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Methods that can both modify a static field and be invoked from untrusted code to modify a static field must synchronize access to that the static field. This is necessary because there is no guarantee that untrusted clients will externally synchronize when accessing the field. Because a Even when client-side locking is a specified requirement of the method, untrusted clients can fail to synchronize (whether inadvertently or maliciously). Because the static field is shared by all clients, untrusted clients may violate the contract by failing to provide suitable locking.

Wiki MarkupAccording to Joshua Block \Bloch [[Bloch 08|AA. Java References#Bloch 08]\Bloch 2008]:

If a method modifies a static field, you must synchronize access to this field, even if the method is typically used only by a single thread. It is not possible for clients to perform external synchronization on such a method because there can be no guarantee that unrelated clients will do likewise.

Documented design intent is irrelevant when dealing with untrusted code because an attacker can always chose choose to ignore the documentation.

Noncompliant Code Example

This noncompliant code example does not fails to synchronize access to the static counter field counter. :

/** This class is not thread-safe! */
public final class CountHits {
  private static int counter;
  
  public void incrementCounter() {
    counter++;
  }
}

This class definition does not violate CON01complies with VNA02-J. Ensure that compound operations on shared variables are atomic, which applies only applies to classes that promise thread-safety. However, this class has a mutable static counter field counter that is modified by the publicly accessible incrementCounter() method. Consequently, this class cannot be used securely used by trusted client code if because untrusted code can purposely fail to externally synchronize access to the field.

Compliant Solution

This compliant solution uses a static private final lock to protect the counter field and consequently , does not depend on any lacks any dependence on external synchronization. This solution also complies with CON04 LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code.

/** This class is thread-safe */
public final class CountHits {
  private static int counter;
  private static final Object lock = new Object();

  public void incrementCounter() {
    synchronized (lock) {
      counter++;
    }
  }
}

Risk Assessment

Failing Failure to internally internally synchronize access to static fields that may can be modified by by untrusted code will result in incorrectly synchronized code if risks incorrect synchronization because the author of the untrusted code chooses to can inadvertently or maliciously ignore the synchronization policy.

Automated Detection

...

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[API 06|AA. Java References#API 06]\] 
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 67: "Avoid excessive synchronization"

Issue Tracking

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||Completed||Priority||Locked||CreatedDate||CompletedDate||Assignee||Name||

Tool	Version	Checker	Description
CodeSonar	Include Page CodeSonar_V CodeSonar_V	JAVA.CONCURRENCY.UG.METH	Unguarded Method (Java)
Parasoft Jtest	Include Page Parasoft_V Parasoft_V	CERT.LCK05.IASF	Inspect accesses to "static" fields which may require synchronization

Related Guidelines

Bibliography

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Image Added Image Added VOID CON06-J. Do not defer a thread that is holding a lock      11. Concurrency (CON)      Image Modified