Set-UID Program Vulnerability Lab

Laboratory for Computer Security Education 1 Set-UID Program Vulnerability Lab

Lab Description

Set-UID is an important security mechanism in Unix operating systems. When a Set-UID program is run, it assumes the owner’s privileges. For example, if the program’s owner is root, then when anyone runs this program, the program gains the root’s privileges during its execution. Set-UID allows us to do many interesting things, but unfortunately, it is also the culprit of many bad things. Therefore, the objective of this lab is two-fold: (1) Appreciate its good side: understand why Set-UID is needed and how it is implemented. (2) Be aware of its bad side: understand its potential security problems.

Lab Tasks

This is an exploration lab. Your main task is to ”play” with the Set-UID mechanism in Linux, and write a lab report to describe your discoveries. You are required to accomplish the following tasks in Linux:

1. Figure out why “passwd”, “chsh”, “su”, and “sudo” commands need to be Set-UID pro- grams. What will happen if they are not? If you are not familiar with these programs, you should first learn what they can do by reading their manuals. Please copy these commands to your own directory; the copies will not be Set-UID programs. Run the copied programs, and observe what happens.
2. Run Set-UID shell programs in Linux, and describe and explain your observations.
   1. (a)  Login as root, copy /bin/zsh to /tmp, and make it a set-root-uid program with permission 4755. Then login as a normal user, and run /tmp/zsh. Will you get root privilege? Please describe your observation. If you cannot find /bin/zsh in your operating system, please use the following command to install it:

      • Note: in our pre-built Ubuntu VM image, zsh is already installed. • For Fedora

                   $ su
                     Password: (enter root password)
      

      # yum install zsh • For Ubuntu

                   $ su
                     Password: (enter root password)
      

                   # apt-get install zsh
      

   2. (b)  Instead of copying /bin/zsh, this time, copy /bin/bash to /tmp, make it a set-root-uid program. Run /tmp/bash as a normal user. will you get root privilege? Please describe and explain your observation.

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3. (Setup for the rest of the tasks) As you can find out from the previous task, /bin/bash has certain built-in protection that prevent the abuse of the Set-UID mechanism. To see the life before such a protection scheme was implemented, we are going to use a different shell program called /bin/zsh. In some Linux distributions (such as Fedora and Ubuntu), /bin/sh is actually a symbolic link to /bin/bash. To use zsh, we need to link /bin/sh to /bin/zsh. The following instructions describe how to change the default shell to zsh.

    $ su
      Password: (enter root password)

    # cd /bin
    # rm sh
    # ln -s zsh sh

4. The PATH environment variable.
The system(const char *cmd) library function can be used to execute a command within a program. The way system(cmd) works is to invoke the /bin/sh program, and then let the shell program to execute cmd. Because of the shell program invoked, calling system() within a Set-UID program is extremely dangerous. This is because the actual behavior of the shell program can be affected by environment variables, such as PATH; these environment variables are under user’s control. By changing these variables, malicious users can control the behavior of the Set-UID program. In bash, you can change the PATH environment variable in the following way (this example adds the directory /home/seed to the beginning of the PATH environment variable):

$ export PATH=/home/seed:$PATH
The Set-UID program below is supposed to execute the /bin/ls command; however, the pro-

grammer only uses the relative path for the ls command, rather than the absolute path:

int main() {

         system("ls");

return 0; }

(a) Can you let this Set-UID program (owned by root) run your code instead of /bin/ls? If you can, is your code running with the root privilege? Describe and explain your observations.

(b) Now, change /bin/sh so it points back to /bin/bash, and repeat the above attack. Can you still get the root privilege? Describe and explain your observations.

5. The difference between system() and execve(). Before you work on this task, please make sure that /bin/sh is pointed to /bin/zsh.
Background: Bob works for an auditing agency, and he needs to investigate a company for a sus- pected fraud. For the investigation purpose, Bob needs to be able to read all the files in the company’s Unix system; on the other hand, to protect the integrity of the system, Bob should not be able to modify any file. To achieve this goal, Vince, the superuser of the system, wrote a special set-root-uid program (see below), and then gave the executable permission to Bob. This program requires Bob to type a file name at the command line, and then it will run /bin/cat to display the specified file.

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Since the program is running as a root, it can display any file Bob specifies. However, since the pro- gram has no write operations, Vince is very sure that Bob cannot use this special program to modify any file.

