Lab 01: Environment Variables & Set-UID Programs Lab

Adapted from SEED Labs: A Hands-on Lab for Security Education.

The learning objective of this lab is for students to understand how environment variables affect program and system behaviors. Environment variables are a set of dynamic named values that can affect the way running processes will behave on a computer. They are used by most operating systems since they were introduced into Unix in 1979. Although environment variables affect program behaviors, how they achieve that is not well understood by many programmers. As a result, if a program uses environment variables, but the programmer does not know that they are used, the program may have vulnerabilities.

In this lab, students will understand how environment variables work, how they are propagated from parent process to child, and how they affect system/program behaviors. We are particularly interested in how environment variables affect the behavior of Set-UID programs, which are usually privileged programs. This lab covers the following topics:

• Environment variables
• Set-UID programs
• How to securely invoke external programs
• The dynamic linker/loader

Resources

• Code related to this lab can be found in our class’s GitHub repository. Specifically, see 01_envvars_setuid/.
• Chapters 1 & 2 in the SEED Textbook.
• A related video lecture (Udemy course) recorded by Kevin Du.
• Checklist for Security of Setuid Programs
• Setuid Demystified, Chen et al.
• How to write a Setuid program, Matt Bishop

Lab Tasks

This lab has been tested on the pre-built SEED VM (Ubuntu 20.04 VM).

Task 1: Manipulating Environment Variables

Task 1.1

Use the printenv or env command to print out the environment variables. If you are interested in viewing particular environment variables, such as PWD, you can use printenv PWD or env | grep PWD.

Task 1.2

Use export and unset to set or unset environment variables. Please show that you can set your own environment variables using export

Task 2: Passing Environment Variables (Parent -> Child)

In this task, we study how a child process gets its environment variables from its parent. In Unix, fork() creates a new process by duplicating the calling process. The new process, referred to as the child, is an exact duplicate of the calling process, referred to as the parent; however, several things are not inherited by the child (please see the manual of fork() by typing the following command: man fork). In this task, we would like to know whether the parent’s environment variables are inherited by the child process or not.

Task 2.1

Please compile and run the following program, and describe your observations. You should save the output of this program to a new file ( ./myprintenv > out1)(NOTE: This file is also in out GitHub repo that you cloned, myprintenv.c

Task 2.2

Now comment out the printenv() statement in the “child process” case, and uncomment the printenv() statement in the “parent process” case. Compile and run the code again, and describe your observation. Save the output in another file (e.g., ./myprintenv > out2.

Task 2.3

Compare the difference of these two files using the diff command. To compare the two outputs with diff, run the command diff out1 out2 Please draw your conclusions. HINT: If you see no ouput running the diff command , that is a valid answer (what does that mean?)

Task 3: Environment Variables and Set-UID Programs

Set-UID is an important security mechanism in Unix operating systems. When a Set-UID program runs, it assumes the owner’s privileges. For example, if the program’s owner is root, when anyone runs this program, the program gains root’s privileges during its execution. Set-UID allows us to do many interesting things, but since it escalates the user’s privilege, it is quite risky. Although the behavior of Set-UID programs is decided by their program logic, not by users, users can indeed affect the behavior via environment variables. To understand how Set-UID programs can be affected, let us first figure out whether environment variables are inherited by a Set-UID program’s process from the user’s process.

Task 3.1

Use the following program that can print out all the environment variables in the current process. Verify that your implementation correctly prints the environment variables.

Task 3.2

Compile the above program, change its ownership to root, and make it a Set-UID program.

$ sudo chown root myenv_environ   # chown = (ch)ange (own)er
$ sudo chmod 4755 myenv_environ   # chmod = (ch)ange file (mod)e bits

Task 3.3

In your shell (you need to be in a normal user account, not the root account), use the export command to set the following environment variables (NOTE: they may exist already!):

• PATH — prepend the current directory symbol to PATH
• LD_LIBRARY_PATH — prepend the current directory symbol to LD_LIBRARY_PATH
• TASK5 — this is a non-standard variable; define this however you want

To be clear, these environment variables should be set in the user’s shell process.

After you have exported the above environment variables into the user’s shell environment, run the Set-UID program from Task 3.2 in your shell. After you type the name of the program in your shell, the shell forks a child process, and uses the child process to run the program. Please check whether all the environment variables you set in the shell process (parent) are inherited in the Set-UID child process. Describe your observations. If there are any surprises to you, describe them.

