fix(shareapi): Check if the returned user id matches the searched one

Assisted-by: ClaudeCode:claude-fable-5
Signed-off-by: David Dreschner <david.dreschner@nextcloud.com>

Author: DerDreschner

build/integration/sharees_features/sharees.feature

@@ -426,6 +426,40 @@ Feature: sharees
     And "exact emails" sharees returned is empty
     And "emails" sharees returned is empty
 
+  Scenario: Search user by system e-mail address with e-mail full match disabled
+    Given As an "test"
+    And parameter "shareapi_restrict_user_enumeration_full_match_email" of app "core" is set to "no"
+    When getting sharees for
+      | search    | sharee2@system.com |
+      | itemType  | file |
+      | shareType | 0 |
+    Then the OCS status code should be "100"
+    And the HTTP status code should be "200"
+    And "exact users" sharees returned is empty
+    # With enumeration allowed the user is still found as non-exact match, as
+    # enumeration also searches in the e-mail address
+    And "users" sharees returned are
+      | Sharee2 | 0 | Sharee2 | sharee2@system.com |
+    And "exact emails" sharees returned is empty
+    And "emails" sharees returned is empty
+
+  Scenario: Search user by system e-mail address with e-mail full match and user enumeration disabled
+    Given As an "test"
+    And parameter "shareapi_restrict_user_enumeration_full_match_email" of app "core" is set to "no"
+    And parameter "shareapi_allow_share_dialog_user_enumeration" of app "core" is set to "no"
+    When getting sharees for
+      | search    | sharee2@system.com |
+      | itemType  | file |
+      | shareType | 0 |
+    Then the OCS status code should be "100"
+    And the HTTP status code should be "200"
+    # The user id full match must not resolve e-mail addresses through the user
+    # backends (login via e-mail address)
+    And "exact users" sharees returned is empty
+    And "users" sharees returned is empty
+    And "exact emails" sharees returned is empty
+    And "emails" sharees returned is empty
+
   Scenario: Search e-mail
     Given As an "test"
     When getting sharees for

lib/private/Collaboration/Collaborators/UserPlugin.php

@@ -118,7 +118,10 @@ public function search($search, $limit, $offset, ISearchResult $searchResult): b
 
 			if ($shareeEnumerationFullMatchUserId) {
 				$user = $this->userManager->get($search);
-				if ($user !== null) {
+				// User backends may also resolve email addresses or other login names here
+				// (e.g. for login via email). Only an actual user id match counts as user id
+				// full match, everything else is governed by the email setting below.
+				if ($user !== null && mb_strtolower($user->getUID()) === $lowerSearch) {
 					$users[$user->getUID()] = ['exact', $user];
 				}
 			}