feat(auth): rate limit registration, atomic login limit

10 failed registrations per IP block it for 15 minutes (429 TOO_MANY_ATTEMPTS with Retry-After). Login and registration now take an attempt before the check, so a burst of parallel requests cannot pass the limit.

src/main/java/auth_service/modules/login/api/LoginController.java

@@ -4,6 +4,7 @@ import auth_service.modules.login.api.dto.request.LoginRequest;
 import auth_service.modules.login.api.dto.response.LoginResponse;
 import auth_service.modules.login.api.mapper.ApiMapper;
 import auth_service.modules.login.service.LoginService;
+import auth_service.shared.api.ClientIp;
 import io.micrometer.core.annotation.Timed;
 import jakarta.servlet.http.HttpServletRequest;
 import jakarta.validation.Valid;
@@ -35,20 +36,7 @@ public class LoginController {
     public ResponseEntity<LoginResponse> login(@Valid @RequestBody LoginRequest request, HttpServletRequest httpRequest) {
 
         var user = ApiMapper.toModel(request);
-        var token = loginService.login(user, clientIp(httpRequest));
+        var token = loginService.login(user, ClientIp.of(httpRequest));
         return ResponseEntity.ok(new LoginResponse("Login successful", token));
     }
-
-    /**
-     * Traefik appends the real client IP as the last X-Forwarded-For entry. Earlier entries come
-     * from the client and can be faked, so only the last one counts.
-     */
-    private static String clientIp(HttpServletRequest request) {
-        String forwardedFor = request.getHeader("X-Forwarded-For");
-        if (forwardedFor == null || forwardedFor.isBlank()) {
-            return request.getRemoteAddr();
-        }
-        String[] hops = forwardedFor.split(",");
-        return hops[hops.length - 1].trim();
-    }
 }

src/main/java/auth_service/modules/login/service/LoginAttemptService.java

@@ -1,13 +1,11 @@
 package auth_service.modules.login.service;
 
 import auth_service.modules.login.exception.TooManyLoginAttemptsException;
-import com.github.benmanes.caffeine.cache.Cache;
-import com.github.benmanes.caffeine.cache.Caffeine;
+import auth_service.shared.security.AttemptLimiter;
 import org.springframework.stereotype.Service;
 
 import java.time.Duration;
 import java.util.Locale;
-import java.util.concurrent.atomic.AtomicInteger;
 
 /**
  * Counts failed logins per email and per client IP in memory. A key is blocked after too many
@@ -20,36 +18,22 @@ public class LoginAttemptService {
     static final int MAX_FAILURES_PER_IP = 20;
     static final Duration LOCK_DURATION = Duration.ofMinutes(15);
 
-    private final Cache<String, AtomicInteger> failures = Caffeine.newBuilder()
-            .expireAfterWrite(LOCK_DURATION)
-            .maximumSize(100_000)
-            .build();
+    private final AttemptLimiter limiter = new AttemptLimiter(LOCK_DURATION);
 
-    public void checkAllowed(String email, String clientIp) {
-        if (count(emailKey(email)) >= MAX_FAILURES_PER_EMAIL || count(ipKey(clientIp)) >= MAX_FAILURES_PER_IP) {
+    public void acquire(String email, String clientIp) {
+        String emailKey = emailKey(email);
+        if (!limiter.tryAcquire(emailKey, MAX_FAILURES_PER_EMAIL)) {
+            throw new TooManyLoginAttemptsException(LOCK_DURATION.toSeconds());
+        }
+        if (!limiter.tryAcquire(ipKey(clientIp), MAX_FAILURES_PER_IP)) {
+            limiter.release(emailKey);
             throw new TooManyLoginAttemptsException(LOCK_DURATION.toSeconds());
         }
     }
 
-    public void recordFailure(String email, String clientIp) {
-        increment(emailKey(email));
-        increment(ipKey(clientIp));
-    }
-
-    public void recordSuccess(String email) {
-        failures.invalidate(emailKey(email));
-    }
-
-    private int count(String key) {
-        AtomicInteger counter = failures.getIfPresent(key);
-        return counter == null ? 0 : counter.get();
-    }
-
-    private void increment(String key) {
-        // put() again so expireAfterWrite restarts the lock window with every failure
-        AtomicInteger counter = failures.get(key, k -> new AtomicInteger());
-        counter.incrementAndGet();
-        failures.put(key, counter);
+    public void recordSuccess(String email, String clientIp) {
+        limiter.reset(emailKey(email));
+        limiter.release(ipKey(clientIp));
     }
 
     private static String emailKey(String email) {

src/main/java/auth_service/modules/login/service/LoginService.java

@@ -39,19 +39,18 @@ public class LoginService {
      */
     public String login(User userToLogin, String clientIp) {
         String email = userToLogin.getEmail();
-        loginAttemptService.checkAllowed(email, clientIp);
+        loginAttemptService.acquire(email, clientIp);
 
