What Is Dev Null Comman In C
I am a new Linux command line user. How do I start or run command in the background so that I can access command prompt immediately? A command that has been scheduled nonsequentially is called background process.You can not see the background processes on screen. Example-1: wget command without any option. The following `wget` command will download the index.html file from the site, linuxhint.com and the file will be stored on the current working directory.‘ls’ command is used here to check the html file is created or not. Jun 05, 2014 What is a null (/dev/null) file in a Linux or Unix-like systems? /dev/null is nothing but a special file that discards all data written to it. The length of the null device is always zero. In this example, first, send output of date command to the screen and later.
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Description
Command injection is an attack in which the goal is execution ofarbitrary commands on the host operating system via a vulnerableapplication. Command injection attacks are possible when an applicationpasses unsafe user supplied data (forms, cookies, HTTP headers etc.) toa system shell. In this attack, the attacker-supplied operating systemcommands are usually executed with the privileges of the vulnerableapplication. Command injection attacks are possible largely due toinsufficient input validation.
This attack differs from Code Injection, inthat code injection allows the attacker to add his own code that is thenexecuted by the application. In Command Injection, the attacker extendsthe default functionality of the application, which execute systemcommands, without the necessity of injecting code.
Examples
Example 1
The following code is a wrapper around the UNIX command cat whichprints the contents of a file to standard output. It is also injectable:
Used normally, the output is simply the contents of the file requested:
However, if we add a semicolon and another command to the end of thisline, the command is executed by catWrapper with no complaint:
If catWrapper had been set to have a higher privilege level than thestandard user, arbitrary commands could be executed with that higherprivilege.
Example 2
The following simple program accepts a filename as a command lineargument, and displays the contents of the file back to the user. Theprogram is installed setuid root because it is intended for use as alearning tool to allow system administrators in-training to inspectprivileged system files without giving them the ability to modify themor damage the system.
Because the program runs with root privileges, the call to system() alsoexecutes with root privileges. If a user specifies a standard filename,the call works as expected. However, if an attacker passes a string ofthe form “;rm -rf /”, then the call to system() fails to execute cat dueto a lack of arguments and then plows on to recursively delete thecontents of the root partition.
Example 3
The following code from a privileged program uses the environmentvariable $APPHOME to determine the application’s installation directory,and then executes an initialization script in that directory.
As in Example 2, the code in this example allows an attacker to executearbitrary commands with the elevated privilege of the application. Inthis example, the attacker can modify the environment variable $APPHOMEto specify a different path containing a malicious version of INITCMD.Because the program does not validate the value read from theenvironment, by controlling the environment variable, the attacker canfool the application into running malicious code.
The attacker is using the environment variable to control the commandthat the program invokes, so the effect of the environment is explicitin this example. We will now turn our attention to what can happen whenthe attacker changes the way the command is interpreted.
Example 4
The code below is from a web-based CGI utility that allows users tochange their passwords. The password update process under NIS includesrunning make in the /var/yp directory. Note that since the programupdates password records, it has been installed setuid root.
The program invokes make as follows:
Linux What Is Dev Null
Unlike the previous examples, the command in this example is hardcoded,so an attacker cannot control the argument passed to system(). However,since the program does not specify an absolute path for make, and doesnot scrub any environment variables prior to invoking the command, theattacker can modify their $PATH variable to point to a malicious binarynamed make and execute the CGI script from a shell prompt. And since theprogram has been installed setuid root, the attacker’s version of makenow runs with root privileges.
The environment plays a powerful role in the execution of systemcommands within programs. Functions like system() and exec() use theenvironment of the program that calls them, and therefore attackers havea potential opportunity to influence the behavior of these calls.
There are many sites that will tell you that Java’s Runtime.exec isexactly the same as C’s system function. This is not true. Both allowyou to invoke a new program/process. However, C’s system function passesits arguments to the shell (/bin/sh) to be parsed, whereas Runtime.exectries to split the string into an array of words, then executes thefirst word in the array with the rest of the words as parameters.Runtime.exec does NOT try to invoke the shell at any point. The keydifference is that much of the functionality provided by the shell thatcould be used for mischief (chaining commands using “&”, “&&”, “ ”,“ ”, etc, redirecting input and output) would simply end up as aparameter being passed to the first command, and likely causing a syntaxerror, or being thrown out as an invalid parameter.
Example 5
The following trivial code snippets are vulnerable to OS commandinjection on the Unix/Linux platform:
:* C:
#include <stdlib.h>#include <stdio.h>#include <string.h>
int main(int argc, char **argv){ char command[256];
if(argc != 2) { printf('Error: Please enter a program to time!n'); return -1; }
memset(&command, 0, sizeof(command));
Dev Null Unix
strcat(command, 'time ./'); strcat(command, argv[1]);
system(command); return 0;}
:* If this were a suid binary, consider the case when an attackerenters the following: ‘ls; cat /etc/shadow’. In the Unix environment,shell commands are separated by a semi-colon. We now can execute systemcommands at will!
:* Java:
There are many sites that will tell you that Java’s Runtime.exec isexactly the same as C’s system function. This is not true. Both allowyou to invoke a new program/process. However, C’s system function passesits arguments to the shell (/bin/sh) to be parsed, whereas Runtime.exectries to split the string into an array of words, then executes thefirst word in the array with the rest of the words as parameters.Runtime.exec does NOT try to invoke the shell at any point. The keydifference is that much of the functionality provided by the shell thatcould be used for mischief (chaining commands using “&”, “&&”, “ ”,“ ”, etc, redirecting input and output) would simply end up as aparameter being passed to the first command, and likely causing a syntaxerror, or being thrown out as an invalid parameter.
Example 6
The following PHP code snippet is vulnerable to a command injectionattack:
The following request and response is an example of a successful attack:
Request
/auto-tune-vocal-chain.html. Response
Auto tune setup for live singer club band. Sanitizing Input
Script Dev Null
Related Attacks
Related Controls
Ideally, a developer should use existing API for their language. Forexample (Java): Rather than use Runtime.exec() to issue a ‘mail’command, use the available Java API located at javax.mail.*
If no such available API exists, the developer should scrub all inputfor malicious characters. Implementing a positive security model wouldbe most efficient. Typically, it is much easier to define the legalcharacters than the illegal characters.