diff --git a/apps/files_sharing/appinfo/info.xml b/apps/files_sharing/appinfo/info.xml
index 03078b54040..0b4ebfd0319 100644
--- a/apps/files_sharing/appinfo/info.xml
+++ b/apps/files_sharing/appinfo/info.xml
@@ -14,7 +14,7 @@
 Turning the feature off removes shared files and folders on the server for all share recipients, and also on the sync clients and mobile apps. More information is available in the Nextcloud Documentation.
 
 	</description>
-	<version>1.24.0</version>
+	<version>1.24.1</version>
 	<licence>agpl</licence>
 	<author>Michael Gapczynski</author>
 	<author>Bjoern Schiessle</author>
diff --git a/apps/files_sharing/composer/composer/autoload_classmap.php b/apps/files_sharing/composer/composer/autoload_classmap.php
index 92751ead730..dda5b5c7f77 100644
--- a/apps/files_sharing/composer/composer/autoload_classmap.php
+++ b/apps/files_sharing/composer/composer/autoload_classmap.php
@@ -81,6 +81,7 @@ return array(
     'OCA\\Files_Sharing\\Migration\\Version24000Date20220208195521' => $baseDir . '/../lib/Migration/Version24000Date20220208195521.php',
     'OCA\\Files_Sharing\\Migration\\Version24000Date20220404142216' => $baseDir . '/../lib/Migration/Version24000Date20220404142216.php',
     'OCA\\Files_Sharing\\Migration\\Version31000Date20240821142813' => $baseDir . '/../lib/Migration/Version31000Date20240821142813.php',
+    'OCA\\Files_Sharing\\Migration\\Version32000Date20251017081948' => $baseDir . '/../lib/Migration/Version32000Date20251017081948.php',
     'OCA\\Files_Sharing\\MountProvider' => $baseDir . '/../lib/MountProvider.php',
     'OCA\\Files_Sharing\\Notification\\Listener' => $baseDir . '/../lib/Notification/Listener.php',
     'OCA\\Files_Sharing\\Notification\\Notifier' => $baseDir . '/../lib/Notification/Notifier.php',
diff --git a/apps/files_sharing/composer/composer/autoload_static.php b/apps/files_sharing/composer/composer/autoload_static.php
index 6ca952007a1..195b4f05836 100644
--- a/apps/files_sharing/composer/composer/autoload_static.php
+++ b/apps/files_sharing/composer/composer/autoload_static.php
@@ -96,6 +96,7 @@ class ComposerStaticInitFiles_Sharing
         'OCA\\Files_Sharing\\Migration\\Version24000Date20220208195521' => __DIR__ . '/..' . '/../lib/Migration/Version24000Date20220208195521.php',
         'OCA\\Files_Sharing\\Migration\\Version24000Date20220404142216' => __DIR__ . '/..' . '/../lib/Migration/Version24000Date20220404142216.php',
         'OCA\\Files_Sharing\\Migration\\Version31000Date20240821142813' => __DIR__ . '/..' . '/../lib/Migration/Version31000Date20240821142813.php',
+        'OCA\\Files_Sharing\\Migration\\Version32000Date20251017081948' => __DIR__ . '/..' . '/../lib/Migration/Version32000Date20251017081948.php',
         'OCA\\Files_Sharing\\MountProvider' => __DIR__ . '/..' . '/../lib/MountProvider.php',
         'OCA\\Files_Sharing\\Notification\\Listener' => __DIR__ . '/..' . '/../lib/Notification/Listener.php',
         'OCA\\Files_Sharing\\Notification\\Notifier' => __DIR__ . '/..' . '/../lib/Notification/Notifier.php',
diff --git a/apps/files_sharing/lib/Migration/Version32000Date20251017081948.php b/apps/files_sharing/lib/Migration/Version32000Date20251017081948.php
new file mode 100644
index 00000000000..15038dc0ec0
--- /dev/null
+++ b/apps/files_sharing/lib/Migration/Version32000Date20251017081948.php
@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2025 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Files_Sharing\Migration;
+
+use Closure;
+use OCP\DB\ISchemaWrapper;
+use OCP\Migration\Attributes\ColumnType;
+use OCP\Migration\Attributes\ModifyColumn;
+use OCP\Migration\IOutput;
+use OCP\Migration\SimpleMigrationStep;
+use Override;
+
+#[ModifyColumn(table: 'share_external', name: 'owner', type: ColumnType::STRING, description: 'Change length to 255 characters')]
+class Version32000Date20251017081948 extends SimpleMigrationStep {
+	/**
+	 * @param Closure $schemaClosure The `\Closure` returns a `ISchemaWrapper`
+	 */
+	#[Override]
+	public function changeSchema(IOutput $output, Closure $schemaClosure, array $options): ?ISchemaWrapper {
+		/** @var ISchemaWrapper $schema */
+		$schema = $schemaClosure();
+
+		$table = $schema->getTable('share_external');
+		$column = $table->getColumn('owner');
+		if ($column->getLength() < 255) {
+			$column->setLength(255);
+			return $schema;
+		}
+		return null;
+	}
+}
diff --git a/lib/private/Federation/CloudIdManager.php b/lib/private/Federation/CloudIdManager.php
index c599d9046a6..b0996d5179c 100644
--- a/lib/private/Federation/CloudIdManager.php
+++ b/lib/private/Federation/CloudIdManager.php
@@ -108,7 +108,7 @@ class CloudIdManager implements ICloudIdManager {
 			// We accept slightly more chars when working with federationId than with a local userId.
 			// We remove those eventual chars from the UserId before using
 			// the IUserManager API to confirm its format.
-			$this->userManager->validateUserId(str_replace('=', '-', $user));
+			$this->validateUser($user, $remote);
 
 			if (!empty($user) && !empty($remote)) {
 				$remote = $this->ensureDefaultProtocol($remote);
@@ -118,6 +118,36 @@ class CloudIdManager implements ICloudIdManager {
 		throw new \InvalidArgumentException('Invalid cloud id');
 	}
 
+	protected function validateUser(string $user, string $remote): void {
+		// Check the ID for bad characters
+		// Allowed are: "a-z", "A-Z", "0-9", spaces and "_.@-'" (Nextcloud)
+		// Additional: "=" (oCIS)
+		if (preg_match('/[^a-zA-Z0-9 _.@\-\'=]/', $user)) {
+			throw new \InvalidArgumentException('Invalid characters');
+		}
+
+		// No empty user ID
+		if (trim($user) === '') {
+			throw new \InvalidArgumentException('Empty user');
+		}
+
+		// No whitespace at the beginning or at the end
+		if (trim($user) !== $user) {
+			throw new \InvalidArgumentException('User contains whitespace at the beginning or at the end');
+		}
+
+		// User ID only consists of 1 or 2 dots (directory traversal)
+		if ($user === '.' || $user === '..') {
+			throw new \InvalidArgumentException('User must not consist of dots only');
+		}
+
+		// User ID is too long
+		if (strlen($user . '@' . $remote) > 255) {
+			// TRANSLATORS User ID is too long
+			throw new \InvalidArgumentException('Cloud id is too long');
+		}
+	}
+
 	public function getDisplayNameFromContact(string $cloudId): ?string {
 		$cachedName = $this->displayNameCache->get($cloudId);
 		if ($cachedName !== null) {