CWE Rule 273

Improper Check for Dropped Privileges

Since R2024a

Description

Rule Description

The product attempts to drop privileges but does not check or incorrectly checks to see if the drop succeeded.

Polyspace Implementation

The rule checker checks for Privilege drop not verified.

Examples

Privilege drop not verified

Issue

This issue occurs when you relinquish privileges using functions such as setuid but do not verify that the privileges were actually dropped before exiting your function.

Risk

If privilege relinquishment fails, an attacker can regain elevated privileges and have more access to your program than intended. This security hole can cause unexpected behavior in your code if left open.

Fix

Before the end of scope, verify that the privileges that you dropped were actually dropped.

Example — Drop Privileges Within a Function

#define _BSD_SOURCE
#include <sys/types.h>
#include <unistd.h>
#include <grp.h>
#include <stdlib.h>
#define fatal_error() abort()
extern int need_more_privileges;

void missingprivilegedropcheck()
{
    /* Code intended to run with elevated privileges */

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

    /* Code intended to run with lower privileges */

    if (need_more_privileges) {
        /* Restore elevated privileges */
        if (seteuid(0) != 0) {
            /* Handle error */
            fatal_error();
        }
        /* Code intended to run with elevated privileges */
    }

    /* ... */

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

    /* Code intended to run with lower privileges */
} //Noncompliant

In this example, privileges are elevated and dropped to run code with the intended privilege level. When privileges are dropped, the privilege level before exiting the function body is not verified. A malicious attacker can regain their elevated privileges.

Correction — Verify Privilege Drop

One possible correction is to use setuid to verify that the privileges were dropped.

#define _BSD_SOURCE
#include <sys/types.h>
#include <unistd.h>
#include <grp.h>
#include <stdlib.h>
#define fatal_error() abort()
extern int need_more_privileges;

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

    /* Code intended to run with elevated privileges   */

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

    /* Code intended to run with lower privileges  */

    if (need_more_privileges) {
        /* Restore elevated Privileges */
        if (seteuid(privid) != 0) {
            /* Handle error */
            fatal_error();
        }
        /* Code intended to run with elevated privileges   */
    }

    /* ... */

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

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

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

    /* Code intended to run with lower privileges; */
}

Check Information

Category: Privilege Issues

Version History

Introduced in R2024a