As reported to the PSRT:

Python's SimpleHTTPServer class is a simple HTTP server, documented as
serving up the content of the pwd and below readonly via GET and HEAD
commands:

 $ python -m SimpleHTTPServer
 Serving HTTP on 0.0.0.0 port 8000 ...

However, by inserting "../" path fragments within the path section of
the URL, it's possible to traverse other directories within the
filesystem.

For example:

 lynx localhost:8000/../../../../..

shows 5 directories above in the directory structure.

I was also able to browse /proc and /sys on this example using:

 lynx localhost:8000/../../../../../../../../proc
 lynx localhost:8000/../../../../../../../../sys

(by browsing to find the correct number of ".." entries to locate the
root directory); arguable this could be leaking much more information
about the host than the administrator might be expecting (e.g. other
programs being executed on the host, command-line arguments of those
programs etc)

This has been fixed in CGIHTTPServer; see
 http://bugs.python.org/issue2254
and:
 http://svn.python.org/view?view=rev&revision=71303

Guido recommended to not make this secret since no one should be using SimpleHTTPServer in production. He also said this should get fixed.