Page History

Wiki MarkupClient code can trivially access {{ public }} {{static}} non-final fields. Neither reads nor writes are checked by a security manager; further, new values cannot be validated programmatically before they are stored in the field. Classes loaded by the same class loaders can access each others' {{public static}} members. For example, consider Java applets \[[Sun 2008|AA. Bibliography#Sun 08]\]:

Normally, if two applets have the same codebase and archive parameters, they will be loaded by the same class loader instance. This behavior is required for backward compatibility and is relied on by several real-world applications. The result is that multiple applets on the same web page may access each others' static variables at the Java language level, effectively allowing the multiple applets to be written as though they comprised a single application.

However, applets loaded by different class loader instances are completely isolated and cannot access each others' public static fields. Further, code from any class can access public members of any class that was loaded by any class loader in the delegation chain of the current class's class loader. In the diagram below, for example, code in classes C4 and C5 can freely access public members of class C2, whereas neither class C2 nor class C4 can access public members of class C5.

Image Removedstatic fields because access to such fields are not checked by a security manager. Furthermore, new values cannot be validated programmatically before they are stored in these fields.

In the presence of multiple threads, non-final nonfinal public static fields can be modified in inconsistent ways . (For example, see guideline TSM01-J. Do not let the ( this ) reference escape during object constructionfor an example).)

Wiki MarkupImproper use of {{ public static }} fields can also result in type -safety issues. For example, untrusted code can supply an unexpected subtype with malicious methods when the variable is defined to be of a more general type, such as {{java.lang.Object}} \[ [Gong 2003|AA. Bibliography#Gong 03]\]]. As a result, classes must not contain nonfinal public static fields.

Noncompliant Code Example

...

This noncompliant code example is adopted taken from JDK v1.4.2 \[ [FT 2008|AA. Bibliography#FT 08]\]. It declares a function table containing a {{ public static }} field.

Code Block
bgColor #FFCCCC
package org.apache.xpath.compiler; public class FunctionTable { public static FuncLoader m_functions; }

An attacker can replace the function table as follows:

Code Block
FunctionTable.m_functions = <newnew_table>table;

Replacing the function table gives the attacker access to XPathContext, which is used to set the reference node for evaluating XPath expressions. Manipulating it XPathContext can allow cause XML fields to be modified in inconsistent ways, resulting in unexpected behavior. Also, because static variables are global across the Java Runtime Environment (JRE), they can be used as a covert communication channel between different application domains (for example, through code loaded by different class loaders).

Note that this This vulnerability was repaired in JDK v1.4.2_05.

Compliant Solution

This compliant solution declares the FuncLoader static field final and treats it like as a constant.:

public static final FuncLoader m_functions;
// Initialize m_functions in a constructorstatic initialization block

Fields declared static and final are also safe for multithreaded use ; see guideline (see TSM03-J. Do not publish partially initialized objectsfor more information). However, remember that simply changing the modifier to final may might not prevent attackers from indirectly retrieving an incorrect value from the static final variable before its initialization ; (see guideline DCL12 DCL00-J. Prevent class initialization cycles for more information). FurtherFurthermore, individual members of the referenced object can also be changed if the object itself is mutable; see guideline OBJ01-J. Do not assume that declaring a reference to be final causes the referenced object to be immutable.

It is also permissible to use a wrapper method to retrieve the value of m_functions. This has encapsulation advantages as it restricts its accessibility of , allowing m_functions to be declared private ; (see guideline OBJ00rule OBJ01-J. Declare data members as private and provide accessible wrapper methodsLimit accessibility of fields for more information).

Noncompliant Code Example (serialVersionUID)

This noncompliant code example uses a public static non-final nonfinal serialVersionUID field in a class designed for serialization. :

class DataSerializer implements Serializable {
  public static long serialVersionUID = 1973473122623778747L;
  // ...
}

Compliant Solution

This compliant solution declares the serialVersionUID field final and limits its accessibility to private.:

class DataSerializer implements Serializable {
  private static final long serialVersionUID = 1973473122623778747L;
}

The serialization mechanism uses the serialVersionUID field internally, so accessible wrapper methods are unnecessary.

Risk Assessment

Unauthorized modifications of public static variables can result in unexpected behavior and violation of class invariants. FurtherFurthermore, because static variables can be visible to code loaded by different class loaders when those class loaders are in the same delegation chain, such variables can be used as a covert communication channel between different application domains in some cases.

Automated Detection

Related Guidelines

...

...

...

...

...

...

...

...

...

Bibliography

...

Image Added Image Added Image Added

Field isn't final but should be refactored to be so

\[[SCG 2007|AA. Bibliography#SCG 07]\] Guideline 3.1, Treat public static fields as constants

Bibliography

\[[FT 2008|AA. Bibliography#FT 08]\] 
\[[Gong 2003|AA. Bibliography#Gong 03]\] 9.3 Static Fields
\[[Nisewanger 2007|AA. Bibliography#Nisewanger 07]\] Antipattern 5, Misusing Public Static Variables
\[[Sterbenz 2006|AA. Bibliography#Sterbenz 06]\] Antipattern 5, Misusing Public Static Variables

OBJ02-J. Do not ignore return values of methods that operate on immutable objects      08. Object Orientation (OBJ)      OBJ04-J. Do not allow access to partially initialized objects