         Optional<User> foundUser = minimalUserRepository.findByEmail(email);
         String hash = foundUser.map(User::getPassword).orElse(dummyHash);
         boolean passwordMatches = passwordEncoder.matches(userToLogin.getPassword(), hash);
 
         if (foundUser.isEmpty() || !passwordMatches || !foundUser.get().isEnabled()) {
-            loginAttemptService.recordFailure(email, clientIp);
             throw new InvalidCredentialsException("Invalid email or password");
         }
 
         User user = foundUser.get();
-        loginAttemptService.recordSuccess(email);
+        loginAttemptService.recordSuccess(email, clientIp);
         minimalUserRepository.updateLoginDate(user.getId());
         return tokenGenerator.generateToken(String.valueOf(user.getId()), user.getEmail(), user.getSteamId());
     }

src/main/java/auth_service/modules/registration/api/RegistrationController.java

@@ -4,7 +4,9 @@ import auth_service.modules.registration.api.dto.mapper.ApiMapper;
 import auth_service.modules.registration.api.dto.request.RegistrationRequest;
 import auth_service.modules.registration.api.dto.response.RegistrationResponse;
 import auth_service.modules.registration.service.RegistrationService;
+import auth_service.shared.api.ClientIp;
 import io.micrometer.core.annotation.Timed;
+import jakarta.servlet.http.HttpServletRequest;
 import jakarta.validation.Valid;
 import lombok.RequiredArgsConstructor;
 import org.springframework.http.ResponseEntity;
@@ -22,8 +24,8 @@ public class RegistrationController {
 
     @PostMapping(value = "/new", version = "1.0", produces = "application/json", consumes = "application/json")
     @Timed(value = "auth.registration.new.requests.time", description = "Time taken to process registration requests")
-    public ResponseEntity<RegistrationResponse> registration(@Valid @RequestBody RegistrationRequest request) {
-        var userId = registrationService.register(ApiMapper.mapToModel(request));
+    public ResponseEntity<RegistrationResponse> registration(@Valid @RequestBody RegistrationRequest request, HttpServletRequest httpRequest) {
+        var userId = registrationService.register(ApiMapper.mapToModel(request), ClientIp.of(httpRequest));
         return ResponseEntity.ok(RegistrationResponse.builder().userId(userId).build());
     }
 }

src/main/java/auth_service/modules/registration/api/RegistrationExceptionController.java

@@ -5,9 +5,11 @@ import auth_service.modules.registration.exception.EmailAlreadyInUseException;
 import auth_service.modules.registration.exception.InvalidInvitationCodeException;
 import auth_service.modules.registration.exception.DisplayNameAlreadyRegisteredException;
 import auth_service.modules.registration.exception.SteamIdAlreadyRegisteredException;
+import auth_service.modules.registration.exception.TooManyRegistrationAttemptsException;
 import lombok.NonNull;
 import lombok.extern.slf4j.Slf4j;
 import org.springframework.core.annotation.Order;
+import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.ExceptionHandler;
@@ -24,6 +26,15 @@ public class RegistrationExceptionController {
                 .body(body);
     }
 
+    @ExceptionHandler(TooManyRegistrationAttemptsException.class)
+    public ResponseEntity<ErrorResponse> clientError(TooManyRegistrationAttemptsException ex) {
+        ErrorResponse body = new ErrorResponse("TOO_MANY_ATTEMPTS", ex.getMessage());
+        return ResponseEntity.status(HttpStatus.TOO_MANY_REQUESTS)
+                .header(HttpHeaders.RETRY_AFTER, String.valueOf(ex.getRetryAfterSeconds()))
+                .contentType(org.springframework.http.MediaType.APPLICATION_JSON)
+                .body(body);
+    }
+
     @ExceptionHandler(EmailAlreadyInUseException.class)
     public ResponseEntity<ErrorResponse> clientError(EmailAlreadyInUseException ex) {
         var body = new ErrorResponse("EMAIL_ALREADY_IN_USE", ex.getMessage());

src/main/java/auth_service/modules/registration/exception/TooManyRegistrationAttemptsException.java

@@ -0,0 +1,13 @@
+package auth_service.modules.registration.exception;
+
+import lombok.Getter;
+
+@Getter
+public class TooManyRegistrationAttemptsException extends RuntimeException {
+    private final long retryAfterSeconds;
+
+    public TooManyRegistrationAttemptsException(long retryAfterSeconds) {
+        super("Too many failed registration attempts. Try again later.");
+        this.retryAfterSeconds = retryAfterSeconds;
+    }
+}

src/main/java/auth_service/modules/registration/service/RegistrationAttemptService.java

