This blog will walk through how to attack insecure Rsync configurations in order to gain a root shell on a Linux system. This should be a fun walkthrough for people new to penetration testing, or those looking for a Rsync refresher. This will be the first of a five part blog series highlighting entry points and local privilege escalation paths commonly found on Linux systems during real network penetration tests.
Below is an overview of what will be covered in this blog:
Rsync is a utility for transferring and synchronizing files between two servers (usually Linux). It determines synchronization by checking file sizes and timestamps. So what’s the problem? Insecurely configured Rsync servers are found during our network penetration tests about a third of the time. The weak configurations often provide unauthorized access to sensitive data, and sometimes the means to obtain a shell on the system. As you might imagine, the access we get is largely dependent on the Rsync configuration.
Remotely accessing directories shared through Rsync requires two things, file share access and file permissions.
File Share Access can be defined in /etc/Rsyncd.confto provide anonymous or authenticated access.
File Permissions can also be defined in /etc/Rsyncd.conf by defining the user that the Rsync service will run as. If Rsync is configured to run as root, then anyone allowed to connect can access the shared files with the privileges of the root user.
Below is an example of Rsyncd.conf file that allows anonymous root access to the entire file system:
Create Sudoers File Entry The /etc/sudoers file contains a list of users that are allowed to run commands as root using the sudo command. It can only be read by root. We are going to modify it to allow the new user to execute any command through sudo.
To inject a entry via Rsync you’ll have to:
Create the user entry to inject.
Download /etc/sudoers. (and backup, just in case)
Append the new user entry to the end of sudoers.
Upload / Overwrite the existing /etc/sudoers file.
Now you can simply log into the server via SSH using your newly created user and sudo sh to root!
Attacking Rsync Demo Video
Below is a video created in a lab environment that shows the process of identifying and exploiting an insecurely configured Rsync server to gain a root shell. While it see too simple to be true, it is based on configurations exploited during real penetration tests.
This blog illustrated one way to obtain a root shell on a remote Linux system using a vulnerability that provided write access. While there are many ways to obtain the same end, I think the moral of the story is to make sure that all network share types are configured with least privilege to help prevent unauthorized access to data and systems. Hopefully this blog will be useful to new pentesters and defenders trying to better understand the potential impacts associated with insecurely configured Rsync servers. Good luck and hack responsibly!
The next blog in the series focuses on NFS and setuid binaries, it can be found here.
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