SEED Labs – Race Condition Vulnerability Lab 1

Race Condition Vulnerability Lab

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1 Overview

The learning objective of this lab is for students to gain the first-hand experience on the race-condition vul-nerability by putting what they have learned about the vulnerability from class into actions. A race conditionoccurs when multiple processes access and manipulate the same data concurrently, and the outcome of theexecution depends on the particular order in which the access takes place. If a privileged program has arace-condition vulnerability, attackers can run a parallel process to “race” against the privileged program,with an intention to change the behaviors of the program.

In this lab, students will be given a program with a race-condition vulnerability; their task is to developa scheme to exploit the vulnerability and gain the root privilege. In addition to the attacks, students will beguided to walk through several protection schemes that can be used to counter the race-condition attacks.Students need to evaluate whether the schemes work or not and explain why. This lab covers the followingtopics:

• Race condition vulnerability• Sticky symlink protection• Principle of least privilege

Readings and videos. Detailed coverage of the race condition attack can be found in the following:

• Chapter 7 of the SEED Book, Computer & Internet Security: A Hands-on Approach, 2nd Edition, byWenliang Du. See details at

• Section 6 of the SEED Lecture at Udemy, Computer Security: A Hands-on Approach, by WenliangDu. See details at

Related topics. There are three more labs related to race condition. One is the Dirty COW attack lab,which exploits a race condition vulnerability inside the OS kernel (Chapter 8 of the SEED book covers thisattack). The other two are Meltdown and Spectre attack labs (Chapters 13 and 14 of the SEED book). Theyexploit race conditions inside CPU. These four labs provide a comprehensive coverage of the race conditionproblem at different levels of a computer system, from application, kernel, to hardware.

Lab environment. This lab has been tested on the SEED Ubuntu 20.04 VM. You can download a pre-builtimage from the SEED website, and run the SEED VM on your own computer. However, most of the SEEDlabs can be conducted on the cloud, and you can follow our instruction to create a SEED VM on the cloud.

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2 Environment Setup

2.1 Turning Off Countermeasures

Ubuntu has a built-in protection against race condition attacks. This scheme works by restricting who canfollow a symlink. According to the documentation, “symlinks in world-writable sticky directories (e.g./tmp) cannot be followed if the follower and directory owner do not match the symlink owner.” Ubuntu20.04 introduces another security mechanism that prevents the root from writing to the files in /tmp that areowned by others. In this lab, we need to disable these protections. You can achieve that using the followingcommands:

// On Ubuntu 20.04, use the following:$ sudo sysctl -w fs.protected_symlinks=0$ sudo sysctl fs.protected_regular=0

// On Ubuntu 16.04, use the following:$ sudo sysctl -w fs.protected_symlinks=0

// On Ubuntu 12.04, use the following:$ sudo sysctl -w kernel.yama.protected_sticky_symlinks=0

2.2 A Vulnerable Program

The following program is a seemingly harmless program. It contains a race-condition vulnerability.

Listing 1: The vulnerable program (vulp.c)#include <stdio.h>#include<unistd.h>

int main(){

char * fn = "/tmp/XYZ";char buffer[60];FILE *fp;

/* get user input */scanf("%50s", buffer );

if(!access(fn, W_OK)){ Àfp = fopen(fn, "a+"); Áfwrite("n", sizeof(char), 1, fp);fwrite(buffer, sizeof(char), strlen(buffer), fp);fclose(fp);

}else printf("No permission n");


The program above is a root-owned Set-UID program; it appends a string of user input to the end ofa temporary file /tmp/XYZ. Since the code runs with the root privilege, i.e., its effective use ID is zero, itcan overwrite any file. To prevent itself from accidentally overwriting other people’s file, the program firstchecks whether the real user ID has the access permission to the file /tmp/XYZ; that is the purpose of the

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access() call in Line À. If the real user ID indeed has the right, the program opens the file in Line Á andappend the user input to the file.