$ sudo ln -sf /bin/zsh /bin/sh   # set shell to zsh - zsh has no set-uid countermeasure
$ sudo ln -sf /bin/dash /bin/sh  # set shell to dash - dash has a set-uid countermeasure

Task 4: Exploiting a SET-UID Program with the system()function

Task 4.1

Task 4.2

Task 5: PATH and Set-UID Programs

In this task, we study how Set-UID programs deal with environment variables. Specifically, we examine the PATH environment variable and its potential impact on Set-UID programs. Because system() runs commands by invoking a shell, calling system() within a Set-UID program is quite dangerous. One concern is that the actual behavior of the shell program can be affected by environment variables, such as PATH; these environment variables are provided by the user, who may be malicious. By changing these variables, malicious users can potentially 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 programmer only uses the relative path for the ls command, rather than the absolute path:

Please compile the above program, change its owner to root, and make it a Set-UID program.

Can you make this Set-UID program run your code (e.g., code that launches a new shell) instead of /bin/ls?

If you can, is your code running with root privileges? Describe and explain your observations.

Task 6: LD_PRELOAD and Set-UID Programs

In this task, we study how Set-UID programs deal with environment variables. Specifically, we examine the LD_PRELOAD environment variable and its potential impact on Set-UID programs. Several environment variables, including LD_PRELOAD, LD_LIBRARY_PATH, and others with the LD_ prefix influence the behavior of the dynamic linker/loader. A dynamic linker/loader is the part of an operating system (OS) that loads an executable from persistent storage to RAM, and links the shared libraries needed by the executable at run time.

In Linux, ld.so or ld-linux.so, are the dynamic linker/loader (ld-linux.so supports ELF, which is a common file format for executables today). Among the environment variables that affect the behavior of the dynamic linker/loader, LD_LIBRARY_PATH and LD_PRELOAD are the two that we are concerned in this lab. In Linux, LD_LIBRARY_PATH is a colon-separated set of directories where libraries should be searched for first, before the standard set of directories. LD_PRELOAD specifies a list of additional, user-specified, shared libraries to be loaded before all others. In this task, we will only focus on LD_PRELOAD.

Task 6.1

First, we will see how these environment variables influence the behavior of the dynamic linker/loader when running a normal program. Please follow these steps:

1. Let us build a dynamically linked library. Create the following program, and name it mylib.c. This program basically overrides the sleep() function in libc:

    #include <stdio.h>
    void sleep (int s)
    {
      /* If this is invoked by a privileged program, you can do damages here! */
      printf("I am not sleeping!\n");
    }
   

2. We can compile the above program using the following commands (in the -lc argument, the second character is the lowercase letter “L”):

   $ gcc -fPIC -g -c mylib.c
   $ gcc -shared -o libmylib.so.1.0.1 mylib.o -lc
   

3. Now, set the LD_PRELOAD environment variable:

   $ export LD_PRELOAD=./libmylib.so.1.0.1
   

4. Finally, compile the following program myprog within the same directory as the dynamically linked library you created above (libmylib.so.1.0.1):

   /* myprog.c */
   #include <unistd.h>
   int main()
   {
     sleep(1);
     return 0;
   }
   

Task 6.2

After you have done Task 6.1, run myprog under the following conditions, and observe what happens.

• 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, export the LD_PRELOAD environment variable again in the root account and run it.

  You can run sudo su to login as root. Make sure to exit when you are done.

Task 6.3

You should be able to observe different behaviors in the scenarios described above, even though you are running the same program. You need to figure out what causes the difference. Environment variables play a role here. Please design an experiment to figure out the main causes, and explain why the behaviors you observed in the previous part are different. (HINT: the child process may not inherit the LD_ environment variables!).

Submission Instructions

The lab report is to help me see that you did the lab and followed the instructions. For each task, you should include a screenshot to show you completed the task. If the task asks you to write down observations, you should also include those in your lab report. For the tasks that requires you to do some thinking and find ways to exploit a program, you should write a brief description about your approach and the steps you took to get your output. This is a lab report taken from a previous offering of this course. This is a good example of how you should format your lab report: https://www.cs.montana.edu/pearsall/classes/spring2023/476/labs/SampleLabReport.pdf
Once you are ready, submit your lab report AS A PDF to the appropriate D2L submission box.