.SPRING_SECURITY_CONTEXT_KEY, context);
-    }
 }
diff --git a/backend/src/test/java/com/recipeapp/household/HouseholdControllerTest.java b/backend/src/test/java/com/recipeapp/household/HouseholdControllerTest.java
index c889963..b7bb1c8 100644
--- a/backend/src/test/java/com/recipeapp/household/HouseholdControllerTest.java
+++ b/backend/src/test/java/com/recipeapp/household/HouseholdControllerTest.java
@@ -1,6 +1,7 @@
 package com.recipeapp.household;
 
 import com.fasterxml.jackson.databind.ObjectMapper;
+import com.recipeapp.auth.AuthService;
 import com.recipeapp.common.GlobalExceptionHandler;
 import com.recipeapp.common.ResourceNotFoundException;
 import com.recipeapp.common.ConflictException;
@@ -36,6 +37,9 @@ class HouseholdControllerTest {
     @Mock
     private HouseholdService householdService;
 
+    @Mock
+    private AuthService authService;
+
     @InjectMocks
     private HouseholdController householdController;