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Client code can trivially access public static fields . Neither reads nor writes because access to such variables fields are not checked by a security manager. Furthermore, new values cannot be validated programmatically before they are stored in the fieldthese fields.

In the presence of multiple threads, non-final nonfinal public static fields can be modified in inconsistent ways . (For an example, see guideline TSM01-J. Do not let the ( this ) reference escape during object construction.) unmigrated-wiki-markup 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}} \[ [Gong 2003|AA. Bibliography#Gong 03]\]]. As a result, classes must not contain nonfinal public static fields.

Noncompliant Code Example

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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
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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Fields declared static and final are also safe for multithreaded use ; see guideline (see TSM03-J. Do not publish partially initialized objectsfor more information).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). Furthermore, individual members of the referenced object can also be changed if the object itself is mutable; see guideline OBJ01-J. Never conflate immutability of a reference with that of the referenced object.

It is also permissible to use a wrapper method to retrieve the value of m_functions. This has encapsulation advantages as it permits , allowing m_functions to be declared private . See guideline OBJ00(see rule 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. 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 in some cases. For more information, see ???.

Related Guidelines

[MITRE CWE|http://cwe.mitre.org/]: [CWE-582|http://cwe.mitre.org/data/definitions/582.html] "Array Declared Public, Final, and Static"
[MITRE CWE|http://cwe.mitre.org/]: [CWE-493|http://cwe.mitre.org/data/definitions/493.html] "Critical Public Variable Without Final Modifier"
[MITRE CWE|http://cwe.mitre.org/]: [CWE-500|http://cwe.mitre.org/data/definitions/500.html] "Public Static Field Not Marked Final"
\[[SCG 2007|AA. Bibliography#SCG 07]\] 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

Automated Detection

Related Guidelines

Bibliography

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Field isn't final but should be refactored to be soOBJ02-J. Sensitive classes must not let themselves be copied      04. Object Orientation (OBJ)      OBJ04-J. Do not allow access to partially initialized objects