Contents:
User accounts can be tied to a person or used for a service known as a logical user.
Users can be organized into groups to apply policy to a set of users for a particular operation, or to provide access to a resource. An improper configuration or change in the user and group settings can leave a host wide open to attack. Traditionally, users and groups on UNIX systems are defined in flat files. In both files, each user or group entry is listed on a single line with the attributes separated by colons.
Not all of these accounts are associated with a person; many are system accounts used for services e. Following the principle of least privilege, most of these service accounts do not have a login shell. The following example lists a password file from a FreeBSD system:.
The UID of zero is reserved for the root user, who has complete control over the system. The names assigned to users are not important; the UID uniquely identifies a user. Thus, any user with a zero UID is considered a root user.
The following example shows a group file from FreeBSD:. The zero GID is usually known as the wheel group or the system group and is reserved for privileged users. The information kept in these two files is referenced by the system when enforcing file permissions and runtime privileges. Changes to these files can significantly undermine the security of a host. Although the cumbersome process of managing multiple hosts has led to the adoption of other means of defining users and groups on UNIX systems, monitoring these files is still important.
In some cases e. If the system becomes mindful of these files, you must make sure that they have not been compromised. It is not uncommon for an attacker to add user accounts as a means of establishing a backdoor into the system. Changes especially suspect include additions to the wheel group or adding another user with a zero UID. Even if your user and groups are defined elsewhere, the changes to user and group files can be the first indication of an attack.
Local access to a host is very important and should be very secure. Once an attacker obtains local access, an abundance of opportunities for misuse and abuse become available. At this point, network defenses become powerless. Perimeter defenses serve to prevent attackers from exploiting software and obtaining unauthorized access; however, eventually a situation will exist where the perimeter can be breached.
The security surrounding passwords has caused the password file to evolve, with most systems now shadowing their passwords. To help secure passwords, they were encrypted and only the encrypted passwords were stored. Upon login, the password presented by the user was encrypted and compared against the entry in the passwd file. Because it was trivial to conduct brute-force attacks on these passwords, it became necessary to remove them from the file altogether. Today, most UNIX systems store encrypted passwords only in a shadow file, and introduce a salt into the encryption process as a means of mitigating dictionary attacks.
The key is to realize that all password authenticating has been susceptible to compromise. Access to user accounts is not well protected, and therefore, monitoring the surrounding elements of user logins is critical in detecting unauthorized access. PAM is basically an interface to various authentication schemes that allow many disparate applications to offload their user authentication handling. PAM is modular, so administrators can, with relative ease, add a new authentication scheme to a host, as long as a PAM module has been implemented for it.
The four module types auth, account, session , and password deal with various aspects of the authentication scheme. The auth type handles verification. The account type is used to place restrictions on access. The password module is responsible for updating authentication tokens. Changes to a PAM configuration file can seriously impact the integrity of a system. Doug Hartmann considers a Professor of mouse click the next webpage at the University of Minnesota.
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The following example lists a password file from a FreeBSD system:. The raw education won while the Web language was visiting your email. Richard Gruneau and David Whitson. Imagine that one of your goals is to protect the data associated with a Web server. Users and Groups Users and groups constitute the backbone of the security model for most operating systems.
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