Privilege drop not verified

Attacker can gain unintended elevated access to program

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Description

This defect 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 you do not verify that privileges were properly dropped after relinquishing them, an attacker may exploit the opportunity to regain elevated access, potentially compromising system security.

Fix

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

Examples

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#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 */
}

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, which indicates they were not properly dropped */
        /* Handle error */
        fatal_error();
    }

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

Result Information

Group: Security
Language: C | C++
Default: Off
Command-Line Syntax: MISSING_PRIVILEGE_DROP_CHECK
Impact: High
PQL Name: std.defects.MISSING_PRIVILEGE_DROP_CHECK

Version History

Introduced in R2016b

See Also

Topics