The program 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 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. 1000 Weakness ChildOf 667 699 Category ChildOf 361 1000 Weakness CanPrecede 367 844 Category ChildOf 853 888 Category ChildOf 894 Implementation Integrity Other Modify application data Alter execution logic 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. It may seem that the following bit of code achieves thread safety while avoiding unnecessary synchronization... 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: 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. David Bacon et al The "Double-Checked Locking is Broken" Declaration http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html Jeremy Manson and Brian Goetz JSR 133 (Java Memory Model) FAQ http://www.cs.umd.edu/~pugh/java/memoryModel/jsr-133-faq.html#dcl Mark Dowd John McDonald Justin Schuh The Art of Software Security Assessment Chapter 13, "Threading Vulnerabilities", Page 815. 1st Edition Addison Wesley 2006 Do not use incorrect forms of the double-checked locking idiom LCK10-J Anonymous Tool Vendor (under NDA) Eric Dalci Cigital 2008-07-01 updated Context_Notes CWE Content Team MITRE 2008-09-08 updated Description, Relationships, Taxonomy_Mappings CWE Content Team MITRE 2008-11-24 updated Demonstrative_Examples CWE Content Team MITRE 2009-01-12 updated Relationships CWE Content Team MITRE 2009-05-27 updated Relationships CWE Content Team MITRE 2009-10-29 updated Taxonomy_Mappings CWE Content Team MITRE 2010-12-13 updated Relationships CWE Content Team MITRE 2011-06-01 updated Common_Consequences, Relationships, Taxonomy_Mappings CWE Content Team MITRE 2012-05-11 updated Demonstrative_Examples, References, Relationships CWE Content Team MITRE 2012-10-30 updated Potential_Mitigations Double Checked Locking