Race Condition Vulnerability

INTRODUCTION

A race condition vulnerability is a critical flaw in software systems that occurs when multiple processes execute concurrently, it often happens in a multi-threaded or multi-processing environment. In such scenarios, threads compete for access to shared resources, which leads to unpredictable and potentially harmful outcomes.

Race Condition Vulnerability.

This vulnerability is particularly prevalent in environments where the same resources are accessed simultaneously by different threads. In a computer system, processes are assigned internal memory to run applications, and when various threads execute concurrently, the stage is set for race conditions to emerge.

EXAMPLE

An illustrative example of a race condition vulnerability is the limit overrun scenario. Consider an online store where users can input a discount code during checkout for a one-time discount on their order. Typically, the application undergoes key steps: checking prior code usage, applying the discount, and updating the database. Under normal conditions, the system prevents code reuse by conducting thorough validation. Now, imagine an attacker attempting to apply a never-used code twice almost simultaneously. This scenario introduces a temporary sub-state, triggered when the server processes the first request and ends upon updating the database.

Two identical requests being executed concurrently.

Within this short race window, the race condition flaw emerges, allowing users to claim the discount multiple times. Despite the initial checks, the concurrency in code validation creates a vulnerability, enabling unintended outcomes due to the overlapping nature of concurrent processes.

DEMONSTRATION

Setting aside the hypothetical scenario, let’s practically attempt to exploit this flaw and examine how an attacker might leverage it to multiply the available discount capacity, enabling the purchase of items at a significantly reduced price.

Scenario

In our E-commerce account, we possess a store credit of $50.00, and we’ve received a promo code (PROMO20) to apply for 20% discount on our total purchase.

Inspection

Now, let's inspect how the web application responds to our requests. The requests are traced in burp suite.

Here, once we enter the promo code and press apply, a POST request is sent to the /cart/coupon endpoint, where the promo code is validated and 20% discount is applied to our total purchase. Now, the specific request is sent to the repeater tab for further inspection. In this phase, we practically attempt to exploit a potential limit overrun race condition vulnerability.

Firstly, we replicate the requests multiple times in the repeater tab, then group them together. The requests are duplicated to enable their simultaneous execution within the same race window.

Once the requests are grouped, we send them in single-packet attack mode.

As the endpoint received the requests, all the concurrent requests falling under the same race window frame were executed simultaneously, thereby deducting 20% multiple times from the total purchase amount. Now, where we should have received a maximum discount of $227, we were able to exploit the limit overrun race condition flaw, tricking the processing unit into applying a discount of $1299. This represents a significant and exploitable flaw in the system.

Ultimately, by exploiting the race condition flaw, we managed to acquire an item of significant value, despite not having a sufficient balance.

REMEDIES

1. Thread-Safe Programming: Design functions to be thread-safe, which allows parallel execution without interference.
2. Architectural Considerations: Ensure shared resources are accessed by one thread at a time, preventing multiple vulnerabilities across functions.

This concise exploration covered the practical impact and few remedies for mitigating race condition vulnerabilities in applications.