CWE

Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

CWE/SANS Top 25 Most Dangerous Software Errors
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ID

CWE-609: Double-Checked Locking

Weakness ID: 609
Abstraction: Base
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
The program uses double-checked locking to access a resource without the overhead of explicit synchronization, but the locking is insufficient.
+ Extended Description
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 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.
+ Relationships

The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

+ Relevant to the view "Research Concepts" (CWE-1000)
NatureTypeIDName
ChildOfBaseBase667Improper Locking
CanPrecedeBaseBase367Time-of-check Time-of-use (TOCTOU) Race Condition
+ Relevant to the view "Development Concepts" (CWE-699)
NatureTypeIDName
MemberOfCategoryCategory3617PK - Time and State
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the software life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

PhaseNote
Implementation
+ Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

Java: (Undetermined Prevalence)

+ Common Consequences

The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

ScopeImpactLikelihood
Integrity
Other

Technical Impact: Modify Application Data; Alter Execution Logic

+ Demonstrative Examples

Example 1

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

(bad)
Example Language: Java 
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.

+ Potential Mitigations

Phase: 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.
+ Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CERT Java Secure CodingLCK10-JDo not use incorrect forms of the double-checked locking idiom
Software Fault PatternsSFP19Missing Lock
+ References
[REF-490] David Bacon et al. "The "Double-Checked Locking is Broken" Declaration". <http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html>.
[REF-491] Jeremy Manson and Brian Goetz. "JSR 133 (Java Memory Model) FAQ". <http://www.cs.umd.edu/~pugh/java/memoryModel/jsr-133-faq.html#dcl>.
[REF-62] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 13, "Threading Vulnerabilities", Page 815.. 1st Edition. Addison Wesley. 2006.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
Anonymous Tool Vendor (under NDA)
Modifications
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigital
updated Context_Notes
2008-09-08CWE Content TeamMITRE
updated Description, Relationships, Taxonomy_Mappings
2008-11-24CWE Content TeamMITRE
updated Demonstrative_Examples
2009-01-12CWE Content TeamMITRE
updated Relationships
2009-05-27CWE Content TeamMITRE
updated Relationships
2009-10-29CWE Content TeamMITRE
updated Taxonomy_Mappings
2010-12-13CWE Content TeamMITRE
updated Relationships
2011-06-01CWE Content TeamMITRE
updated Common_Consequences, Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITRE
updated Demonstrative_Examples, References, Relationships
2012-10-30CWE Content TeamMITRE
updated Potential_Mitigations
2014-07-30CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Double Checked Locking

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Page Last Updated: November 14, 2017