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Command executed from externally controlled path

Path argument from an unsecure source

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Description

This defect occurs when the path to a command executed in the program is constructed from external sources.

Risk

An attacker can:

  • Change the command that the program executes, possibly to a command that only the attack can control.

  • Change the environment in which the command executes, by which the attacker controls what the command means and does.

Fix

Before calling the command, validate the path to make sure that it is the intended location.

Extend Checker

By default, Polyspace® assumes that data from external sources are tainted. See Sources of Tainting in a Polyspace Analysis. To consider any data that does not originate in the current scope of Polyspace analysis as tainted, use the command line option -consider-analysis-perimeter-as-trust-boundary.

Examples

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#include <stdlib.h>
#include <stdio.h>
#include <string.h>

enum {
    SIZE10  =  10,
    SIZE100 = 100,
    SIZE128 = 128
};

void bug_taintedpathcmd() {
    char cmd[SIZE128] = "";
    char* userpath = getenv("MYAPP_PATH");

    strncpy(cmd, userpath, SIZE100);//Noncompliant
    strcat(cmd, "/ls *");
    /* Launching command */
    system(cmd);//Noncompliant 
}

This example obtains a path from an environment variable MYAPP_PATH. The path string is tainted. Polyspace flags its use in the strncopy function. system runs a command from the tainted path without checking the value of the path. If the path is not the intended path, your program executes in the wrong location.

Correction — Use Trusted Path

One possible correction is to use a list of allowed paths to match against the environment variable path.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

enum {
    SIZE10  =  10,
    SIZE100 = 100,
    SIZE128 = 128
};

/* Function to sanitize a string */
/* Any defect is localized here */
int sanitize_str(char* s, size_t n) {
    int res = 0;
    /* String is ok if */
    if (s && n>0 && n<SIZE128) {
        /* - string is not null                     */
        /* - string has a positive and limited size */
        s[n-1] = '\0';  /* Add a security \0 char at end of string *///Noncompliant
        /* Tainted pointer detected above, used as "firewall" */
        res = 1;
    }
    return res;
}

/* Authorized path ids */
enum { PATH0=1, PATH1, PATH2 };

void taintedpathcmd() {
    char cmd[SIZE128] = "";

    char* userpathid = getenv("MYAPP_PATH_ID");
    if (sanitize_str(userpathid, SIZE100)) {
        int pathid = atoi(userpathid);


        char path[SIZE128] = "";
        switch(pathid) {
            case PATH0:
                strcpy(path, "/usr/local/my_app0");
                break;
            case PATH1:
                strcpy(path, "/usr/local/my_app1");
                break;
            case PATH2:
                strcpy(path, "/usr/local/my_app2");
                break;
            default:
                /* do nothing */
		break;
        }
        if (strlen(path)>0) {
            strncpy(cmd, path, SIZE100);
            strcat(cmd, "/ls *");
            system(cmd);    
        }
    }
}

Result Information

Group: Tainted Data
Language: C | C++
Default: Off
Command-Line Syntax: TAINTED_PATH_CMD
Impact: Medium

Version History

Introduced in R2015b

See Also

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