At first glance the program does not seem to have any problem. However, there is a race conditionvulnerability in this program: due to the time window between the check (access) and the use (fopen),there is a possibility that the file used by access() is different from the file used by fopen(), eventhough they have the same file name /tmp/XYZ. If a malicious attacker can somehow makes /tmp/XYZ asymbolic link pointing to a protected file, such as /etc/passwd, inside the time window, the attacker cancause the user input to be appended to /etc/passwd, and can thus gain the root privilege. The vulnerableprogram runs with the root privilege, so it can overwrite any file.

Set up the Set-UID program. We first compile the above code, and turn its binary into a Set-UIDprogram that is owned by the root. The following commands achieve this goal:

$ gcc vulp.c -o vulp$ sudo chown root vulp$ sudo chmod 4755 vulp

3 Task 1: Choosing Our Target

We would like to exploit the race condition vulnerability in the program. We choose to target the passwordfile /etc/passwd, which is not writable by normal users. By exploiting the vulnerability, we would liketo add a record to the password file, with a goal of creating a new user account that has the root privilege.Inside the password file, each user has an entry, which consists of seven fields separated by colons (:). Theentry for the root user is listed below.


For the root user, the third field (the user ID field) has a value zero. Namely, when the root user logsin, its process’s user ID is set to zero, giving the process the root privilege. Basically, the power of the rootaccount does not come from its name, but instead from the user ID field. If we want to create an accountwith the root privilege, we just need to put a zero in this field.

Each entry also contains a password field, which is the second field. In the example above, the field isset to "x", indicating that the password is stored in another file called /etc/shadow (the shadow file).If we follow this example, we have to use the race condition vulnerability to modify both password andshadow files, which is not very hard to do. However, there is a simpler solution. Instead of putting "x"in the password file, we can simply put the password there, so the operating system will not look for thepassword from the shadow file.

The password field does not hold the actual password; it holds the one-way hash value of the password.To get such a value for a given password, we can add a new user in our own system using the addusercommand, and then get the one-way hash value of our password from the shadow file. Or we can simply copythe value from the seed user’s entry, because we know its password is dees. Interestingly, there is a magicvalue used in Ubuntu live CD for a password-less account, and the magic value is U6aMy0wojraho (the6th character is zero, not letter O). If we put this value in the password field of a user entry, we only need tohit the return key when prompted for a password.

Task. To verify whether the magic password works or not, we manually (as a superuser) add the followingentry to the end of the /etc/passwd file. Please report whether you can log into the test account

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without typing a password, and check whether you have the root privilege.


After this task, please remove this entry from the password file. In the next task, we need to achieve thisgoal as a normal user. Clearly, we are not allowed to do that directly to the password file, but we can exploita race condition in a privileged program to achieve the same goal.

Warning. In the past, some students accidentally emptied the /etc/passwd file during the attack (thiscould be caused by some race condition problems inside the OS kernel). If you lose the password file, youwill not be able to log in again. To avoid this trouble, please make a copy of the original password file ortake a snapshot of the VM. This way, you can easily recover from the mishap.

4 Task 2: Launching the Race Condition Attack

The goal of this task is to exploit the race condition vulnerability in the vulnerable Set-UID programlisted earlier. The ultimate goal is to gain the root privilege. The most critical step of the attack, making/tmp/XYZ point to the password file, must occur within the window between check and use; namelybetween the access and fopen calls in the vulnerable program.

4.1 Task 2.A: Simulating a Slow Machine

Let us pretend that the machine is very slow, and there is a 10-second time window between the access()and fopen() calls. To simulate that, we add a sleep(10) between them. The program will look likethe following:

if (!access(fn, W_OK)) {sleep(10);fp = fopen(fn, "a+");…

With this addition, the vulp program (when re-compiled) will pause and yield control to the operatingsystem for 10 seconds. Your job is to manually do something, so when the program resumes after 10seconds, the program can help you add a root account to the system. Please demonstrate how you wouldachieve this.

