Testing for Race Conditions (OWASP-AT-010)

Brief Summary
A race condition is a flaw that produces an unexpected result when the timing of actions impact other actions. An example may be seen on a multithreaded application where actions are being performed on the same data. Race conditions, by their very nature, are difficult to test for.

Description of the Issue
Race conditions may occur when a process is critically or unexpectedly dependent on the sequence or timings of other events. In a web application environment, where multiple requests can be processed at a given time, developers may leave concurrency to be handled by the framework, server or programming language. The following simplified example illustrates a potential concurrency problem in a transactional web application and relates to a joint savings account in which both users (threads) are logged into the same account and attempting a transfer. Account A has 100 credits. Account B has 100 credits. Both User 1 and User 2 want to transfer 10 credits from Account A to Account B. If the transaction was correct the outcome should be: Account A has 80 credits. Account B has 120 credits. However, due to concurrency issues, the following result could be obtained: User 1 checks the value of Account A (=100 credits) User 2 checks the value of Account A (=100 credits) User 2 takes 10 credits from Account A (=90 credits) and put it in Account B (=110 credits) User 1 takes 10 credits from Account A (Still believed to contain 100 credits) (=90 credits) and puts it into Account B (=120 credits). Result: Account A has 90 credits. Account B has 120 credits.

Another example can be seen in OWASP's WebGoat project under the Thread Safety lesson and shows how a shopping cart can be manipulated to purchase items for less than their advertised price. This, as with the example above, is due to the data changing between the time of check and its time of use.

Black Box testing and example
Testing for race conditions is problematic due to their nature, and external influences on testing including server load, network latency etc will all play a part in the presence and detection of the condition. However, testing can be focused at specific transactional areas of the application, where time of read to time of use of specific data variables could be adversely affected by concurrency issues. Black Box testing attempts to force a race condition may include the ability to make multiple simultaneous requests while observing the outcome for unexpected behavior. Examples of such areas are illustrated in the paper "On Race Vulnerabilities in Web Applications", cited in the further reading section. The authors suggest that it may be possible in certain circumstances to:

Testers should be aware of the security implications of race conditions and their factors surrounding their difficulty of testing.
 * Create multiple user accounts with the same username.
 * Bypass account lockouts against brute forcing.

Gray Box testing and example
Code review may reveal likely areas of concern for concurrency issues. More information on reviewing code for concurrency issues can be seen at OWASP Code Review Guide's Reviewing Code for Race Conditions