diff --git a/IONOS b/IONOS
index 35650b0e17105..9decc9bcd4f9d 160000
--- a/IONOS
+++ b/IONOS
@@ -1 +1 @@
-Subproject commit 35650b0e17105902e4c0b4d15c8b51f204249010
+Subproject commit 9decc9bcd4f9defbb9206d3e69805658496a4753
diff --git a/core/Controller/ClientFlowLoginV2Controller.php b/core/Controller/ClientFlowLoginV2Controller.php
index e6e1c282d2b6b..7ad91fcc40c3f 100644
--- a/core/Controller/ClientFlowLoginV2Controller.php
+++ b/core/Controller/ClientFlowLoginV2Controller.php
@@ -10,6 +10,7 @@
 
 use OC\Core\Db\LoginFlowV2;
 use OC\Core\Exception\LoginFlowV2NotFoundException;
+use OC\Core\Exception\LoginFlowV2ClientForbiddenException;
 use OC\Core\ResponseDefinitions;
 use OC\Core\Service\LoginFlowV2Service;
 use OCP\AppFramework\Controller;
@@ -106,6 +107,8 @@ public function showAuthPickerPage(string $user = '', int $direct = 0): Standalo
 			$flow = $this->getFlowByLoginToken();
 		} catch (LoginFlowV2NotFoundException $e) {
 			return $this->loginTokenForbiddenResponse();
+		} catch (LoginFlowV2ClientForbiddenException $e) {
+			return $this->loginTokenForbiddenClientResponse();
 		}
 
 		$stateToken = $this->random->generate(
@@ -149,6 +152,8 @@ public function grantPage(?string $stateToken, int $direct = 0): StandaloneTempl
 			$flow = $this->getFlowByLoginToken();
 		} catch (LoginFlowV2NotFoundException $e) {
 			return $this->loginTokenForbiddenResponse();
+		} catch (LoginFlowV2ClientForbiddenException $e) {
+			return $this->loginTokenForbiddenClientResponse();
 		}
 
 		/** @var IUser $user */
@@ -185,6 +190,8 @@ public function apptokenRedirect(?string $stateToken, string $user, string $pass
 			$this->getFlowByLoginToken();
 		} catch (LoginFlowV2NotFoundException $e) {
 			return $this->loginTokenForbiddenResponse();
+		} catch (LoginFlowV2ClientForbiddenException $e) {
+			return $this->loginTokenForbiddenClientResponse();
 		}
 
 		$loginToken = $this->session->get(self::TOKEN_NAME);
@@ -230,6 +237,8 @@ public function generateAppPassword(?string $stateToken): Response {
 			$this->getFlowByLoginToken();
 		} catch (LoginFlowV2NotFoundException $e) {
 			return $this->loginTokenForbiddenResponse();
+		} catch (LoginFlowV2ClientForbiddenException $e) {
+			return $this->loginTokenForbiddenClientResponse();
 		}
 
 		$loginToken = $this->session->get(self::TOKEN_NAME);
@@ -329,6 +338,7 @@ private function stateTokenForbiddenResponse(): StandaloneTemplateResponse {
 	/**
 	 * @return LoginFlowV2
 	 * @throws LoginFlowV2NotFoundException
+	 * @throws LoginFlowV2ClientForbiddenException
 	 */
 	private function getFlowByLoginToken(): LoginFlowV2 {
 		$currentToken = $this->session->get(self::TOKEN_NAME);
@@ -352,6 +362,19 @@ private function loginTokenForbiddenResponse(): StandaloneTemplateResponse {
 		return $response;
 	}
 
+	private function loginTokenForbiddenClientResponse(): StandaloneTemplateResponse {
+		$response = new StandaloneTemplateResponse(
+			$this->appName,
+			'403',
+			[
+				'message' => $this->l10n->t('Please use original client'),
+			],
+			'guest'
+		);
+		$response->setStatus(Http::STATUS_FORBIDDEN);
+		return $response;
+	}
+
 	private function getServerPath(): string {
 		$serverPostfix = '';
 
