In Unix-like operating systems, /etc/shadow is used to increase the security level of passwords by restricting all but highly privileged users’ access to hashed password data. Typically, that data is kept in files owned by and accessible only by the super user.
- Systems administrators can reduce the likelihood of brute force attacks by making the list of hashed passwords unreadable by unprivileged users. The obvious way to do this is to make the passwd database itself readable only by the root user. However, this would restrict access to other data in the file such as username-to-userid mappings, which would break many existing utilities and provisions.
- One solution is a “shadow” password file to hold the password hashes separate from the other data in the world-readable passwd file. For local files, this is usually /etc/shadow on Linux and Unix systems, or /etc/master.passwd on BSD systems; each is readable only by root. (Root access to the data is considered acceptable since on systems with the traditional “all-powerful root” security model, the root user would be able to obtain the information in other ways in any case). Virtually all recent Unix-like operating systems use shadowed passwords. The shadow password file does not entirely solve the problem of attacker access to hashed passwords, as some network authentication schemes operate by transmitting the hashed password over the network (sometimes in clear text, e.g., Telnet), making it vulnerable to interception. Copies of system data, such as system backups written to tape or optical media, can also become a means for illicitly obtaining hashed passwords. In addition, the functions used by legitimate password-checking programs need to be written in such a way that malicious programs cannot make large numbers of authentication checks at high rates of speed.
On a system without shadowed passwords (typically older Unix systems dating from before 1990 or so), the passwd file holds the following user information for each user account:
- Salt combined with the current hash of the user’s password (usually produced from
a cryptographic hash function)
- Password expiration information
- User ID (UID)
- Default group ID (GID)
- Full name
- Home directory path
- Login shell
The passwd file is readable by all users so that name service switch can work (e.g., to ensure that user names are shown when the user lists the contents of a folder), but only the root user can write to it. This means that an attacker with unprivileged access to the system can obtain the hashed form of every user’s password. Those values can be used to mount a brute force attack offline, testing
possible passwords against the hashed passwords relatively quickly without alerting system security arrangements designed to detect an abnormal number of failed login attempts. Users oftenselect passwords vulnerable to such password cracking techniques.
- With a shadowed password scheme in use, the /etc/passwd file typically shows a character such as ‘*’, or ‘x’ in the password field for each user instead of the hashed password, and /etc/shadow usually contains the following user information:
- User login name
- salt and hashed password OR a status exception value e.g.:
o “$id$salt$hashed”, where “$id” is the algorithm used (On GNU/Linux,
“$1$” stands for MD5, “$2$” is Blowfish, “$5$” is SHA-256 and
“$6$” is SHA-512, crypt(3) manpage, other Unix may have different
values, like NetBSD).
o “NP” or “!” or null – No password, the account has no password.
o “LK” or “*” – the account is Locked, user will be unable to log-in
o “!!” – the password has expired
- Days since epoch of last password change
- Days until change allowed
- Days before change required
- Days warning for expiration
- Days before account inactive
- Days since Epoch when account expires
- The format of the shadow file is simple, and basically identical to that of the password file, to wit, one line per user, ordered fields on each line, and fields separated by colons. Many systems require the order of user lines in the shadow file be identical to the order of the corresponding users in the password file.
- To modify the contents of the shadow file on most systems, users generally invoke the passwd program, which in turn largely depends on PAM. For example, the type of hash used is dictated by the configuration of the pam_unix.so module. By default, the MD5 hash has been used, while current modules are also capable of stronger hashes such as blowfish, SHA256 and SHA512
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