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

  int main(int argc, char *argv[])
  {

char *v[3];

    if(argc < 2) {
      printf("Please type a file name.n");
      return 1;

    }
    v[0] = "/bin/cat"; v[1] = argv[1]; v[2] = 0;

    /* Set q = 0 for Question a, and q = 1 for Question b */
    int q = 0;
    if (q == 0){

      char *command = malloc(strlen(v[0]) + strlen(v[1]) + 2);
      sprintf(command, "%s %s", v[0], v[1]);
      system(command);

    }
    else execve(v[0], v, 0);

return 0 ; }

1. (a)  Set q = 0 in the program. This way, the program will use system() to invoke the command. Is this program safe? If you were Bob, can you compromise the integrity of the system? For example, can you remove any file that is not writable to you? (Hint: remember that system() actually invokes /bin/sh, and then runs the command within the shell environment. We have tried the environment variable in the previous task; here let us try a different attack. Please pay attention to the special characters used in a normal shell environment).
2. (b)  Set q = 1 in the program. This way, the program will use execve() to invoke the command. Do your attacks in task (a) still work? Please describe and explain your observations.

6. The LD PRELOAD environment variable.
To make sure Set-UID programs are safe from the manipulation of the LD PRELOAD environ- ment variable, the runtime linker (ld.so) will ignore this environment variable if the program is a Set-UID root program, except for some conditions. We will figure out what these conditions are in this task.

(a) Let us build a dynamic link library. Create the following program, and name it mylib.c. It basically overrides the sleep() function in libc:

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        #include <stdio.h>
        void sleep (int s)
        {

          printf("I am not sleeping!n");
        }

2. (b)  We can compile the above program using the following commands (in the -Wl argument, the third character is l, not one; in the -lc argment, the second character is l):

           % gcc -fPIC -g -c mylib.c
           % gcc -shared -Wl,-soname,libmylib.so.1
   

                 -o libmylib.so.1.0.1 mylib.o -lc
   

3. (c)  Now, set the LD PRELOAD environment variable:
   % export LD_PRELOAD=./libmylib.so.1.0.1
4. (d)  Finally, compile the following program myprog (put this program in the same directory as libmylib.so.1.0.1):

           /* myprog.c */
           int main()
           {
   

   sleep(1);

   return 0; }

Please run myprog under the following conditions, and observe what happens. Based on your ob- servations, tell us when the runtime linker will ignore the LD PRELOAD environment variable, and explain why.

• Make myprog a regular program, and run it as a normal user.
• Make myprog a Set-UID root program, and run it as a normal user.
• Make myprog a Set-UID root program, and run it in the root account.
• Make myprog a Set-UID user1 program (i.e., the owner is user1, which is another user ac- count), and run it as a different user (not-root user).

  7. Relinquishing privileges and cleanup.
  To be more secure, Set-UID programs usually call setuid() system call to permanently relin- quish their root privileges. However, sometimes, this is not enough. Compile the following program, and make the program a set-root-uid program. Run it in a normal user account, and describe what you have observed. Will the file /etc/zzz be modified? Please explain your observation.

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    #include <stdio.h>
    #include <stdlib.h>
    #include <sys/types.h>
    #include <sys/stat.h>
    #include <fcntl.h>

    void main()
    { int fd;

      /* Assume that /etc/zzz is an important system file,
       * and it is owned by root with permission 0644.
       * Before running this program, you should creat
       * the file /etc/zzz first. */

      fd = open("/etc/zzz", O_RDWR | O_APPEND);
      if (fd == -1) {

         printf("Cannot open /etc/zzzn");

exit(0); }

      /* Simulate the tasks conducted by the program */
      sleep(1);

      /* After the task, the root privileges are no longer needed,
         it’s time to relinquish the root privileges permanently. */

      setuid(getuid());  /* getuid() returns the real uid */

      if (fork()) { /* In the parent process */
        close (fd);
        exit(0);

      } else { /* in the child process */
        /* Now, assume that the child process is compromised, malicious

           attackers have injected the following statements
           into this process */

        write (fd, "Malicious Datan", 15);

close (fd); }

}

Submission

You need to submit a detailed lab report to describe what you have done and what you have observed; you also need to provide explanation to the observations that are interesting or surprising.