@@ -0,0 +1,30 @@
+package auth_service.modules.registration.service;
+
+import auth_service.modules.registration.exception.TooManyRegistrationAttemptsException;
+import auth_service.shared.security.AttemptLimiter;
+import org.springframework.stereotype.Service;
+
+import java.time.Duration;
+
+/**
+ * Blocks a client IP after too many failed registrations. Every rejection counts, so guessing the
+ * invitation code and probing which emails have an account are both limited.
+ */
+@Service
+public class RegistrationAttemptService {
+
+    static final int MAX_FAILURES_PER_IP = 10;
+    static final Duration LOCK_DURATION = Duration.ofMinutes(15);
+
+    private final AttemptLimiter limiter = new AttemptLimiter(LOCK_DURATION);
+
+    public void acquire(String clientIp) {
+        if (!limiter.tryAcquire(clientIp, MAX_FAILURES_PER_IP)) {
+            throw new TooManyRegistrationAttemptsException(LOCK_DURATION.toSeconds());
+        }
+    }
+
+    public void release(String clientIp) {
+        limiter.release(clientIp);
+    }
+}

src/main/java/auth_service/modules/registration/service/RegistrationService.java

@@ -17,8 +17,20 @@ public class RegistrationService {
     private final CodeValidationService codeValidationService;
     private final RegisteredUserRepository registeredUserRepository;
     private final PasswordEncoder passwordEncoder;
+    private final RegistrationAttemptService registrationAttemptService;
 
-    public long register(Registration registration) {
+    public long register(Registration registration, String clientIp) {
+        registrationAttemptService.acquire(clientIp);
+        validate(registration);
+        registrationAttemptService.release(clientIp);
+
+        var rawPassword = registration.getRawPassword();
+        registration.setEncryptedPassword(passwordEncoder.encode(rawPassword));
+
+        return registeredUserRepository.save(registration);
+    }
+
+    private void validate(Registration registration) {
         if (!codeValidationService.isCodeValid(registration.getInviteCode())) {
             throw new InvalidInvitationCodeException("The provided invite code is invalid or has already been used.");
         }
@@ -34,10 +46,5 @@ public class RegistrationService {
         if (registration.getDisplayName() != null && registeredUserRepository.existsByDisplayName(registration.getDisplayName())) {
             throw new DisplayNameAlreadyRegisteredException("The display name is already in use.");
         }
-
-        var rawPassword = registration.getRawPassword();
-        registration.setEncryptedPassword(passwordEncoder.encode(rawPassword));
-
-        return registeredUserRepository.save(registration);
     }
 }

src/main/java/auth_service/shared/api/ClientIp.java

@@ -0,0 +1,23 @@
+package auth_service.shared.api;
+
+import jakarta.servlet.http.HttpServletRequest;
+
+public final class ClientIp {
+
+    private ClientIp() {
+    }
+
+    /**
+     * Traefik appends the real client IP as the last X-Forwarded-For entry. Earlier entries come
+     * from the client and can be faked, so only the last one counts. Without a usable last entry
+     * (no proxy in front) the connection address counts.
+     */
+    public static String of(HttpServletRequest request) {
+        String forwardedFor = request.getHeader("X-Forwarded-For");
+        if (forwardedFor == null) {
+            return request.getRemoteAddr();
+        }
+        String lastHop = forwardedFor.substring(forwardedFor.lastIndexOf(',') + 1).trim();
+        return lastHop.isEmpty() ? request.getRemoteAddr() : lastHop;
+    }
+}

src/main/java/auth_service/shared/security/AttemptLimiter.java

@@ -0,0 +1,49 @@
+package auth_service.shared.security;
+
+import com.github.benmanes.caffeine.cache.Cache;
+import com.github.benmanes.caffeine.cache.Caffeine;
+
+import java.time.Duration;
+import java.util.concurrent.atomic.AtomicInteger;
+
+/**
+ * Counts attempts per key in memory. Callers take an attempt before the check and give it back on
+ * success, so only failures stay counted. A key is forgotten lockDuration after its last counted
+ * attempt. A restart resets all counters.
+ */
+public class AttemptLimiter {
+
+    private final Cache<String, AtomicInteger> counters;
+
+    public AttemptLimiter(Duration lockDuration) {
+        this.counters = Caffeine.newBuilder()
+                .expireAfterWrite(lockDuration)
+                .maximumSize(100_000)
+                .build();
+    }
+
+    /**
+     * Check and count are one atomic step, so parallel requests cannot pass more than max.
+     */
+    public boolean tryAcquire(String key, int max) {
+        AtomicInteger counter = counters.get(key, k -> new AtomicInteger());
+        if (counter.incrementAndGet() > max) {
+            counter.decrementAndGet();
+            return false;
+        }
+        // put() again so expireAfterWrite restarts the lock window with every counted attempt
+        counters.put(key, counter);
+        return true;
+    }
+
+    public void release(String key) {
+        AtomicInteger counter = counters.getIfPresent(key);
+        if (counter != null) {
+            counter.updateAndGet(n -> Math.max(0, n - 1));
+        }
+    }
+
+    public void reset(String key) {
+        counters.invalidate(key);
+    }
+}

