Client code can trivially access public static 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, nonfinal public static fields can be modified in inconsistent ways (see TSM01-J. Do not let the this reference escape during object construction for an example).

Improper 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]. As a result, classes must not contain nonfinal public static fields.

Noncompliant Code Example

This noncompliant code example is taken from JDK v1.4.2 [FT 2008]. It declares a function table containing a public static

Callers can trivially access and modify {{public}} {{static}} non-final fields. Neither accesses nor any modifications are automatically checked by a security manager, and newly set values cannot be validated. Classes loaded by the same class loader can access each others' {{public static}} members, unless appropriate protection is installed. For example, consider Java applets \[[Sun 08|AA. Java References#Sun 08]\]:

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.

In the presence of multiple threads non-final {{public static}} fields can be modified in inconsistent ways. Improper use of {{public static}} fields can also result in type safety issues. For example, untrusted code may supply an unexpected subtype when the variable is defined to be of a more general type such as {{java.lang.Object}}. \[[Gong 03|AA. Java References#Gong 03]\]

Noncompliant Code Example

Wiki MarkupThis noncompliant code example is adopted from JDK v1.4.2 \[[FT 08|AA. Java References#FT 08]\]. It declares a function table containing a {{public static}} field.

package org.apache.xpath.compiler;

public class FunctionTable {
  public static FuncLoader m_functions;
}

An attacker can replace the function table as follows.:

FunctionTable.m_functions = <newnew_table>table;

Replacing the function table gives the attacker access to the XPathContext. The 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 (e.g.for example, through code loaded by different class loaders).

This vulnerability was repaired in JDK v1.4.2_05.

Compliant Solution

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

public static final FuncLoader m_functions;

// Initialize m_functions in a static initialization block

Fields declared static and final are also safe for multithreaded use (see TSM03-J. Do not publish partially initialized objects for more information). However, remember that As a cautionary note, however, 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 MSC07(see DCL00-J. Eliminate Prevent class initialization cycles and OBJ01-J. Be aware that a final reference may not always refer to immutable data for more details about such problems for more information). Furthermore, individual members of the referenced object can also be changed if the object itself is mutable.

It is also permissible to use a wrapper method to retrieve the value of m_functions, allowing m_functions to be declared private (see rule OBJ01-J. Limit 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 as 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. Furthermore, 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.

References

\[[FT 08|AA. Java References#FT 08]\] 
\[[Sterbenz 06|AA. Java References#Sterbenz 06]\] Antipattern 5, Misusing Public Static Variables
\[[Nisewanger 07|AA. Java References#Nisewanger 07]\] Antipattern 5, Misusing Public Static Variables
\[[SCG 07|AA. Java References#SCG 07]\] Guideline 3.1, Treat public static fields as constants
\[[Gong 03|AA. Java References#Gong 03]\] 9.3 Static Fields
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 582|http://cwe.mitre.org/data/definitions/582.html] "Array Declared Public, Final, and Static", [CWE ID 493|http://cwe.mitre.org/data/definitions/493.html] "Critical Public Variable Without Final Modifier", [CWE ID 500|http://cwe.mitre.org/data/definitions/500.html] "Public Static Field Not Marked Final"

Automated Detection

Related Guidelines

Bibliography

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Field isn't final but should be refactored to be soOBJ02-J. Do not ignore return values of methods that operate on immutable objects      08. Object Orientation (OBJ)      OBJ04-J. Do not allow partially initialized objects to be accessed