diff --git a/core/Exception/LoginFlowV2ClientForbiddenException.php b/core/Exception/LoginFlowV2ClientForbiddenException.php
new file mode 100644
index 0000000000000..459c30fb4dc45
--- /dev/null
+++ b/core/Exception/LoginFlowV2ClientForbiddenException.php
@@ -0,0 +1,12 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2025 STRATO GmbH
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+namespace OC\Core\Exception;
+
+class LoginFlowV2ClientForbiddenException extends \Exception {
+}
diff --git a/core/Service/LoginFlowV2Service.php b/core/Service/LoginFlowV2Service.php
index 89c89f50b3ec1..a30d8b2cb5ea6 100644
--- a/core/Service/LoginFlowV2Service.php
+++ b/core/Service/LoginFlowV2Service.php
@@ -15,6 +15,7 @@
 use OC\Core\Data\LoginFlowV2Tokens;
 use OC\Core\Db\LoginFlowV2;
 use OC\Core\Db\LoginFlowV2Mapper;
+use OC\Core\Exception\LoginFlowV2ClientForbiddenException;
 use OC\Core\Exception\LoginFlowV2NotFoundException;
 use OCP\AppFramework\Db\DoesNotExistException;
 use OCP\AppFramework\Utility\ITimeFactory;
@@ -74,13 +75,33 @@ public function poll(string $pollToken): LoginFlowV2Credentials {
 	 * @param string $loginToken
 	 * @return LoginFlowV2
 	 * @throws LoginFlowV2NotFoundException
+	 * @throws LoginFlowV2ClientForbiddenException
 	 */
 	public function getByLoginToken(string $loginToken): LoginFlowV2 {
+		/** @var LoginFlowV2|null $flow */
+		$flow = null;
+
 		try {
-			return $this->mapper->getByLoginToken($loginToken);
+			$flow = $this->mapper->getByLoginToken($loginToken);
 		} catch (DoesNotExistException $e) {
 			throw new LoginFlowV2NotFoundException('Login token invalid');
 		}
+
+		$allowedAgents = $this->config->getSystemValue('core.login_flow_v2.allowed_user_agents', []);
+
+		if (empty($allowedAgents)) {
+			return $flow;
+		}
+
+		$flowClient = $flow->getClientName();
+
+		foreach ($allowedAgents as $allowedAgent) {
+			if (preg_match($allowedAgent, $flowClient) === 1) {
+				return $flow;
+			}
+		}
+
+		throw new LoginFlowV2ClientForbiddenException('Client not allowed');
 	}
 
 	/**
diff --git a/tests/Core/Controller/ClientFlowLoginV2ControllerTest.php b/tests/Core/Controller/ClientFlowLoginV2ControllerTest.php
index eefa2982c74bb..8fe31e1a300cf 100644
--- a/tests/Core/Controller/ClientFlowLoginV2ControllerTest.php
+++ b/tests/Core/Controller/ClientFlowLoginV2ControllerTest.php
@@ -11,6 +11,7 @@
 use OC\Core\Controller\ClientFlowLoginV2Controller;
 use OC\Core\Data\LoginFlowV2Credentials;
 use OC\Core\Db\LoginFlowV2;
+use OC\Core\Exception\LoginFlowV2ClientForbiddenException;
 use OC\Core\Exception\LoginFlowV2NotFoundException;
 use OC\Core\Service\LoginFlowV2Service;
 use OCP\AppFramework\Http;
@@ -56,6 +57,12 @@ protected function setUp(): void {
 		$this->random = $this->createMock(ISecureRandom::class);
 		$this->defaults = $this->createMock(Defaults::class);
 		$this->l = $this->createMock(IL10N::class);
+		$this->l
+			->expects($this->any())
+			->method('t')
+			->willReturnCallback(function ($text, $parameters = []) {
+				return vsprintf($text, $parameters);
+			});
 		$this->controller = new ClientFlowLoginV2Controller(
 			'core',
 			$this->request,
@@ -150,6 +157,22 @@ public function testShowAuthPickerInvalidLoginToken() {
 		$this->assertSame(Http::STATUS_FORBIDDEN, $result->getStatus());
 	}
 
+	public function testShowAuthPickerForbiddenUserClient() {
+		$this->session->method('get')
+			->with('client.flow.v2.login.token')
+			->willReturn('loginToken');
+
+		$this->loginFlowV2Service->method('getByLoginToken')
+			->with('loginToken')
+			->willThrowException(new LoginFlowV2ClientForbiddenException());
+
+		$result = $this->controller->showAuthPickerPage();
+
+		$this->assertInstanceOf(Http\StandaloneTemplateResponse::class, $result);
+		$this->assertSame(Http::STATUS_FORBIDDEN, $result->getStatus());
+		$this->assertSame('Please use original client', $result->getParams()['message']);
+	}
+
 	public function testShowAuthPickerValidLoginToken() {
 		$this->session->method('get')
 			->with('client.flow.v2.login.token')
@@ -206,6 +229,29 @@ public function testGrantPageInvalidLoginToken() {
 		$this->assertSame(Http::STATUS_FORBIDDEN, $result->getStatus());
 	}
 
