CWE-543: Use of Singleton Pattern Without Synchronization in a Multithreaded Context
Use of Singleton Pattern Without Synchronization in a Multithreaded Context
Weakness ID: 543 (Weakness Variant)
Status: Incomplete
Description
Description Summary
The software uses the singleton pattern when creating a resource within a multithreaded environment.
Extended Description
The use of a singleton pattern may not be thread-safe.
Time of Introduction
Implementation
Applicable Platforms
Languages
Java
C++
Common Consequences
Scope
Effect
Other
Integrity
Technical Impact: Other; Modify application
data
Demonstrative Examples
Example 1
This method is part of a singleton pattern, yet the following
singleton() pattern is not thread-safe. It is possible that the method will
create two objects instead of only one.
(Bad Code)
Example
Language: Java
private static NumberConverter singleton;
public static NumberConverter get_singleton() {
if (singleton == null) {
singleton = new NumberConverter();
}
return singleton;
}
Consider the following course of events:
Thread A enters the method, finds singleton to be null, begins the
NumberConverter constructor, and then is swapped out of
execution.
Thread B enters the method and finds that singleton remains null.
This will happen if A was swapped out during the middle of the
constructor, because the object reference is not set to point at the
new object on the heap until the object is fully initialized.
Thread B continues and constructs another NumberConverter object
and returns it while exiting the method.
Thread A continues, finishes constructing its NumberConverter
object, and returns its version.
At this point, the threads have created and returned two different
objects.
Potential Mitigations
Phase: Architecture and Design
Use the Thread-Specific Storage Pattern. See References.
Phase: Implementation
Do not use member fields to store information in the Servlet. In
multithreading environments, storing user data in Servlet member fields
introduces a data access race condition.
Phase: Implementation
Avoid using the double-checked locking pattern in language versions that cannot guarantee thread safety. This pattern may be used to avoid the overhead of a synchronized call, but in certain versions of Java (for example), this has been shown to be unsafe because it still introduces a race condition (CWE-209).
Description