src/test/java/auth_service/modules/login/service/LoginAttemptServiceTest.java

@@ -5,8 +5,18 @@ import auth_service.shared.test.AbstractUnitTest;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.CyclicBarrier;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+
 import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 class LoginAttemptServiceTest extends AbstractUnitTest {
 
@@ -20,33 +30,91 @@ class LoginAttemptServiceTest extends AbstractUnitTest {
     @Test
     void blocksEmailAfterFiveFailures() {
         for (int i = 0; i < LoginAttemptService.MAX_FAILURES_PER_EMAIL; i++) {
-            attempts.checkAllowed("a@b.de", "1.1.1." + i);
-            attempts.recordFailure("a@b.de", "1.1.1." + i);
+            attempts.acquire("a@b.de", "1.1.1." + i);
         }
 
-        assertThrows(TooManyLoginAttemptsException.class, () -> attempts.checkAllowed("a@b.de", "9.9.9.9"));
-        assertThrows(TooManyLoginAttemptsException.class, () -> attempts.checkAllowed("A@B.DE", "9.9.9.9"));
-        assertDoesNotThrow(() -> attempts.checkAllowed("other@b.de", "9.9.9.9"));
+        assertThrows(TooManyLoginAttemptsException.class, () -> attempts.acquire("a@b.de", "9.9.9.9"));
+        assertThrows(TooManyLoginAttemptsException.class, () -> attempts.acquire("A@B.DE", "9.9.9.9"));
+        assertDoesNotThrow(() -> attempts.acquire("other@b.de", "9.9.9.9"));
     }
 
     @Test
     void blocksIpAfterTwentyFailures() {
         for (int i = 0; i < LoginAttemptService.MAX_FAILURES_PER_IP; i++) {
-            attempts.recordFailure("user" + i + "@b.de", "1.2.3.4");
+            attempts.acquire("user" + i + "@b.de", "1.2.3.4");
         }
 
-        assertThrows(TooManyLoginAttemptsException.class, () -> attempts.checkAllowed("new@b.de", "1.2.3.4"));
-        assertDoesNotThrow(() -> attempts.checkAllowed("new@b.de", "5.6.7.8"));
+        assertThrows(TooManyLoginAttemptsException.class, () -> attempts.acquire("new@b.de", "1.2.3.4"));
+        assertDoesNotThrow(() -> attempts.acquire("new@b.de", "5.6.7.8"));
+    }
+
+    @Test
+    void blockedIpDoesNotUseUpAnEmailAttempt() {
+        for (int i = 0; i < LoginAttemptService.MAX_FAILURES_PER_IP; i++) {
+            attempts.acquire("user" + i + "@b.de", "1.2.3.4");
+        }
+        for (int i = 0; i < LoginAttemptService.MAX_FAILURES_PER_EMAIL; i++) {
+            assertThrows(TooManyLoginAttemptsException.class, () -> attempts.acquire("victim@b.de", "1.2.3.4"));
+        }
+
+        assertDoesNotThrow(() -> attempts.acquire("victim@b.de", "5.6.7.8"));
     }
 
     @Test
     void successResetsEmailCounter() {
         for (int i = 0; i < LoginAttemptService.MAX_FAILURES_PER_EMAIL - 1; i++) {
-            attempts.recordFailure("a@b.de", "1.1.1.1");
+            attempts.acquire("a@b.de", "1.1.1.1");
         }
-        attempts.recordSuccess("a@b.de");
-        attempts.recordFailure("a@b.de", "1.1.1.1");
+        attempts.acquire("a@b.de", "1.1.1.1");
+        attempts.recordSuccess("a@b.de", "1.1.1.1");
+        attempts.acquire("a@b.de", "1.1.1.1");
 