You won’t be able to modify the file name /tmp/XYZ, because it is hardcoded in the program, butyou can use symbolic links to change the meaning of this name. For example, you can make /tmp/XYZ asymbolic link to the /dev/null file. When you write to /tmp/XYZ, the actual content will be written to/dev/null. See the following example (the "f" option means that if the link exists, remove the old onefirst):

$ ln -sf /dev/null /tmp/XYZ$ ls -ld /tmp/XYZlrwxrwxrwx 1 seed seed 9 Dec 25 22:20 /tmp/XYZ -> /dev/null

4.2 Task 2.B: The Real Attack

In the previous task, we have kind of “cheated” by asking the vulnerable program to slow down, so we canlaunch the attack. This is definitely not a real attack. In this task, we will launch the real attack. Before

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doing this task, make sure that the sleep() statement is removed from the vulp program.The typical strategy in race condition attacks is to run the attack program in parallel to the target program,

hoping to be able to do the critical step within that time window. Unfortunately, perfect timing is very hardto achieve, so the success of attack is only probabilistic. The probability of a successful attack might bequite low if the window is small, but we can run the attack many many times. We just need to hit the racecondition window once.

Writing the attack program. In the simulated attack, we use the "ln -s" command to make/changesymbolic links. Now we need to do it in a program. We can use symlink() in C to create symboliclinks. Since Linux does not allow one to create a link if the link already exists, we need to delete the oldlink first. The following C code snippet shows how to remove a link and then make /tmp/XYZ point to/etc/passwd. Please write your attack program.


Running the vulnerable program and monitoring results. Since we need to run the vulnerable programfor many times, we will write a program to automate this process. To avoid manually typing an input to thevulnerable program vulp, we can use input redirection. Namely, we save our input in a file, and ask vulpto get the input from this file using "vulp < inputFile". We can also use pipe (an example will begiven later).

It may take a while before our attack can successfully modify the password file, so we need a way toautomatically detect whether the attack is successful or not. There are many ways to do that; an easy way isto monitor the timestamp of the file. The following shell script runs the "ls -l" command, which outputsseveral piece of information about a file, including the last modified time. By comparing the outputs of thecommand with the ones produced previously, we can tell whether the file has been modified or not.

The following shell script runs the vulnerable program (vulp) in a loop, with the input given by theecho command (via a pipe). You need to decide what should be the actual input. If the attack is successful,i.e., the passwd is modified, the shell script will stop. You do need to be a little bit patient. Normally, youshould be able to succeed within 5 minutes.


CHECK_FILE="ls -l /etc/passwd"old=$($CHECK_FILE)new=$($CHECK_FILE)while [ "$old" == "$new" ] ÙCheck if /etc/passwd is modifieddo

echo "your input" | ./vulp ÙRun the vulnerable programnew=$($CHECK_FILE)

doneecho "STOP… The passwd file has been changed"

Verifying success When your script terminates, test the success of your exploit by logging in as the testuser and verifying root privileges. Then terminate the attack program by pressing Ctrl-C in the Terminalwindow in which you started the program.

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A Note. If after 10 minutes, your attack is still not successful, you can stop the attack, and check theownership of the /tmp/XYZ file. If the owner of this file becomes root, manually delete this file, and tryyour attack again, until your attack becomes successful. Please document this observation in your lab report.In Task 2.C, we will explain the reason and provide an improved attack method.

4.3 Task 2.C: An Improved Attack Method

In Task 2.B, if you have done everything correctly, but still could not succeed in the attack, check theownership of /tmp/XYZ. You will find out that /tmp/XYZ’s owner has become root (normally, it shouldbe seed). If this happens, your attack will never succeed, because your attack program, running with theseed privilege, can no longer remove or unlink() it. This is because the /tmp folder has a “sticky” biton, meaning that only the owner of the file can delete the file, even though the folder is world-writable.

In Task 2.B, we let you use the root’s privilege to delete /tmp/XYZ, and then try your attack again. Theundesirable condition happens randomly, so by repeating the attack (with the “help” from root), you willeventually succeed in Task 2.B. Obviously, getting help from root is not a real attack. We would like to getrid of that, and do it without root’s help.

