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The synchronized keyword is used to acquire a mutual-exclusion lock so that no other thread can acquire the lock, while it is being held by the executing thread. Recall that there There are two ways to synchronize access to shared mutable variables, : method synchronization and block synchronization. The synchronized keyword always uses the object's intrinsic 'monitor' lock. This lock is available to any code that the object itself is available to; consequently any code can lock on the object, and potentially cause Methods declared as synchronized and blocks that synchronize on the this reference both use the object as a monitor (that is, its intrinsic lock). An attacker can manipulate the system to trigger contention and deadlock by obtaining and indefinitely holding the intrinsic lock of an accessible class, consequently causing a denial of service (DoS). Excessive synchronization can induce a DoS vulnerability because another class whose member locks on the object, can fail to release the lock promptly. However, this requires the victim class to be accessible from the hostile class.

The _private lock object_ idiom can be used to prevent this vulnerability. The idiom consists of a {{private}} object declared as an instance field. The {{private}} object must be explicitly used for locking purposes in {{synchronized}} blocks, within the class's methods. This lock object belongs to an instance of the object and is not associated with the class itself. Consequently, there is no lock contention between a class method and a method of a hostile class when both try to lock on the object. \[[Bloch 01|AA. Java References#Bloch 01]\]  

One technique for preventing this vulnerability is the private lock object idiom [Bloch 2001]. This idiom uses the intrinsic lock associated with the instance of a private final java.lang.Object declared within the class instead of the intrinsic lock of the object itself. This idiom requires the use of synchronized blocks within the class's methods rather than the use of synchronized methods. Lock contention between the class's methods and those of a hostile class becomes impossible because the hostile class cannot access the private final lock object.

Static methods and state also share this vulnerability. When a static method is declared synchronized, it acquires the intrinsic lock of the class object before any statements in its body are executed, and it releases the intrinsic lock when the method completes. Untrusted code that has access to an object of the class, or of a subclass, can use the getClass() method to gain access to the class object and consequently manipulate the class object's intrinsic lock. Protect static data by locking on a private static final Object. Reducing the accessibility of the class to package-private provides further protection against untrusted callers.

The private lock object idiom is also suitable for classes that are designed for inheritance. When a superclass This idiom can also be suitably used by classes designed for inheritance. If a superclass thread requests a lock on the object's monitor, a subclass thread can interfere with its operation.

An object should use a private internal final lock object rather than its own intrinsic lock unless the class can guarantee that untrusted code may not:

. For example, a subclass may use the superclass object's intrinsic lock for performing unrelated operations, causing lock contention and deadlock. Separating the locking strategy of the superclass from that of the subclass ensures that they do not share a common lock and also permits fine-grained locking by supporting the use of multiple lock objects for unrelated operations. This increases the overall responsiveness of the application.

Objects that require synchronization must use the private lock object idiom rather than their own intrinsic lock in any case where untrusted code could:

• Subclass the class.Subclass the class (However, trusted code may subclass the class.)
• Create an object of the class ( or of a subclass).
• Access or acquire an object instance of the class ( or of a subclass)
  Furthermore, the class must also ensure that no superclasses that it inherits from use any synchronization..

Subclasses whose superclasses use the private lock object idiom must themselves use the idiom. However, when a class uses intrinsic synchronization on the class object without documenting its locking policy, subclasses must not use intrinsic synchronization on their own class object. When the superclass documents its policy by stating that client-side locking is supported, the subclasses have the option to choose between intrinsic locking and using the private lock object idiom. Subclasses must document their locking policy regardless of which locking option is chosen. See rule TSM00-J. Do not override thread-safe methods with methods that are not thread-safe for related information.

When any of these restrictions are violatedIf these restrictions are not met, the object's intrinsic lock is not trustworthy. If all conditions are satisfied, then the object gains no significant security from using a private internal lock object, and may synchronize using its own intrinsic lock.

Likewise, if a static method has the synchronized keyword, the intrinsic lock of the Class object is obtained, and released when the method completes. The same restrictions listed above apply to static methods because any untrusted code that can access an object of the class, or a subclass, can use the getClass() method to obtain access to the Class object. Furthermore, hostile code must not be able to access the Class object. This may be accomplished, for instance, by making the class package-private.