-        assertDoesNotThrow(() -> attempts.checkAllowed("a@b.de", "1.1.1.1"));
+        assertDoesNotThrow(() -> attempts.acquire("a@b.de", "1.1.1.1"));
+    }
+
+    @Test
+    void successDoesNotUseUpAnIpAttempt() {
+        for (int i = 0; i < LoginAttemptService.MAX_FAILURES_PER_IP * 2; i++) {
+            attempts.acquire("user" + i + "@b.de", "1.2.3.4");
+            attempts.recordSuccess("user" + i + "@b.de", "1.2.3.4");
+        }
+
+        assertDoesNotThrow(() -> attempts.acquire("new@b.de", "1.2.3.4"));
+    }
+
+    @Test
+    void concurrentBurstCannotExceedEmailLimit() throws Exception {
+        int requests = 50;
+        CyclicBarrier allChecked = new CyclicBarrier(requests);
+        AtomicInteger passed = new AtomicInteger();
+        ExecutorService pool = Executors.newFixedThreadPool(requests);
+        List<Future<?>> results = new ArrayList<>();
+
+        try {
+            for (int i = 0; i < requests; i++) {
+                String ip = "10.0.0." + i;
+                results.add(pool.submit(() -> {
+                    boolean allowed = true;
+                    try {
+                        attempts.acquire("a@b.de", ip);
+                    } catch (TooManyLoginAttemptsException e) {
+                        allowed = false;
+                    }
+                    allChecked.await(5, TimeUnit.SECONDS);
+                    if (allowed) {
+                        passed.incrementAndGet();
+                    }
+                    return null;
+                }));
+            }
+            for (Future<?> result : results) {
+                result.get(10, TimeUnit.SECONDS);
+            }
+        } finally {
+            pool.shutdownNow();
+        }
+
+        assertTrue(passed.get() <= LoginAttemptService.MAX_FAILURES_PER_EMAIL,
+                "attempts that passed the limit: " + passed.get());
     }
 }

src/test/java/auth_service/modules/login/service/LoginServiceTest.java

@@ -76,7 +76,8 @@ class LoginServiceTest extends AbstractUnitTest {
 
         // Assert
         assertEquals("mock-token", result);
-        verify(loginAttemptService).recordSuccess("test@example.com");
+        verify(loginAttemptService).acquire("test@example.com", IP);
+        verify(loginAttemptService).recordSuccess("test@example.com", IP);
     }
 
     @Test
@@ -88,7 +89,8 @@ class LoginServiceTest extends AbstractUnitTest {
         assertThrows(InvalidCredentialsException.class, () -> loginService.login(user, IP));
         // bcrypt still runs, so the response time does not reveal that the email is unknown
         verify(passwordEncoder).matches("password", "dummy-hash");
-        verify(loginAttemptService).recordFailure("test@example.com", IP);
+        verify(loginAttemptService).acquire("test@example.com", IP);
+        verify(loginAttemptService, never()).recordSuccess(any(), any());
     }
 
     @Test
@@ -104,7 +106,8 @@ class LoginServiceTest extends AbstractUnitTest {
 
         // Act & Assert
         assertThrows(InvalidCredentialsException.class, () -> loginService.login(user, IP));
-        verify(loginAttemptService).recordFailure("test@example.com", IP);
+        verify(loginAttemptService).acquire("test@example.com", IP);
+        verify(loginAttemptService, never()).recordSuccess(any(), any());
     }
 
     @Test
@@ -127,7 +130,7 @@ class LoginServiceTest extends AbstractUnitTest {
     @Test
     void login_TooManyAttempts() {
         // Arrange
-        doThrow(new TooManyLoginAttemptsException(900)).when(loginAttemptService).checkAllowed("test@example.com", IP);
+        doThrow(new TooManyLoginAttemptsException(900)).when(loginAttemptService).acquire("test@example.com", IP);
 
         // Act & Assert
         assertThrows(TooManyLoginAttemptsException.class, () -> loginService.login(user, IP));

src/test/java/auth_service/modules/registration/api/RegistrationControllerTest.java

@@ -4,6 +4,7 @@ import auth_service.configuration.security.JwtAuthenticationFilter;
 import auth_service.configuration.security.JwtService;
 import auth_service.modules.registration.api.dto.request.RegistrationRequest;
 import auth_service.modules.registration.exception.EmailAlreadyInUseException;
+import auth_service.modules.registration.exception.TooManyRegistrationAttemptsException;
 import auth_service.modules.registration.model.Registration;
 import auth_service.modules.registration.service.RegistrationService;
 import auth_service.shared.test.AbstractWebMvcTest;
@@ -21,12 +22,14 @@ import org.springframework.test.context.bean.override.mockito.MockitoBean;
 import org.springframework.test.web.servlet.MockMvc;
 
 import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.eq;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.verifyNoInteractions;
 import static org.mockito.Mockito.when;
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.post;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.header;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.jsonPath;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
 
