Reviewing Code for Race Conditions

OWASP Code Review Guide Table of Contents

Introduction
Race Conditions occur when a piece of code does not work as it is supposed to (like many security issues). They are the result of an unexpected ordering of events, which can result in the finite state machine of the code to transition to a undefined state, and also give rise to contention of more than one thread of execution over the same resource. Multiple threads of execution acting or manipulating the same area in memory or persisted data which gives rise to integrity issues.

How They Work
With competing tasks manipulating the same resource, we can easily get a race condition as the resource is not in step-lock or utilises a token based multi-use system such as semaphores.

Say we have two processes (Thread 1, T1) and (Thread 2, T2). The code in question adds 10 to an integer X. The initial value of X is 5.

X = X + 10

With no controls surrounding this code in a multithreaded environment, we get the following problem:

T1 places X into a register in thread 1 T2 places X into a register in thread 2 T1 adds 10 to the value in T1's register resulting in 15 T2 adds 10 to the value in T2's register resulting in 15 T1 saves the register value (15) into X. T1 saves the register value (15) into X.

The value should actually be 25, as each thread added 10 to the initial value of 5. But the actual value is 15 due to T2 not letting T1 save into X before it takes a value of X for its addition.

.NET
Look for code which used multithreaded environments:

Thread System.Threading ThreadPool System.Threading.Interlocked

Java
java.lang.Thread start stop destroy init synchronized wait notify notifyAll

Classic ASP
Multithreading is not a directly supported feature of classic ASP, so this kind of race condition could be present only when using COM objects.

Vulnerable Patterns for Race Conditions
Static methods (One per class, not one per object) are an issue particularly if there is a shared state among multiple threads. For example, in Apache, struts static members should not be used to store information relating to a particular request. The same instance of a class can be used by multiple threads, and the value of the static member can not be guaranteed.

Instances of classes do not need to be thread safe as one is made per operation/request. Static states must be thread safe.
 * 1) References to static variables, these must be thread locked.
 * 2) Releasing a lock in places other then finally{} may cause issues
 * 3) Static methods that alter static state

Related Articles
http://msdn2.microsoft.com/en-us/library/f857xew0(vs.71).aspx