POS37-C. Ensure that privilege relinquishment is successful

The POSIX setuid() function has complex semantics and platform-specific behavior [Open Group 2004].

If the process has appropriate privileges, setuid() shall set the real user ID, effective user ID, and the saved set-user-ID of the calling process to uid.

If the process does not have appropriate privileges, but uid is equal to the real user ID or the saved set-user-ID, setuid() shall set the effective user ID to uid; the real user ID and saved set-user-ID shall remain unchanged.

The meaning of "appropriate privileges" varies from platform to platform. For example, on Solaris, appropriate privileges for setuid() means that the PRIV_PROC_SETID privilege is in the effective privilege set of the process. On BSD, it means that the effective user ID (EUID) is zero (that is, the process is running as root) or that uid=geteuid(). On Linux, it means that the process has CAP_SETUID capability and that setuid(geteuid()) will fail if the EUID is not equal to 0, the real user ID (RUID), or the saved set-user ID (SSUID).

Because of this complex behavior, desired privilege drops sometimes may fail. For example, the range of Linux Kernel versions (2.2.0–2.2.15) is vulnerable to an insufficient privilege attack wherein setuid(getuid() did not drop privileges as expected when the capability bits were set to zero. As a precautionary measure, subtle behavior and error conditions for the targeted implementation must be carefully noted.

Noncompliant Code Example

This noncompliant code example compiles cleanly on most POSIX systems, but no explicit checks are made to ensure that privilege relinquishment has succeeded. This may be dangerous depending on the sequence of the preceding privilege changes.

/* Code intended to run with elevated privileges */

/* Temporarily drop privileges */
if (seteuid(getuid()) != 0) {
  /* Handle error */
}

/* Code intended to run with lower privileges */

if (need_more_privileges) {
  /* Restore privileges */
  if (seteuid(0) != 0) {
    /* Handle error */
  }

  /* Code intended to run with elevated privileges */
}

/* ... */

/* Permanently drop privileges */
if (setuid(getuid()) != 0) {
  /* Handle error */
}

/*
 * Code intended to run with lower privileges,
 * but if privilege relinquishment failed,
 * attacker can regain elevated privileges!
 */

If the program is run as a setuid root program, over time, the state of the UIDs might look like the following:

If the program fails to restore privileges, it will be unable to permanently drop them later:

Compliant Solution

This compliant solution was implemented in sendmail, a popular mail transfer agent, to determine if superuser privileges were successfully dropped [Wheeler 2003]. If the setuid() call succeeds after (supposedly) dropping privileges permanently, then the privileges were not dropped as intended.

/* Code intended to run with elevated privileges   */

/* Temporarily drop privileges */
if (seteuid(getuid()) != 0) {
  /* Handle error */
}

/* Code intended to run with lower privileges */

if (need_more_privileges) {
  /* Restore Privileges */
  if (seteuid(0) != 0) {
    /* Handle error */
  }

  /* Code intended to run with elevated privileges */
}

/* ... */

/* Permanently drop privileges */
if (setuid(getuid()) != 0) {
  /* Handle error */
}

if (setuid(0) != -1) {
  /* Privileges can be restored, handle error */
}

/*
 * Code intended to run with lower privileges;
 * attacker cannot regain elevated privileges
 */

Compliant Solution

A better solution is to ensure that proper privileges exist before attempting to perform a permanent drop:

/* Store the privileged ID for later verification */
uid_t privid = geteuid();

/* Code intended to run with elevated privileges   */

/* Temporarily drop privileges */
if (seteuid(getuid()) != 0) {
  /* Handle error */
}

/* Code intended to run with lower privileges  */

if (need_more_privileges) {
  /* Restore Privileges */
  if (seteuid(privid) != 0) {
    /* Handle error */
  }

  /* Code intended to run with elevated privileges   */
}

/* ... */

/* Restore privileges if needed */
if (geteuid() != privid) {
  if (seteuid(privid) != 0) {
    /* Handle error */
  }
}

/* Permanently drop privileges */
if (setuid(getuid()) != 0) {
  /* Handle error */
}

if (setuid(0) != -1) {
  /* Privileges can be restored, handle error */
}

/*
 * Code intended to run with lower privileges;
 * attacker cannot regain elevated privileges
 */

Supplementary Group IDs

A process may have a number of supplementary group IDs, in addition to its effective group ID, and the supplementary groups can allow privileged access to files. The getgroups() function returns an array that contains the supplementary group IDs and can also contain the effective group ID. The setgroups() function can set the supplementary group IDs and can also set the effective group ID on some systems. Using setgroups() usually requires privileges. Although POSIX defines the getgroups() function, it does not define setgroups().

Under normal circumstances, setuid() and related calls do not alter the supplementary group IDs. However, a setuid-root program can alter its supplementary group IDs and then relinquish root privileges, in which case, it maintains the supplementary group IDs but lacks the privilege necessary to relinquish them. Consequently, it is recommended that a program immediately relinquish supplementary group IDs before relinquishing root privileges.

POS36-C. Observe correct revocation order while relinquishing privileges discusses how to drop supplementary group IDs. To ensure that supplementary group IDs are indeed relinquished, you can use the following eql_sups function:

/* Returns nonzero if the two group lists are equivalent (taking into
   account that the lists may differ wrt the egid */
int eql_sups(const int cursups_size, const gid_t* const cursups_list,
	     const int targetsups_size, const gid_t* const targetsups_list) {
  int i;
  int j;
  const int n = targetsups_size;
  const int diff = cursups_size - targetsups_size;
  const gid_t egid = getegid();
  if (diff > 1 || diff < 0 ) {
    return 0;
  }
  for (i=0, j=0; i < n; i++, j++) {
    if (cursups_list[j] != targetsups_list[i]) {
      if (cursups_list[j] == egid) {
	i--; /* skipping j */
      } else {
	return 0;
      }
    }
  }
  /* If reached here, we're sure i==targetsups_size. Now, either
     j==cursups_size (skipped the egid or it wasn't there), or we didn't
     get to the egid yet because it's the last entry in cursups */
  return j == cursups_size ||
    (j+1 == cursups_size && cursups_list[j] == egid);
}

System-Specific Capabilities

Many systems have nonportable privilege capabilities that, if unchecked, can yield privilege escalation vulnerabilities. The following section describes one such capability.

File System Access Privileges (Linux)

Processes on Linux have two additional values called fsuid and fsgid. These values indicate the privileges used when accessing files on the file system. They normally shadow the effective user ID and effective group ID, but the setfsuid() and setfsgid() functions allow them to be changed. Because changes to the euid and egid normally also apply to fsuid and fsgid, a program relinquishing root privileges need not be concerned with setting fsuid or fsgid to safe values. However, there has been at least one kernel bug that violated this invariant ([Chen 2002] and [Tsafrir 2008]). Consequently, a prudent program checks that fsuid and fsgid have harmless values after relinquishing privileges.

Risk Assessment

If privilege relinquishment conditions are left unchecked, any flaw in the program may lead to unintended system compromise corresponding to the more privileged user or group account.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography