Transferring Files

Related Sites:

• Impacket SMBServer

• PowerUp

There are many situations when transferring files to or from a target system is necessary. Let's imagine the following scenario:

Setting the Stage

During an engagement, we gain remote code execution (RCE) on an IIS web server via an unrestricted file upload vulnerability. We upload a web shell initially and then send ourselves a reverse shell to enumerate the system further in an attempt to escalate privileges. We attempt to use PowerShell to transfer PowerUp.ps1 (a PowerShell script to enumerate privilege escalation vectors), but PowerShell is blocked by the Application Control Policy. We perform our local enumeration manually and find that we have SeImpersonatePrivilege. We need to transfer a binary to our target machine to escalate privileges using the PrintSpoofer tool. We then try to use Certutil to download the file we compiled ourselves directly from our own GitHub, but the organization has strong web content filtering in place. We cannot access websites such as GitHub, Dropbox, Google Drive, etc., that can be used to transfer files. Next, we set up an FTP Server and tried to use the Windows FTP client to transfer files, but the network firewall blocked outbound traffic for port 21 (TCP). We tried to use the Impacket smbserver tool to create a folder, and we found that outgoing traffic to TCP port 445 (SMB) was allowed. We used this file transfer method to successfully copy the binary onto our target machine and accomplish our goal of escalating privileges to an administrator-level user.

Understanding different ways to perform file transfers and how networks operate can help us accomplish our goals during an assessment. We must be aware of host controls that may prevent our actions, like application whitelisting or AV/EDR blocking specific applications or activities. File transfers are also affected by network devices such as Firewalls, IDS, or IPS which can monitor or block particular ports or uncommon operations.

File transfer is a core feature of any operating system, and many tools exist to achieve this. However, many of these tools may be blocked or monitored by diligent administrators, and it is worth reviewing a range of techniques that may be possible in a given environment.