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.” 
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.
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:- https://github.com/SecureAuthCorp/impacket/blob/cb6d43a677c338db930bc4e9161620832c1ec624/impacket/smbserver.py#L3485
- https://github.com/SecureAuthCorp/impacket/blob/cb6d43a677c338db930bc4e9161620832c1ec624/impacket/smbserver.py#L876
- https://github.com/SecureAuthCorp/impacket/blob/cb6d43a677c338db930bc4e9161620832c1ec624/impacket/smbserver.py#L2008
- https://github.com/SecureAuthCorp/impacket/blob/cb6d43a677c338db930bc4e9161620832c1ec624/impacket/smbserver.py#L2958
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