@@ -67,7 +70,7 @@ class RegistrationControllerTest extends AbstractWebMvcTest {
         long expectedUserId = 42L;
 
         // Mock the service call, ignoring the exact static mapping output
-        when(registrationService.register(any(Registration.class))).thenReturn(expectedUserId);
+        when(registrationService.register(any(Registration.class), any())).thenReturn(expectedUserId);
 
         // Act & Assert
         mockMvc.perform(post("/api/registration/new")
@@ -87,7 +90,7 @@ class RegistrationControllerTest extends AbstractWebMvcTest {
                 .steamId("76561198000000001")
                 .build();
 
-        when(registrationService.register(any(Registration.class)))
+        when(registrationService.register(any(Registration.class), any()))
                 .thenThrow(new EmailAlreadyInUseException("The provided email address is already in use."));
 
         // Act & Assert
@@ -108,7 +111,7 @@ class RegistrationControllerTest extends AbstractWebMvcTest {
                 .inviteCode("code")
                 .steamId("76561198000000001")
                 .build();
-        when(registrationService.register(any(Registration.class))).thenReturn(42L);
+        when(registrationService.register(any(Registration.class), any())).thenReturn(42L);
 
         // Act
         mockMvc.perform(post("/api/registration/new")
@@ -118,7 +121,7 @@ class RegistrationControllerTest extends AbstractWebMvcTest {
 
         // Assert
         var captor = ArgumentCaptor.forClass(Registration.class);
-        verify(registrationService).register(captor.capture());
+        verify(registrationService).register(captor.capture(), any());
         assertEquals(76561198000000001L, captor.getValue().getSteamId());
     }
 
@@ -160,7 +163,7 @@ class RegistrationControllerTest extends AbstractWebMvcTest {
                 .password("SuperSecret123!")
                 .inviteCode("code")
                 .build();
-        when(registrationService.register(any(Registration.class)))
+        when(registrationService.register(any(Registration.class), any()))
                 .thenThrow(new DataIntegrityViolationException("users_email_key"));
 
         // Act & Assert: the user module's 409 mapping must not apply to registration
@@ -183,7 +186,7 @@ class RegistrationControllerTest extends AbstractWebMvcTest {
     @Test
     void register_PassesDisplayName() throws Exception {
         // Arrange
-        when(registrationService.register(any(Registration.class))).thenReturn(42L);
+        when(registrationService.register(any(Registration.class), any())).thenReturn(42L);
 
         // Act
         mockMvc.perform(post("/api/registration/new")
@@ -194,7 +197,38 @@ class RegistrationControllerTest extends AbstractWebMvcTest {
 
         // Assert
         var captor = ArgumentCaptor.forClass(Registration.class);
-        verify(registrationService).register(captor.capture());
+        verify(registrationService).register(captor.capture(), any());
         assertEquals("Lobo", captor.getValue().getDisplayName());
     }
+
+    @Test
+    void register_UsesLastForwardedForHopAsClientIp() throws Exception {
+        // Arrange
+        when(registrationService.register(any(Registration.class), eq("203.0.113.7"))).thenReturn(42L);
+
+        // Act & Assert
+        mockMvc.perform(post("/api/registration/new")
+                        .header("X-Forwarded-For", "10.0.0.1, 203.0.113.7")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("""
+                                {"email":"test@example.com","password":"SuperSecret123!","inviteCode":"code"}"""))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$.userId").value(42L));
+    }
+
+    @Test
+    void register_TooManyAttempts() throws Exception {
+        // Arrange
+        when(registrationService.register(any(Registration.class), any()))
+                .thenThrow(new TooManyRegistrationAttemptsException(900));
+
+        // Act & Assert
+        mockMvc.perform(post("/api/registration/new")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("""
+                                {"email":"test@example.com","password":"SuperSecret123!","inviteCode":"code"}"""))
+                .andExpect(status().isTooManyRequests())
+                .andExpect(header().string("Retry-After", "900"))
+                .andExpect(jsonPath("$.code").value("TOO_MANY_ATTEMPTS"));
+    }
 }

src/test/java/auth_service/modules/registration/service/RegistrationAttemptServiceTest.java

