Callers can trivially access and modify public non-final static fields. Neither accesses nor modifications can be checked by a SecurityManager, and newly set values cannot be validated. Furthermore, multiple threads can modify non-final public static data in ways that are not consistent.

Improper use of public static fields can also lead to type safety issues. For example, untrusted code might supply an unexpected subtype when the variable is defined to be of a more general type such as java.lang.Object. [[Gong 03]]

Noncompliant Code Example

This is an example from the JDK 1.4.2 software [[Function Table]].

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 = <new_table>;

Replacing the function table gives the attacker access to the XPathContext used to evaluate XPath expressions. The XPathContext is used to set the reference node for evaluating XPath expressions. Manipulating this can cause unexpected behavior and XML fields can be modified in inconsistent ways. Also, static variables are global across the Java Runtime Environment (JRE). They can be used as a communication channel between different application domains (e.g., through code loaded by different class loaders).

Compliant Solution

Treat public static fields as constants and declare them as final. Consider the use of enum types in the following example.

public class MyClass {
    public static final int LEFT  = 1;
    public static final int RIGHT = 2;
}

The FuncLoader static instance in the noncompliant example could have been declared as final as shown below.

...
public static final FuncLoader m_functions;
...

Compliant Solution

Additionally, for mutable static state one can define assessor methods and add appropriate security checks. Note that this is a different example, wherein the scope of the static variable has been changed to private.

public class MyClass {

  private static byte[] data;
  
  public static byte[] getData() {
    return data.clone();
  }

  public static void setData(byte[] b) {
    securityCheck();
    data = b.clone();
  }
}

As a cautionary note however, simply changing the modifier to final may not prevent attackers from indirectly retrieving an incorrect value from the static final variable before its initialization. See MSC00-J. Eliminate class initialization cycles and OBJ03-J. Be careful about final reference for more on this problem.

Risk Assessment

Unauthorized modifications to public static variables can result in unexpected behavior and can bypass important security checks and/or invoke malicious code.

References

[[FT 08]]
[[Nisewanger 07]] Antipattern 5, Misusing Public Static Variables
[[SCG 07]] Guideline 3.1, Treat public static fields as constants
[[Gong 03]] 9.3 Static Fields

OBJ03-J. Be careful about final reference      06. Objects Orientation (OBJ)      OBJ32-J. Do not allow partially initialized objects to be accessed