...

cannot be trusted. But when these restrictions are obeyed, the private lock object idiom fails to add any additional security. Consequently, objects that comply with all of the restrictions are permitted to synchronize using their own intrinsic lock. However, block synchronization using the private lock object idiom is superior to method synchronization for methods that contain nonatomic operations that could either use a more fine-grained locking scheme involving multiple private final lock objects or that lack a requirement for synchronization. Nonatomic operations can be decoupled from those that require synchronization and can be executed outside the synchronized block. Both for this reason and for simplification of maintenance, block synchronization using the private lock object idiom is generally preferred over intrinsic synchronization.

Noncompliant Code Example (Method Synchronization)

This noncompliant code example exposes instances of the object someObject SomeObject class to untrusted code.

public class SomeObject {

  // Locks on the object's monitor
  public synchronized void changeValue() { 
    // Locks on the object's monitor ...
  }
 
  public static SomeObject lookup(String name) {
    // ...   
  }
}

// Untrusted code
String name = // ...
SomeObject someObject = SomeObject.lookup(name);
if (someObject == null) {
  // ... handle error
}
synchronized (someObject) {
  while (true) {
    // Indefinitely lock someObject
    Thread.sleep(Integer.MAX_VALUE); // Indefinitely delay someObject
  }
}

The untrusted code attempts to acquire a lock on the object's monitor and, upon succeeding, introduces an indefinite delay which holds up that prevents the synchronized changeValue() method from acquiring the same lock. Furthermore, the object locked is publicly available via the lookup() method.

Alternatively, an attacker could create a private SomeObject object and make it available to trusted code to use it before the attacker code grabs and holds the lock.

Note that in the untrusted code also violates CON20, the attacker intentionally violates rule LCK09-J. Do not perform operations that may can block while holding a lock.

Noncompliant Code Example (

...

Public Non-final

...

Lock Object)

This noncompliant code example locks on a public non-final nonfinal object in an attempt to use a lock different from SomeObjectother than {{SomeObject}}'s own intrinsic lock.

public class SomeObject {
  public Object lock = new Object();

  public void changeValue() {
    synchronized (lock) {
      // ...
    }
  }
}

However, it is possible for untrusted code to change This change fails to protect against malicious code. For example, untrusted or malicious code could disrupt proper synchronization by changing the value of the lock object and foil all attempts to synchronize.

Noncompliant Code Example (

...

Publicly Accessible Non-

...

final

...

Lock Object)

This noncompliant code example synchronizes on a private publicly accessible but nonfinal field. The lock field is declared volatile so that changes are visible to other threads.

public class SomeObject {
  private volatile Object lock = new Object();

  public void changeValue() {
    synchronized (lock) {
      // ...
    }
  }

  public void setLock(IntegerObject lockvaluelockValue) {
    lock = lockValue;
  }
}

Any thread can modify the field's value to refer to some other a different object in the presence of an accessor such as setLock(). This That modification might cause two threads that intend to lock on the same object to lock on different objects, enabling thereby permitting them to execute the two critical sections in an unsafe manner. For example, if the lock were changed when one thread was in its critical section, a second thread would lock on the new object instead of the old one and would enter its critical section erroneously.

A class that lacks accessible methods to change the lock is secure against untrusted manipulation. However, it remains susceptible to inadvertent modification by the programmer.

Noncompliant Code Example (Public Final Lock Object)

This noncompliant code example uses a public final lock object.

public class SomeObject {
  public final Object lock = new Object();

  public void changeValue() {
    synchronized (lock) {
      // ...
    }
  }
}

This noncompliant code example also violates rule OBJ01-J. Limit accessibility of fields.

Compliant Solution (

...

