Use of getlogin() in Multithreaded Application

Description

The getlogin() function returns a pointer to a string that contains the name of the user associated with the calling process. The function is not reentrant, meaning that if it is called from another process, the contents are not locked out and the value of the string can be changed by another process. This makes it very risky to use because the username can be changed by other processes, so the results of the function cannot be trusted.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following code relies on getlogin() to determine whether or not a user is trusted. It is easily subverted.

pwd = getpwnam(getlogin());
if (isTrustedGroup(pwd->pw_gid)) {
  allow();
} else {
  deny();
}

See Also