@@ -0,0 +1,98 @@
+package auth_service.modules.registration.service;
+
+import auth_service.modules.registration.exception.TooManyRegistrationAttemptsException;
+import auth_service.shared.test.AbstractUnitTest;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.CyclicBarrier;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+class RegistrationAttemptServiceTest extends AbstractUnitTest {
+
+    private RegistrationAttemptService attempts;
+
+    @BeforeEach
+    void setUp() {
+        attempts = new RegistrationAttemptService();
+    }
+
+    @Test
+    void blocksIpAfterTenFailures() {
+        for (int i = 0; i < RegistrationAttemptService.MAX_FAILURES_PER_IP; i++) {
+            attempts.acquire("1.2.3.4");
+        }
+
+        TooManyRegistrationAttemptsException blocked =
+                assertThrows(TooManyRegistrationAttemptsException.class, () -> attempts.acquire("1.2.3.4"));
+        assertEquals(RegistrationAttemptService.LOCK_DURATION.toSeconds(), blocked.getRetryAfterSeconds());
+        assertDoesNotThrow(() -> attempts.acquire("5.6.7.8"));
+    }
+
+    @Test
+    void successDoesNotUseUpAnAttempt() {
+        for (int i = 0; i < RegistrationAttemptService.MAX_FAILURES_PER_IP * 2; i++) {
+            attempts.acquire("1.2.3.4");
+            attempts.release("1.2.3.4");
+        }
+
+        assertDoesNotThrow(() -> attempts.acquire("1.2.3.4"));
+    }
+
+    @Test
+    void blockedAttemptIsNotCounted() {
+        for (int i = 0; i < RegistrationAttemptService.MAX_FAILURES_PER_IP; i++) {
+            attempts.acquire("1.2.3.4");
+        }
+        assertThrows(TooManyRegistrationAttemptsException.class, () -> attempts.acquire("1.2.3.4"));
+        attempts.release("1.2.3.4");
+
+        assertDoesNotThrow(() -> attempts.acquire("1.2.3.4"));
+    }
+
+    @Test
+    void concurrentBurstCannotExceedLimit() throws Exception {
+        int requests = 50;
+        CyclicBarrier allChecked = new CyclicBarrier(requests);
+        AtomicInteger passed = new AtomicInteger();
+        ExecutorService pool = Executors.newFixedThreadPool(requests);
+        List<Future<?>> results = new ArrayList<>();
+
+        try {
+            for (int i = 0; i < requests; i++) {
+                results.add(pool.submit(() -> {
+                    boolean allowed = true;
+                    try {
+                        attempts.acquire("1.2.3.4");
+                    } catch (TooManyRegistrationAttemptsException e) {
+                        allowed = false;
+                    }
+                    allChecked.await(5, TimeUnit.SECONDS);
+                    if (allowed) {
+                        passed.incrementAndGet();
+                    }
+                    return null;
+                }));
+            }
+            for (Future<?> result : results) {
+                result.get(10, TimeUnit.SECONDS);
+            }
+        } finally {
+            pool.shutdownNow();
+        }
+
+        assertTrue(passed.get() <= RegistrationAttemptService.MAX_FAILURES_PER_IP,
+                "attempts that passed the limit: " + passed.get());
+    }
+}

src/test/java/auth_service/modules/registration/service/RegistrationServiceTest.java

@@ -4,6 +4,7 @@ import auth_service.modules.registration.exception.EmailAlreadyInUseException;
 import auth_service.modules.registration.exception.InvalidInvitationCodeException;
 import auth_service.modules.registration.exception.DisplayNameAlreadyRegisteredException;
 import auth_service.modules.registration.exception.SteamIdAlreadyRegisteredException;
+import auth_service.modules.registration.exception.TooManyRegistrationAttemptsException;
 import auth_service.modules.registration.model.Registration;
 import auth_service.modules.registration.persistence.RegisteredUserRepository;
 import auth_service.shared.test.AbstractUnitTest;
@@ -23,6 +24,8 @@ import static org.mockito.Mockito.*;
  */
 class RegistrationServiceTest extends AbstractUnitTest {
 
+    private static final String IP = "203.0.113.7";
+
     @Mock
     private CodeValidationService codeValidationService;
 
@@ -32,6 +35,9 @@ class RegistrationServiceTest extends AbstractUnitTest {
     @Mock
     private PasswordEncoder passwordEncoder;
 
+    @Mock
+    private RegistrationAttemptService registrationAttemptService;
+
     @InjectMocks
     private RegistrationService registrationService;
 
@@ -58,13 +64,15 @@ class RegistrationServiceTest extends AbstractUnitTest {
         when(registeredUserRepository.save(any(Registration.class))).thenReturn(expectedUserId);
 
         // Act
-        long result = registrationService.register(validRegistration);
+        long result = registrationService.register(validRegistration, IP);
 
         // Assert
         assertEquals(expectedUserId, result);
         verify(passwordEncoder).encode("SuperSecret123!");
         verify(registeredUserRepository).save(validRegistration);
         assertEquals("hashedPasswordXYZ", validRegistration.getEncryptedPassword());
+        verify(registrationAttemptService).acquire(IP);
+        verify(registrationAttemptService).release(IP);
     }
 
