Race Condition Vulnerability

Last Updated : 15 Apr, 2025

Race condition occurs when multiple threads read and write the same variable i.e. they have access to some shared data and they try to change it at the same time. In such a scenario threads are “racing” each other to access/change the data. This is a major security vulnerability.

What is Race Condition?

A race condition is a situation that may occur inside a critical section. This happens when the result of multiple thread execution in a critical section differs according to the order in which the threads execute. Race conditions in critical sections can be avoided if the critical section is treated as an atomic instruction. Also, proper thread synchronization using locks or atomic variables can prevent race conditions.

This is a major security vulnerability [CWE-362], and by manipulating the timing of actions anomalous results might appear. This vulnerability arises during a TOCTOU (time-of-check, time-of-use) window.

Flow of File Access during its TOCTOU Window So, what if we lock the file during this TOCTOU window itself?

General Misconception - A trivial cure to this vulnerability could be locking the file itself during this check-and-use window because then no other process can use the file during the time window. Seems easy, then why isn’t this practical? Why can’t we use this approach to solve the race condition problem? The answer is simply that such a vulnerability could not be prevented by just locking the file.
Problems while locking the file - A file is locked out for other processes only if it is already in the open state. This process is called the check-and-open process and during this time it is impossible to lock a file. Any locks created can be ignored by the attacking or the malicious process. What happens is that the call to Open() does not block an attack on a locked file. When the file is available for a check-and-open process, the file is open to any access/ change. So it’s impossible to lock a file at this point in time. This makes any kind of lock virtually non-existent to the malicious processes. Internally it is using the sleep_time which doubles at every attempt. More commonly this is referred to as a spinlock or the busy form of waiting. Also, there is always a possibility of the file getting locked indefinitely i.e. danger of getting stuck in a deadlock.
What would happen even if we were somehow able to lock the file? Let’s try to lock the file and see what could be the possible drawbacks. The most common locking mechanism that is available is atomic file locking. It is done using a lock file to create a unique file on the same filesystem. We make use of link() to make a link to the lock file for any kind of access to the file.
- If link() returns 0, the lock is successful.
The most common fix available is to store the PID of the application in the lock file, which is checked against the active PID at that time. Then again a flaw with this fix is that PID may have been reused.
Actual Solutions - A better solution is to rather than creating locks on the file as a whole, lock the parts of the file to different processes. Example - When a process wants to write into a file, it first asks the kernel to lock that file or a part of it. As long as the process keeps the lock, no other process can ask to lock the same part of the file. Hence you could see that the issue with concurrency is getting resolved like this. In the same way, a process asks for locking before reading the content of a file, which ensures no changes will be made as long as the lock is kept. Differentiating this different kind of locks is done by the system itself. The system has the capability to distinguish between the locks required for file reading and those required for file writing. This kind of locking system is achieved by the flock() system call. Flock() call can have different values :
- LOCK_SH (lock for reading)
- LOCK_EX (for writing)
- LOCK_UN (release of the lock)
Using these separate calls we can tell what kind of locks are necessary. A point to note here is that many processes can be benefited from a reading lock simultaneously since no one will attempt to change the file content. However, only one process can benefit from a lock for writing at a given time which is currently using it. Thus no other lock can be allowed at the same time, even for reading. This kind of carefully crafted system works well with applications that can ask the kernel to reserve their access (their lock) before reading or writing to an important system file. Hence this way of selectively locking the file is much practical than our initial approach. So, while you are trying to implement your file system for directories you could take advantage of this secure coding technique to prevent a potential CWE-362 (Race Condition Vulnerability).

Real-Time Examples of Race Conditions

Example 1 - Consider an ATM Withdrawal

Imagine Ram and his friend Sham both have access to the same bank account. They both try to withdraw Rs,500 at the same time from different ATMs. The system checks the balance and sees there’s enough money for both withdrawals. Without proper synchronization, the system might allow both transactions to go through, even if the balance is only enough for one, leaving the account overdrawn.

Example 2 - Consider a Printer Queue

Imagine two people sending print jobs at the same time. If the printer isn’t managed properly, the print jobs could get mixed up, with pages from one person’s document being printed in the middle of another’s.

Key Terms in a Race Condition

Critical Section: A code part where the shared resources are accessed. It is critical as multiple processes enter this section at same time leading to data corruption and errors.
Synchronization: It is the process of controlling how and when multiple processes or threads access the shared resources ensuring that only one can enter the critical section at same time.
Mutual Exclusion (Mutex): A mutex is like a lock that ensures only one process can access a resource at a time. If a process holds a lock, others must wait their turn preventing race conditions.
Deadlock: A situation where two or more processes are stuck waiting for each other's resources, causing a deadlock(standstill).

