diff --git a/src/zope/pluggableauth/plugins/httpplugins.py b/src/zope/pluggableauth/plugins/httpplugins.py
index b455486..f69ef66 100644
--- a/src/zope/pluggableauth/plugins/httpplugins.py
+++ b/src/zope/pluggableauth/plugins/httpplugins.py
@@ -80,6 +80,14 @@ def extractCredentials(self, request):
           >>> print(plugin.extractCredentials(TestRequest('/')))
           None
 
+        According to RFC 2617, password can contain one or more colons;
+        user ID can't contain any colon.
+
+          >>> request = TextRequest(
+          ...     environ={'HTTP_AUTHORIZATION': u'Basic bWdyOm1ncnB3OndpdGg6Y29sb24='})
+          >>> pprint(plugin.extractCredentials(request))
+          {'login': u'mgr', 'password': u'mgrpw:with:colon'}
+
         """
         if not IHTTPRequest.providedBy(request):
             return None