OBJ03-J. Do not use public static non-final variables

Callers can trivially access and modify public static non-final fields. Neither accesses nor modifications are checked by a security manager, and newly set values cannot be validated. Classes loaded by the same or different class loaders can access each others' public static members, unless appropriate protection is installed. For example, consider Java applets [[Sun 2008]]:

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. (For example, see guideline TSM01-J. Do not let the (this) reference escape during object construction.)

Improper use of public static fields can also result in type safety issues. For example, untrusted code can supply an unexpected subtype when the variable is defined to be of a more general type, such as java.lang.Object [[Gong 2003]].

Noncompliant Code Example

This noncompliant code example is adopted from JDK v1.4.2 [[FT 2008]]. 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 = <new_table>;

Replacing the function table gives the attacker access to the XPathContext. The XPathContext is used to set the reference node for evaluating XPath expressions. Manipulating it can allow 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).

Compliant Solution

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

public static final FuncLoader m_functions;
// Initialize m_functions in a constructor

Fields declared static final are also safe for multithreaded use. (See guideline TSM03-J. Do not publish partially initialized objects.) On 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 guideline MSC07-J. Eliminate class initialization cycles.) Individual members of the referenced object can also be changed if it 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 to private. (See guideline OBJ00-J. Declare data members as private and provide accessible wrapper methods.) However, it is unnecessary when m_functions needs to be treated like a world-accessible constant.

Noncompliant Code Example (serialVersionUID)

This noncompliant code example uses a public static non-final 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 internally uses the serialVersionUID field, so no accessible wrapper methods are required.

Risk Assessment

Unauthorized modifications of public static variables can result in unexpected behavior and violation of class invariants. Further, 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).

Related Guidelines

MITRE CWE: CWE-582 "Array Declared Public, Final, and Static"

MITRE CWE: CWE-493 "Critical Public Variable Without Final Modifier"

MITRE CWE: CWE-500 "Public Static Field Not Marked Final"

[[SCG 2007]] Guideline 3.1, Treat public static fields as constants

Bibliography

[[FT 2008]]
[[Gong 2003]] 9.3 Static Fields
[[Nisewanger 2007]] Antipattern 5, Misusing Public Static Variables
[[Sterbenz 2006]] 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