What is a Race Condition Vulnerability?

Race Condition Vulnerability is a situation where two or more processes or threads in a system simultaneously access the shared resources. If the processes lacks coordination, it leads to unexpected behaviour or security issues.

Let's understand this concept by taking a real-life scenario . Imagine there are 2 people named A and B , trying to write on the same notebook at same time without agreeing on who needs to write first followed by other person which results into writings getting mixed up leading to confusion.

Now let's understand the same concept in computer system. Imagine a system where one process's duty is to read a file from the system and another process's duty is to write to the system. If both of them try to access the same file at same time without a proper control leads to incorrect/incomplete data leading to errors.

Common Vulnerabilities Leading to Race Conditions

In systems where multiple processes access the shared memory, failure to control how memory is accessed can lead to conflicting operations, resulting in incorrect data being read or written to the system.
If multiple processes access the same file at same time without proper access mechanism, the file's content can become inconsistent.
In some scenarios, a certain operations need to be happened in a specific order. If the sequence is not followed and multiple processes run out of order, race conditions can occur leading to error or vulnerabilities.

By identifying these common vulnerabilities, developers can reduce the chance of race conditions that affect the system by proper locking mechanism, careful sequencing and strong synchronization practices which can help to almost reduce/eliminate these issues.

Race Condition, Deadlock and Threat Block

1. Race Condition

A race condition happens when two or more processes try to access the same resource at the same time without proper coordination. This “race” can lead to incorrect results or unpredictable behavior because the order of execution is not controlled.

Example: Two people trying to edit the same document at the same time, causing one’s changes to overwrite the other’s.

2. Deadlock

A deadlock occurs when two or more processes are waiting for each other to release resources, and none can proceed. It’s like a standstill where each process is holding a resource the other needs, leading to a complete freeze.

Example: Two cars stuck in a narrow road from opposite directions, each refusing to move back and waiting for the other.

3. Thread Block

Thread blocking happens when a thread is unable to continue its work because it’s waiting for a resource that’s currently unavailable. It pauses until the resource is free.

Example: A cashier waiting for the customer ahead to finish paying before attending to the next person in line.

How to Detect Race Conditions?

Review the Code : Carefully inspecting of the code can help to identify the areas where shared resources are accessed without proper locking or synchronization.
Static Analysis Tools : Specialized tools analyze the code to automatically detect potential race conditions by identifying unsafe access to shared resources.
Testing with Multiple Threads/Processes: Simulate scenarios with many threads or processes running simultaneously. If unexpected behaviors like data inconsistencies or crashes occur, a race condition might be present.
Logging and Monitoring: Adding logs to track resource access can reveal out-of-order operations, signaling a race condition

How to Prevent Race Condition Attacks?

Use Locks: Implement locks (like mutexes) to ensure that only one process or thread can access a resource at a time, preventing conflicting operations.
Proper Synchronization: Ensure processes or threads work in a coordinated sequence when accessing shared data. Techniques like semaphores help achieve this.
Avoid Time-of-Check to Time-of-Use (TOCTOU) Vulnerabilities: Reduce the gap between checking a condition (like permissions) and acting on it, minimizing opportunities for an attacker to change the state in between.
Priority Management: Prioritize certain processes or threads so they get controlled access to critical resources, preventing uncoordinated access.

Conclusion

In conclusion, race conditions, deadlocks, and thread blocks are all issues that arise when multiple processes or threads compete for shared resources. Without proper management, these can lead to data errors, system freezes, or performance slowdowns. By understanding these concepts and applying techniques like synchronization, locking, and careful resource management, we can design systems that avoid these pitfalls and run smoothly.

Critical Section in Synchronization

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Article Tags :

Race Condition Vulnerability

What is Race Condition?

Real-Time Examples of Race Conditions

Example 1 - Consider an ATM Withdrawal

Example 2 - Consider a Printer Queue

Key Terms in a Race Condition

What is a Race Condition Vulnerability?

Common Vulnerabilities Leading to Race Conditions

Race Condition, Deadlock and Threat Block

1. Race Condition

2. Deadlock

3. Thread Block

How to Detect Race Conditions?

How to Prevent Race Condition Attacks?

Conclusion

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