The Acme Company has a strict policy about encryption and security for all sensitive information. In an attempt to provide a method for secure file transfer between itself and its customers and vendors, the Acme Company set up a Linux server, placed it behind a firewall, opened only TCP port 22 to the Internet, and created regular user accounts for each vendor and a shared regular user account for all customers.
The data transferred between Acme and its customers was nonsensitive and related to technical support. The technical support username and password were given out only to customers with technical support questions and only over the phone, never via email or other insecure means.
The data that was transferred between Acme and its vendors, however, was highly sensitive and contained detailed information on product designs for future products as well as sensitive financial and employee payroll data. All of these data types were accessible via the user permissions for the respective vendor and, in a perfect world, were protected by the user account and permissions design. The individual vendor usernames and passwords were also given out only over the phone and were never sent via unencrypted means, such as email.
While this may seem like a good start (not really), Slartibartfast, a disgruntled customer who had been scorned by an impatient technical support representative, quickly discovered that he could not only upload files but also log into the system using an SSH command shell, run system commands, and execute arbitrary binaries that he uploaded to the system.
Within minutes, he determined the distribution, kernel, installed software, and patch level of the system. A quick search on http://www.cve.mitre.org/cve/ revealed that the system had multiple vulnerabilities due to uninstalled patches and updates. Slartibartfast then browsed to http://www.packetstormsecurity.org and downloaded several exploits, one or more of which would give him root access to the system through any of the multiple vulnerabilities identified. Ten minutes later, he was reviewing plans and schematics for products coming out the next year while figuring out how he could profit from the employee payroll information he had also found.
Possibly the second most important rule in information security is "Treat shell access like it is physical access!" Shell accounts are effectively the same as local system access, even when coming in from SSH or some other remote service. If attackers obtain access to even unprivileged shell accounts, it may only be a matter of time until they find a way to upload tools and exploits to the system and are able to gain root-level access.
Therefore, most of the principles that apply in physical security (PHYSEC) also apply in communication security (COMMSEC). It could be said that COMMSEC is PHYSEC minus hardware controls plus network concerns. So, as if things weren't complicated enough already, we're about to add another entire dimension.
Additionally, the concept of "providing only the access needed" carries over seamlessly to the network perspective, but we add to that, "limiting access by what is disabled or closed, instead of implementing access controls that limit what is already enabled." In other words, no service should be running, or ports listening, unless they are supposed to be.
For example, it is better to disable services and filter ports than to add authentication mechanisms or white lists. This remains true to the goal of least access and brings continuity to the physical and network configurations. Throughout this chapter, we will discuss the data, traffic, and attack vectors that travel across network segments, their danger, possible abuse, and protection strategies. For best understanding, it is essential to have a basic understanding of the Seven Layer OSI model.
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