CVE-2021-31800: How We Used Impacket to Hack Itself

Share on facebook
Share on twitter
Share on linkedin
According to its official documentation, Impacket is a collection of Python classes for working with network protocols. Impacket is focused on providing low-level programmatic access to the packets for some protocols (e.g. SMB1-3 and MSRPC), and for others, the protocol implementation itself. After investigating Impacket, the Checkmarx Security Research Team discovered a Path Traversal vulnerability, which could allow an attacker to write malicious files to any path on the target and achieve Remote Code Execution (RCE). This vulnerability, assigned CVE-2021-31800, was found not only in the Impacket library, but also in other tools that implement the vulnerable code such as CrackMapExec and “Honeypots.”

CVSS Score

CVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H CVS Score: 9.9 (Critical)

Impact Summary

If the vulnerability is exploited, an attacker could write files to any location on the affected computer. This could be elevated to an RCE in a variety of ways depending on the environment and the operating system. This will be demonstrated later in this write-up.

Description

Impacket’s smbserver is vulnerable to Path Traversal Attacks. An attacker that connects to a running smbserver instance can list and write arbitrary files to any location on the attacked host (not just the chosen workdir for the server). The reason for that is that while there is validation against relative paths in filenames in smbclient (and of course Windows Client), the Impacket SMB server implementation, smbserver does not validate it, which allows an attacker with an edited version of smbclient to exploit anyone who deploys an instance of smbserver.

Vulnerability Analysis

The following 2 lines take the filename path from the received SMB request and try to list its directory without making sure that the path is inside the intended working directory. This could enable an attacker to read the contents of the file system: The following 2 lines take the filename path and contents from the received SMB request and try write the contents to the received path without making sure that the path is inside the intended working directory. This could enable an attacker to arbitrarily write files:

Proof-of-Concept

To exploit this vulnerability, we have edited Impacket’s own smbclient.py to exploit its server implementation with 2 extra functions: do_ls_exploit and do_put_exploit. The functions do the exact same thing as do_ls and do_put (which is list directory and put file), only they do not sanitize the file paths on the client. Since this is not enforced on the server, when combined with a path containing Path Traversal strings (../), these customized functions allow an attacker to read directories and write files anywhere on the file system. Below are the differences between the two smbclient functions and their original counterparts: Below is a Proof-of-Concept flow of the attack: Creating a file for injection, connecting to a running smbserver instance (run locally for demonstration purposes) using the edited smbclient.py script and connecting to WORKDIR Listing the directory contents (empty directory) Listing /tmp using Path Traversal exploit Injecting poc.txt and confirming using the exploit again.

CrackMapExec

Another tool that we found containing the same vulnerability is CrackMapExec. CrackMapExec (or CME for short) is a post-exploitation tool used primarily for offensive purposes and is heavily based on the Impacket library. We have found three attack modules that use the vulnerable smbserver which are: netripper, get_timedscreenshot and get_keystrokes. When an attacker attacks a victim with one of these modules, CME opens an smbserver instance in order to collect the results. As we’ve seen before, this could be exploited by a third party.

Proof-of-Concept

Exploitation of the get_timedscreenshot module of CrackMapExec using an injected PHP Webshell in /var/www/html.

Honeypots and Chameleon

Finally, we found two honeypot-based tools (used for creating intentionally vulnerable environments that act as tripwires for incoming attacks) called "honeypots" and “Chameleon,” which again contain the same vulnerability. “Honeypots” is a python library which contains 16 different honeypots (including Impacket’s smbserver) in a single PyPI package for monitoring network traffic. Chameleon is a project from the same creators which features a GUI for handling multiple honeypots (imported from the “honeypots” library), which again contains the same vulnerability.

Proof-of-Concept

Exploitation of the “honeypots” smbserver using an injected SSH Key.

Recommendations

To avoid issues like this, update the Impacket package to the latest version.

Summary of Disclosure and Events

After notifying SecureAuthCorp, they informed us that they were aware that there might be possible vulnerabilities in the implementation, as can be seen by this 10 year old comment. Nevertheless, they did acknowledge the vulnerability and promptly accepted our fix. We would like to thank SecureAuthCorp for their fast response and continuous contribution to the security and research community.

Timeline of Disclosure

  • Apr 20, 2021 – Disclosure
  • Apr 20, 2021 – Issue acknowledged by SecureAuthCorp
  • Apr 26, 2021 – Fix committed
  • Apr 26, 2021 – CVE-2021-31800 assigned
  • May 3, 2021 – Fixed version released
  • June 16, 2021 – Public disclosure

Final Words

The research and resulting findings with Impacket serve as a valuable reminder that almost any piece of code that could be deployed in an organization must be tested regularly and made compliant, even if it does something as benign as serve some files via an SMB share. In this case, multiple tools that can be deployed by security experts in the company, either for testing or entrapment (honeypots), would actually be leaving themselves vulnerable. Impacket is very popular amongst security professionals, and so are the derivatives uses of the vulnerable code within it. The Checkmarx Security Research Team is dedicated to performing investigations of this nature to better improve the security posture of tools and organizations more broadly.

Copyright Notices

Impacket

Copyright (c) 2000 The Apache Software Foundation.  All rights reserved. This product includes software developed by SecureAuth Corporation (https://www.secureauth.com/)

CrackMapExec

Copyright (c) 2020, byt3bl33d3r All rights reserved. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Chameleon

Copyright (C) 2007 Free Software Foundation, Inc. https://fsf.org/  Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.

Honeypots

Copyright (C) 2007 Free Software Foundation, Inc. https://fsf.org/  Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.

Latest Blog Posts

Follow Us

Latest from Our Blog

Skip to content