     @Test
@@ -73,10 +81,11 @@ class RegistrationServiceTest extends AbstractUnitTest {
         when(codeValidationService.isCodeValid(validRegistration.getInviteCode())).thenReturn(false);
 
         // Act & Assert
-        assertThrows(InvalidInvitationCodeException.class, () -> registrationService.register(validRegistration));
+        assertThrows(InvalidInvitationCodeException.class, () -> registrationService.register(validRegistration, IP));
 
         verify(registeredUserRepository, never()).existsByEmail(anyString());
         verify(registeredUserRepository, never()).save(any());
+        verify(registrationAttemptService, never()).release(anyString());
     }
 
     @Test
@@ -86,10 +95,11 @@ class RegistrationServiceTest extends AbstractUnitTest {
         when(registeredUserRepository.existsByEmail(validRegistration.getEmail())).thenReturn(true);
 
         // Act & Assert
-        assertThrows(EmailAlreadyInUseException.class, () -> registrationService.register(validRegistration));
+        assertThrows(EmailAlreadyInUseException.class, () -> registrationService.register(validRegistration, IP));
 
         verify(registeredUserRepository, never()).existsBySteamId(anyLong());
         verify(registeredUserRepository, never()).save(any());
+        verify(registrationAttemptService, never()).release(anyString());
     }
 
     @Test
@@ -100,9 +110,10 @@ class RegistrationServiceTest extends AbstractUnitTest {
         when(registeredUserRepository.existsBySteamId(validRegistration.getSteamId())).thenReturn(true);
 
         // Act & Assert
-        assertThrows(SteamIdAlreadyRegisteredException.class, () -> registrationService.register(validRegistration));
+        assertThrows(SteamIdAlreadyRegisteredException.class, () -> registrationService.register(validRegistration, IP));
 
         verify(registeredUserRepository, never()).save(any());
+        verify(registrationAttemptService, never()).release(anyString());
     }
 
     @Test
@@ -115,7 +126,7 @@ class RegistrationServiceTest extends AbstractUnitTest {
         when(registeredUserRepository.save(any(Registration.class))).thenReturn(42L);
 
         // Act
-        long result = registrationService.register(validRegistration);
+        long result = registrationService.register(validRegistration, IP);
 
         // Assert
         assertEquals(42L, result);
@@ -132,7 +143,19 @@ class RegistrationServiceTest extends AbstractUnitTest {
         when(registeredUserRepository.existsByDisplayName("Lobo")).thenReturn(true);
 
         // Act & Assert
-        assertThrows(DisplayNameAlreadyRegisteredException.class, () -> registrationService.register(validRegistration));
+        assertThrows(DisplayNameAlreadyRegisteredException.class, () -> registrationService.register(validRegistration, IP));
         verify(registeredUserRepository, never()).save(any());
+        verify(registrationAttemptService, never()).release(anyString());
+    }
+
+    @Test
+    void register_BlockedIp_ChecksNothingElse() {
+        // Arrange
+        doThrow(new TooManyRegistrationAttemptsException(900)).when(registrationAttemptService).acquire(IP);
+
+        // Act & Assert
+        assertThrows(TooManyRegistrationAttemptsException.class, () -> registrationService.register(validRegistration, IP));
+        verifyNoInteractions(codeValidationService, registeredUserRepository);
+        verify(registrationAttemptService, never()).release(anyString());
     }
 }

src/test/java/auth_service/shared/api/ClientIpTest.java

@@ -0,0 +1,28 @@
+package auth_service.shared.api;
+
+import auth_service.shared.test.AbstractUnitTest;
+import org.junit.jupiter.api.Test;
+import org.springframework.mock.web.MockHttpServletRequest;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class ClientIpTest extends AbstractUnitTest {
+
+    @Test
+    void commaOnlyForwardedForFallsBackToRemoteAddr() {
+        MockHttpServletRequest request = new MockHttpServletRequest();
+        request.setRemoteAddr("198.51.100.9");
+        request.addHeader("X-Forwarded-For", ",");
+
+        assertEquals("198.51.100.9", ClientIp.of(request));
+    }
+
+    @Test
+    void blankLastHopFallsBackToRemoteAddr() {
+        MockHttpServletRequest request = new MockHttpServletRequest();
+        request.setRemoteAddr("198.51.100.9");
+        request.addHeader("X-Forwarded-For", "10.0.0.1, ");
+
+        assertEquals("198.51.100.9", ClientIp.of(request));
+    }
+}