diff --git a/src/IDF/Form/Admin/ProjectCreate.php b/src/IDF/Form/Admin/ProjectCreate.php
index 0c7aec2..3af968b 100644
--- a/src/IDF/Form/Admin/ProjectCreate.php
+++ b/src/IDF/Form/Admin/ProjectCreate.php
@@ -133,6 +133,16 @@ class IDF_Form_Admin_ProjectCreate extends Pluf_Form
                           'IDF_Form_Admin_ProjectCreate', $params);
     }
 
+    public function clean_owners()
+    {
+        return IDF_Form_MembersConf::checkBadLogins($this->cleaned_data['owners']);
+    }
+
+    public function clean_members()
+    {
+        return IDF_Form_MembersConf::checkBadLogins($this->cleaned_data['members']);
+    }
+
     public function clean_svn_remote_url()
     {
         $this->cleaned_data['svn_remote_url'] = (!empty($this->cleaned_data['svn_remote_url'])) ? $this->cleaned_data['svn_remote_url'] : '';
diff --git a/src/IDF/Form/Admin/ProjectUpdate.php b/src/IDF/Form/Admin/ProjectUpdate.php
index 30cc4cb..cd9995e 100644
--- a/src/IDF/Form/Admin/ProjectUpdate.php
+++ b/src/IDF/Form/Admin/ProjectUpdate.php
@@ -61,6 +61,16 @@ class IDF_Form_Admin_ProjectUpdate extends Pluf_Form
                                             ));
     }
 
+    public function clean_owners()
+    {
+        return IDF_Form_MembersConf::checkBadLogins($this->cleaned_data['owners']);
+    }
+
+    public function clean_members()
+    {
+        return IDF_Form_MembersConf::checkBadLogins($this->cleaned_data['members']);
+    }
+
     public function save($commit=true)
     {
         if (!$this->isValid()) {
diff --git a/src/IDF/Form/MembersConf.php b/src/IDF/Form/MembersConf.php
index 35c0413..69552b3 100644
--- a/src/IDF/Form/MembersConf.php
+++ b/src/IDF/Form/MembersConf.php
@@ -67,6 +67,46 @@ class IDF_Form_MembersConf extends Pluf_Form
         $this->project->membershipsUpdated();
     }
 
+    public function clean_owners()
+    {
+        return self::checkBadLogins($this->cleaned_data['owners']);
+    }
+
+    public function clean_members()
+    {
+        return self::checkBadLogins($this->cleaned_data['members']);
+    }
+
+    /**
+     * From the input, find the bad logins.
+     *
+     * Throw a Pluf_Form_Invalid exception when bad logins are found.
+     *
+     * @param string Comma, new line delimited list of logins
+     * @return string Comma, new line delimited list of logins
+     */
+    public static function checkBadLogins($logins)
+    {
+        $bad = array();
+        foreach (preg_split("/\015\012|\015|\012|\,/", $logins, -1, PREG_SPLIT_NO_EMPTY) as $login) {
+            $sql = new Pluf_SQL('login=%s', array(trim($login)));
+            try {
+                $user = Pluf::factory('Pluf_User')->getOne(array('filter'=>$sql->gen()));
+                if (null == $user) {
+                    $bad[] = $login;
+                }
+            } catch (Exception $e) {
+                $bad[] = $login;
+            }
+        }
+        $n = count($bad);
+        if ($n) {
+            $badlogins = Pluf_esc(implode(', ', $bad));
+            throw new Pluf_Form_Invalid(sprintf(_n('The following login is invalid: %s.', 'The following login are invalids: %s.', $n), $badlogins));
+        }
+        return $logins;
+    }
+
     /**
      * The update of the memberships is done in different places. This
      * avoids duplicating code.
diff --git a/src/IDF/Form/TabsConf.php b/src/IDF/Form/TabsConf.php
index 9b4eb30..a9087c9 100644
--- a/src/IDF/Form/TabsConf.php
+++ b/src/IDF/Form/TabsConf.php
@@ -86,6 +86,11 @@ class IDF_Form_TabsConf extends Pluf_Form
                                             ));
     }
 
+    public function clean_authorized_users()
+    {
+        return IDF_Form_MembersConf::checkBadLogins($this->cleaned_data['authorized_users']);
+    }
+
     public function save($commit=true)
     {
         if (!$this->isValid()) {