+	public function testGrantPageForbiddenUserClient() {
+		$this->session->method('get')
+			->willReturnCallback(function ($name) {
+				if ($name === 'client.flow.v2.state.token') {
+					return 'stateToken';
+				}
+				if ($name === 'client.flow.v2.login.token') {
+					return 'loginToken';
+				}
+				return null;
+			});
+
+		$this->loginFlowV2Service->method('getByLoginToken')
+			->with('loginToken')
+			->willThrowException(new LoginFlowV2ClientForbiddenException());
+
+		$result = $this->controller->grantPage('stateToken');
+
+		$this->assertInstanceOf(Http\StandaloneTemplateResponse::class, $result);
+		$this->assertSame(Http::STATUS_FORBIDDEN, $result->getStatus());
+		$this->assertSame('Please use original client', $result->getParams()['message']);
+	}
+
 	public function testGrantPageValid() {
 		$this->session->method('get')
 			->willReturnCallback(function ($name) {
@@ -266,6 +312,29 @@ public function testGenerateAppPassworInvalidLoginToken() {
 		$this->assertSame(Http::STATUS_FORBIDDEN, $result->getStatus());
 	}
 
+	public function testGenerateAppPasswordForbiddenUserClient() {
+		$this->session->method('get')
+			->willReturnCallback(function ($name) {
+				if ($name === 'client.flow.v2.state.token') {
+					return 'stateToken';
+				}
+				if ($name === 'client.flow.v2.login.token') {
+					return 'loginToken';
+				}
+				return null;
+			});
+
+		$this->loginFlowV2Service->method('getByLoginToken')
+			->with('loginToken')
+			->willThrowException(new LoginFlowV2ClientForbiddenException());
+
+		$result = $this->controller->generateAppPassword('stateToken');
+
+		$this->assertInstanceOf(Http\StandaloneTemplateResponse::class, $result);
+		$this->assertSame(Http::STATUS_FORBIDDEN, $result->getStatus());
+		$this->assertSame('Please use original client', $result->getParams()['message']);
+	}
+
 	public function testGenerateAppPassworValid() {
 		$this->session->method('get')
 			->willReturnCallback(function ($name) {
diff --git a/tests/Core/Service/LoginFlowV2ServiceUnitTest.php b/tests/Core/Service/LoginFlowV2ServiceUnitTest.php
index 2e6407d4c00d2..dc9e8427d3e59 100644
--- a/tests/Core/Service/LoginFlowV2ServiceUnitTest.php
+++ b/tests/Core/Service/LoginFlowV2ServiceUnitTest.php
@@ -14,6 +14,7 @@
 use OC\Core\Data\LoginFlowV2Tokens;
 use OC\Core\Db\LoginFlowV2;
 use OC\Core\Db\LoginFlowV2Mapper;
+use OC\Core\Exception\LoginFlowV2ClientForbiddenException;
 use OC\Core\Exception\LoginFlowV2NotFoundException;
 use OC\Core\Service\LoginFlowV2Service;
 use OCP\AppFramework\Db\DoesNotExistException;
@@ -237,6 +238,57 @@ public function testGetByLoginTokenLoginTokenInvalid() {
 		$this->subjectUnderTest->getByLoginToken('test_token');
 	}
 
+	public function testGetByLoginTokenClientForbidden() {
+		$this->expectException(LoginFlowV2ClientForbiddenException::class);
+		$this->expectExceptionMessage('Client not allowed');
+
+		$allowedClients = [
+			'/Custom Allowed Client/i'
+		];
+
+		$this->config->expects($this->exactly(1))
+			->method('getSystemValue')
+			->willReturn($this->returnCallback(function ($key) use ($allowedClients) {
+				// Note: \OCP\IConfig::getSystemValue returns either an array or string.
+				return $key == 'core.login_flow_v2.allowed_user_agents' ? $allowedClients : '';
+			}));
+
+		$loginFlowV2 = new LoginFlowV2();
+		$loginFlowV2->setClientName('Rogue Curl Client/1.0');
+
+		$this->mapper->expects($this->once())
+			->method('getByLoginToken')
+			->willReturn($loginFlowV2);
+
+		$this->subjectUnderTest->getByLoginToken('test_token');
+	}
+
+	public function testGetByLoginTokenClientAllowed() {
+		$allowedClients = [
+			'/Foo Allowed Client/i',
+			'/Custom Allowed Client/i'
+		];
+
+		$loginFlowV2 = new LoginFlowV2();
+		$loginFlowV2->setClientName('Custom Allowed Client Curl Client/1.0');
+
+		$this->config->expects($this->exactly(1))
+			->method('getSystemValue')
+			->willReturn($this->returnCallback(function ($key) use ($allowedClients) {
+				// Note: \OCP\IConfig::getSystemValue returns either an array or string.
+				return $key == 'core.login_flow_v2.allowed_user_agents' ? $allowedClients : '';
+			}));
+
+		$this->mapper->expects($this->once())
+			->method('getByLoginToken')
+			->willReturn($loginFlowV2);
+
+		$result = $this->subjectUnderTest->getByLoginToken('test_token');
+
+		$this->assertTrue($result instanceof LoginFlowV2);
+		$this->assertEquals('Custom Allowed Client Curl Client/1.0', $result->getClientName());
+	}
+
 	/*
 	 * Tests for startLoginFlow
 	 */