diff --git a/gluon/tests/test_tools.py b/gluon/tests/test_tools.py
index 79f2db03..c8009bcc 100644
--- a/gluon/tests/test_tools.py
+++ b/gluon/tests/test_tools.py
@@ -1081,12 +1081,38 @@ class TestToolsFunctions(unittest.TestCase):
         self.assertEqual(prettydate(d='invalid_date'), '[invalid date]')
 
 
-
 pjoin = os.path.join
 
 def have_symlinks():
     return os.name == 'posix'
 
+
+class Test_Expose__in_base(unittest.TestCase):
+
+    def test_in_base(self):
+        are_under = [ # (sub, base)
+            ('/foo/bar', '/foo'),
+            ('/foo', '/foo'),
+            ('/foo', '/'),
+            ('/', '/'),
+        ]
+        for sub, base in are_under:
+            self.assertTrue( Expose._Expose__in_base(subdir=sub, basedir=base, sep='/'),
+                             '%s is not under %s' % (sub, base) )
+
+    def test_not_in_base(self):
+        are_not_under = [ # (sub, base)
+            ('/foobar', '/foo'),
+            ('/foo', '/foo/bar'),
+            ('/bar', '/foo'),
+            ('/foo/bar', '/bar'),
+            ('/', '/x'),
+        ]
+        for sub, base in are_not_under:
+            self.assertFalse( Expose._Expose__in_base(subdir=sub, basedir=base, sep='/'),
+                              '%s should not be under %s' % (sub, base) )
+
+
 class TestExpose(unittest.TestCase):
 
     def setUp(self):
diff --git a/gluon/tools.py b/gluon/tools.py
index 5138d076..2734118c 100644
--- a/gluon/tools.py
+++ b/gluon/tools.py
@@ -6294,15 +6294,20 @@ class Expose(object):
                                 for folder in self.folders], **dict(_class="table")))
         return ''
 
+    @staticmethod
+    def __in_base(subdir, basedir, sep=os.path.sep):
+        """True if subdir/ is under basedir/"""
+        s = lambda f: '%s%s' % (f.rstrip(sep), sep) # f -> f/
+        # The trailing '/' is for the case of '/foobar' in_base of '/foo':
+        # - becase '/foobar'  starts with        '/foo'
+        # - but    '/foobar/' doesn't start with '/foo/'
+        return s(subdir).startswith(s(basedir))
+
     def in_base(self, f):
         """True if f/ is under self.base/
         Where f ans slef.base are normalized paths
         """
-        s = lambda f: '%s%s' % (f, os.path.sep) # f -> f/
-        # The trailing '/' is for the case of '/foobar' in_base of '/foo':
-        # - becase '/foobar'  starts with        '/foo'
-        # - but    '/foobar/' doesn't start with '/foo/'
-        return s(self.normalize_path(f)).startswith(s(self.base))
+        return self.__in_base(self.normalize_path(f), self.base)
 
     def normalize_path(self, f):
         if self.follow_symlink_out: