PAM—Securing Linux Boxes Everywhere

If you are into British detective fiction and names like Sherlock Holmes, Sexton Blake, Mr. J. G. Reeder, Miss Marple, Hercule Poirot, Father Brown, Dr. John Evelyn Thorndyke and Lord Peter Wimsey mean anything to you, you also probably will recognize E. W. Hornung's (brother-in-law to Sir Arthur Conan Doyle, the creator of Sherlock Holmes) character: the white-glove thief, Raffles. In the “A Jubilee Present” short story, the thief is fascinated with an antique gold cup, displayed at the British Museum. Upon finding only one guard, Raffles questions him on the perceived lack of security and gets the confident answer, “You see, sir, it's early as yet; in a few minutes these here rooms will fill up; and there's safety in numbers, as they say.” With Linux, rather than security by numbers (which eventually is no good for the poor guard; see Resources for a link to the complete story), security is managed by Pluggable Authentication Modules (PAM). In this article, we study PAM's features, configuration and usage.

Let's start at the beginning and consider how an application authenticates a user. Without a common, basic mechanism, each application would need to be programmed with particular authentication logic, such as checking the /etc/passwd for a valid user and password. But, what if you have several different applications that need authentication? Do you include the same specific logic in all of them? And, what if your security requirements vary? Would you then have to modify and recompile all those applications? This wouldn't be a practical method and surely would become a vulnerability. How would you be sure that all applications were duly updated and correctly implemented your new specifications?

The PAM Project provides a solution by adding an extra layer. Programs that need authentication use a standard library or API (Application Programming Interface), and system administrators can configure what checks will be done by that library separately. (Checks are implemented via independent modules; you even can program your own modules.) This way, you can change your security checks dynamically, and all utilities will follow your new rules automatically. In other words, you can modify the authentication mechanism used by any PAM-aware application, without ever touching the application itself. For programmers, this also is a good thing, because they need not be concerned with the mechanisms that will be used. Simply by using the PAM libraries, whenever the application is run, the appropriate checks will be made (Figure 1).

The PAM library breaks down authentication in four areas or groups (Table 1). Note that all applications won't always require the four previous actions. For example, the passwd command will require only the last group. (Quick tip: how can you learn whether an application uses PAM? Use ldd to print the shared libraries required by the program, and check for libpam.so; see Listing 1 for an example.)

Figure 1. Whenever an application does an authentication request, the PAM library executes whatever modules are specified in the configuration file and decides whether to approve (success) or reject (failure) the request.