Index: management/univention-directory-manager-modules/modules/univention/admin/syntax.py
===================================================================
--- management/univention-directory-manager-modules/modules/univention/admin/syntax.py	(Revision 73141)
+++ management/univention-directory-manager-modules/modules/univention/admin/syntax.py	(Arbeitskopie)
@@ -865,9 +865,32 @@
 	min_length=1
 	max_length=16
 	_re = re.compile('(?u)(^\w[\w -.]*\w$)|\w*$')
+	# https://wiki.debian.org/SystemGroups#Groups_with_an_associated_user
+	default_debian_users = (
+		'root',
+		'daemon',
+		'games',
+		'man',
+		'mail',
+		'news',
+		'proxy',
+		'postgres',
+		'www-data',
+		'backup',
+		'list',
+		'irc',
+		'sync',
+		'uucp',
+		'operator',
+		'bin',
+		'sys',
+		'nobody',
+	)
 
 	@classmethod
 	def parse(self, text):
+		if text in self.default_debian_users:
+			raise univention.admin.uexceptions.valueError, _("Username must not match a default debian user!")
 		if " " in text:
 			raise univention.admin.uexceptions.valueError, _("Spaces are not allowed in the username!")
 		if self._re.match(text.decode("utf-8")) != None and text != 'admin':