Private Final Lock Object)

Thread-safe public classes that use the intrinsic synchronization of their respective objects may be protected by using the private lock object idiom and adapting them may interact with untrusted code must use a private final lock object. Existing classes that use intrinsic synchronization must be refactored to use block synchronization on such an object. In this compliant solution, if the method calling changeValue() is called, the lock is obtained obtains a lock on a private final Object instance that is inaccessible from the callerto callers that are outside the class's scope.

public class SomeObject {
  private final Object lock = new Object(); // private final lock object

  public void changeValue() {
    synchronized (lock) { // Locks on the private Object
      // ...
    }
  }
}

Using a A private final lock may only be achieved object can be used only with block synchronization. Block synchronization is sometimes preferred over method synchronization , because operations that do not require without a requirement for synchronization can be moved outside the synchronized region which reduces the overall execution time, reducing lock contention and blocking. Note that it is unnecessary to declare the lock field volatile because of the strong visibility semantics of final fields. When granularity issues require the use of multiple locks, declare and use multiple private final lock objects to satisfy the granularity requirements rather than using a mutable reference to a lock object along with a setter method.

Noncompliant Code Example (

...

Static)

This noncompliant code example exposes the class object of someObject SomeObject to untrusted code.

public class SomeObject {
  public static synchronized void ChangeValue() { //changeValue Lockslocks on the class object's monitor
  public static synchronized void changeValue() { 
    // ...   
  }
}

// Untrusted code
synchronized (someObjectSomeObject.getClass(class)) {
  while (true) {
    Thread.sleep(Integer.MAX_VALUE); // Indefinitely delay someObject
  }
}

The untrusted code attempts to acquire a lock on the class object''s monitor and, upon succeeding, introduces an indefinite delay which holds up that prevents the synchronized changeValue() method from acquiring the same lock. Note that

A compliant solution must also comply with rule LCK05-J. Synchronize access to static fields that can be modified by untrusted code.
In the untrusted code also violates CON20, the attacker intentionally violates rule LCK09-J. Do not perform operations that may can block while holding a lock.

Compliant Solution (

...

Static)

Thread-safe public classes that both use intrinsic synchronization of over the class object and may be protected by using interact with untrusted code must be refactored to use a static private final lock object and block synchronization.

public class SomeObject {
  private static final Object lock = new Object(); // private lock object

  public static void ChangeValuechangeValue() {
    synchronized (lock) { // Locks on the private Object
      // ...
    }
  }
}

In this compliant solution, if the method ChangeValuechangeValue() is called, the lock is obtained obtains a lock on a private static private Object that is inaccessible from to the caller.

Using a private lock may only be achieved with block synchronization, as static method synchronization always uses the intrinsic lock of the object's class. However, block synchronization is also preferred over method synchronization, because it is easy to move operations out of the synchronized block when they might take a long time and they are not truly a critical section.

Exceptions

Exceptions

LCK00-J-EX0EX1: A class may violate this guideline, if rule when all of the following conditions are met:

• it It sufficiently documents that callers must not pass objects of this class to untrusted code,.
• The class cannot invoke methods, directly or indirectly, on objects of trusted callers do not use any untrusted classes that violate this guideline this rule.
• The synchronization policy of the class is documented properly.

Clients are permitted to use a class that violates this rule when all of the following conditions are met:

• Neither the client class nor any other class in the system passes objects of the violating class to untrusted code.
• The violating class cannot invoke methods, directly or indirectly, from untrusted classes that violate this rule.the synchronization policy of the class is properly documented

LCK00-J-EX1: When a superclass of the class documents that it supports client-side locking and synchronizes on its class object, the class can support client-side locking in the same way and document this policy.

LCK00-J-EX2: Package-private classes that are never exposed to untrusted code are exempt from this rule because their accessibility protects against untrusted callers. However, use of this exemption should be documented explicitly to ensure that trusted code within the same package neither reuses the lock object nor changes the lock object inadvertently.

Risk Assessment

Exposing the class lock object to untrusted code can result in denial-of-serviceDoS.

Related Vulnerabilities

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

References

\[[Bloch 01|AA. Java References#Bloch 01]\] Item 52: "Document Thread Safety"

Automated Detection

Related Guidelines

Bibliography

Image Added      Image Added      Image AddedCON03-J. Do not use background threads during class initialization      11. Concurrency (CON)      CON05-J. Ensure that threads are started properly