diff -naur python-cherrypy-2.2.1.org/cherrypy/filters/sessionfilter.py python-cherrypy-2.2.1/cherrypy/filters/sessionfilter.py
--- python-cherrypy-2.2.1.org/cherrypy/filters/sessionfilter.py	2008-01-18 16:21:27.000000000 +0100
+++ python-cherrypy-2.2.1/cherrypy/filters/sessionfilter.py	2008-01-18 16:23:36.000000000 +0100
@@ -326,6 +326,8 @@
             raise SessionStoragePathNotConfiguredError()
         fileName = self.SESSION_PREFIX + id
         file_path = os.path.join(storage_path, fileName)
+        if not os.path.normpath(file_path).startswith(storage_path):
+            raise cherrypy.HTTPError(400, "Invalid session id in cookie.")
         return file_path
     
     def _lock_file(self, path):