The main reason for that undesirable situation is that our attack program has a problem, a race conditionproblem, the exact problem that we are trying to exploit in the victim program. How ironic! In the past,when we saw that problem, we simply advised students to delete the file and try the attack again. Thanksto one of my students, who was determined to figure out what the problem was. Because of his effort, wefinally understand why and have an improved solution.

The main reason for the situation to happen is that the attack program is context switched out right afterit removes /tmp/XYZ (i.e., unlink()), but before it links the name to another file (i.e., symlink().Remember, the action to remove the existing symbolic link and create a new one is not atomic (it involvestwo separate system calls), so if the context switch occurs in the middle (i.e., right after the removal of/tmp/XYZ), and the target Set-UID program gets a chance to run its fopen(fn, "a+") statement, itwill create a new file with root being the owner. After that, your attack program can no longer make changesto /tmp/XYZ.

Basically, using the unlink() and symlink() approach, we have a race condition in our attackprogram. Therefore, while we are trying to exploit the race condition in the target program, the targetprogram may accidentally “exploit” the race condition in our attack program, defeating our attack.

To solve this problem, we need to make unlink() and symlink() atomic. Fortunately, there is asystem call that allows us to achieve that. More accurately, it allows us to atomically swap two symboliclinks. The following program first makes two symbolic links /tmp/XYZ and /tmp/ABC, and then usingthe renameat2 system call to atomically switch them. This allows us to change what /tmp/XYZ pointsto without introducing any race condition.

#define _GNU_SOURCE

#include <stdio.h>#include <unistd.h>int main(){

unsigned int flags = RENAME_EXCHANGE;

unlink("/tmp/XYZ"); symlink("/dev/null", "/tmp/XYZ");unlink("/tmp/ABC"); symlink("/etc/passwd", "/tmp/ABC");

renameat2(0, "/tmp/XYZ", 0, "/tmp/ABC", flags);

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return 0;}

Tasks. Please revise your attack program using this new strategy, and try your attack again. If everythingis done correctly, your attack should be able to succeed.

5 Task 3: Countermeasures

5.1 Task 3.A: Applying the Principle of Least Privilege

The fundamental problem of the vulnerable program in this lab is the violation of the Principle of LeastPrivilege. The programmer does understand that the user who runs the program might be too powerful, sohe/she introduced access() to limit the user’s power. However, this is not the proper approach. A betterapproach is to apply the Principle of Least Privilege; namely, if users do not need certain privilege, theprivilege needs to be disabled.

We can use seteuid system call to temporarily disable the root privilege, and later enable it if neces-sary. Please use this approach to fix the vulnerability in the program, and then repeat your attack. Will yoube able to succeed? Please report your observations and provide explanation.

5.2 Task 3.B: Using Ubuntu’s Built-in Scheme

Ubuntu 10.10 and later come with a built-in protection scheme against race condition attacks. In this task,you need to turn the protection back on using the following commands:

// On Ubuntu 16.04 and 20.04, use the following command:$ sudo sysctl -w fs.protected_symlinks=1

// On Ubuntu 12.04, use the following command:$ sudo sysctl -w kernel.yama.protected_sticky_symlinks=1

Conduct your attack after the protection is turned on. Please describe your observations. Please alsoexplain the followings: (1) How does this protection scheme work? (2) What are the limitations of thisscheme?

6 Submission

You need to submit a detailed lab report, with screenshots, to describe what you have done and what youhave observed. You also need to provide explanation to the observations that are interesting or surprising.Please also list the important code snippets followed by explanation. Simply attaching code without anyexplanation will not receive credits.

  • Overview
  • Environment Setup
    • Turning Off Countermeasures
    • A Vulnerable Program
  • Task 1: Choosing Our Target
  • Task 2: Launching the Race Condition Attack
    • Task 2.A: Simulating a Slow Machine
    • Task 2.B: The Real Attack
    • Task 2.C: An Improved Attack Method
  • Task 3: Countermeasures
    • Task 3.A: Applying the Principle of Least Privilege
    • Task 3.B: Using Ubuntu's Built-in Scheme
  • Submission

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