CWE-609: Double-Checked Locking

Description

The product uses double-checked locking to access a resource without the overhead of explicit synchronization, but the locking is insufficient.

Double-checked locking refers to the situation where a programmer checks to see if a resource has been initialized, grabs a lock, checks again to see if the resource has been initialized, and then performs the initialization if it has not occurred yet. This should not be done, as it is not guaranteed to work in all languages and on all architectures. In summary, other threads may not be operating inside the synchronous block and are not guaranteed to see the operations execute in the same order as they would appear inside the synchronous block.

Potential Impact

Integrity, Other

Modify Application Data, Alter Execution Logic

Demonstrative Examples

It may seem that the following bit of code achieves thread safety while avoiding unnecessary synchronization...

Bad

if (helper == null) {
                        synchronized (this) {if (helper == null) {helper = new Helper();}}
                     }return helper;

The programmer wants to guarantee that only one Helper() object is ever allocated, but does not want to pay the cost of synchronization every time this code is called.

Suppose that helper is not initialized. Then, thread A sees that helper==null and enters the synchronized block and begins to execute:

Bad

helper = new Helper();

If a second thread, thread B, takes over in the middle of this call and helper has not finished running the constructor, then thread B may make calls on helper while its fields hold incorrect values.

Mitigations & Prevention

Implementation

While double-checked locking can be achieved in some languages, it is inherently flawed in Java before 1.5, and cannot be achieved without compromising platform independence. Before Java 1.5, only use of the synchronized keyword is known to work. Beginning in Java 1.5, use of the "volatile" keyword allows double-checked locking to work successfully, although there is some debate as to whether it achieves sufficient performance gains. See references.

Detection Methods

Automated Static Analysis — Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then sea

Taxonomy Mappings

The CERT Oracle Secure Coding Standard for Java (2011): LCK10-J — Do not use incorrect forms of the double-checked locking idiom
Software Fault Patterns: SFP19 — Missing Lock

Frequently Asked Questions

What is CWE-609?

CWE-609 (Double-Checked Locking) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. The product uses double-checked locking to access a resource without the overhead of explicit synchronization, but the locking is insufficient.

How can CWE-609 be exploited?

Attackers can exploit CWE-609 (Double-Checked Locking) to modify application data, alter execution logic. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-609?

Key mitigations include: While double-checked locking can be achieved in some languages, it is inherently flawed in Java before 1.5, and cannot be achieved without compromising platform independence. Before Java 1.5, only use

What is the severity of CWE-609?

CWE-609 is classified as a Base-level weakness (Medium abstraction). Its actual severity depends on the specific context and how the weakness manifests in your application.