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Over the last week we've seen the compromise of a number of supercomputers through their SSH service. Given the increase in Work From Home (WFH), the security of remote, administrative based services is more important than ever. If we look at Shodan (Figure 1), we can see ~22 million SSH services. Given the sheer volume of SSH services on the Internet, it's hardly surprising that attacks are on the rise and that they are successful.
Figure 1: SSH Services by as identified by Shodan
It's also a popular service for the SpiderLabs penetration test team when looking to gain entry into a host or environment. Given our long history compromising the service we wanted to give you some advice on how to properly secure the service from threat actors.
The configuration file for SSH, sshd_config, has a large number of configurable options. We would always recommend that options are considered for their security and context within an environment. We'll list what we believe are the most important recommendations for securing the SSH service.
Figure 2: OpensSSH Vulnerabilities by Year (src: https://www.cvedetails.com/product/585/Openbsd-Openssh.html?vendor_id=97)
As a dedicated penetration testing team, we see many different types of SSH configurations. Some of these configurations we believe add little or no value to the overall security posture of the service.
The SSH administrative service is a key component when looking to gain access to an environment, even more so with the WFH movement. Ensuring its security is paramount when looking to create a robust and secure environment.
Ed Williams is VP, SpiderLabs at Trustwave, with over 10 years of experience directly focused on penetration testing and consultancy for Government and private sector organizations. Follow Ed on LinkedIn.
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