POS37-C. Ensure that privilege relinquishment is successful

POSIX defines setuid() in a rather non-intuitive way [[Open Group 04]]

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.

Because of this, there may be cases where the desired privilege drops are unsuccessful. 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.

Non-Compliant Code Example

The following non-compliant code example compiles cleanly on most POSIX based systems, however no explicit checks have been made to ensure that privilege relinquishment is carried out successfully. This may be dangerous depending on the sequence of the preceding privilege changes.

/*  Code intended to run with elevated privileges   */

/* Temporary Drop */
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   */
}

/* Permanent Drop */
if (setuid(getuid()) != 0) {
  /* Handle Error */
}

/*  Code intended to run with lower privileges  */ 

If the program is run with the setuid-0 flag, the state of the UID's over time might be:

If, for some reason, 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 [[Wheeler 03]]. It checks whether superuser privileges were dropped successfully. Note that if the setuid() call succeeds after the setuid(getuid()) operation, privileges were not dropped as was originally intended.

/*  Code intended to run with elevated privileges   */

/* Temporary Drop */
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   */
}

/* Permanent Drop */
if (setuid(getuid()) != 0) {
  /* Handle Error */
}

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

/*  Code intended to run with lower privileges  */ 

Non-Compliant Code Example

The function in this non-compliant code example correctly follows the principle of least privilege, however, due to inconsistencies and implementation defined behavior of certain functions (such as setuid()) across various operating systems, the final result may be unexpected. Here, when privileges are given up temporarily for the final time, the effective user ID of the process is set to the real user ID. Unexpectedly, the call to setuid(realuid) that follows, does not affect the saved set-user-ID since effective UID is no longer 0 (Except on FreeBSD and NetBSD). If a seteuid(0) gets executed maliciously after this statement, root privileges would be recovered from the saved set-user-ID.

void doSomething(void) {
  uid_t realuid = getuid();
  seteuid(realuid);      /* Give up privileges temporarily  */
  
  seteuid(0);            /* Regain superuser privileges  */

  /* Carry out the privileged task */ 

  seteuid(realuid);      /* Give up privileges temporarily */
 
  setuid(realuid);       /* Failed attempt at giving up privileges permanently */
}

Compliant Solution

The following code shows how the effective UID should be obtained and compared against 0 (superuser's EUID) to make sure privileges can be successfully dropped permanently. This constitutes a more portable and safe solution.

void doSomething(void) {
  uid_t realuid = getuid();
  seteuid(realuid);      /* Give up privileges temporarily */
  
  seteuid(0);            /* Regain superuser privileges */

  /* Carry out privileged task */ 

  seteuid(realuid);      /* Give up privileges temporarily */
 
  if (!geteuid()) {         /*  Check if the effective uid is still that of the superuser  */
    setuid(realuid);     /*  Go ahead and give up privileges permanently  */
  }
  else {
    /* Handle the possible implementation defined behavior */
  } 
}

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.

Related Vulnerabilities

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

References

[[CWE - 273]] Failure to Check Whether Privileges Were Dropped Successfully
[[Dowd 06]] Chapter 9, "Unix I: Privileges and Files"
[[Open Group 04]] setuid(), getuid(), seteuid()
[[Wheeler 03]] Section 7.4, "Minimize Privileges"

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