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A non-empty array is always mutable, so Do not expose references to mutable objects to client code. Never initialize such a field to a client-provided object reference or return the object reference from an accessor. Exposing a public static final array makes no sense; clients will be able object allows clients to modify the contents of the array object (although they will not be able to change the array object itself, as it is final).

This rule does not address private mutable objects, see rule OBJ05-J. Do not return references to private mutable class members for more information.

Noncompliant Code Example

Suppose that SomeType is immutable.

public static final SomeType [] SOMETHINGS = { ... };

With this declaration, {{SOMETHINGS\[1\]}}, etc. can be modified by clients of the code.

Compliant Solution

Even though SomeType is immutable, this declaration allows the SOMETHINGS array to be modified by untrusted clients of the code. Any element of the array can be assigned a new value, namely a reference to a new SomeType object.

This noncompliant code example also violates OBJ01-J. Limit accessibility of fields.

Noncompliant Code Example (getter method)

This noncompliant code example complies with OBJ01-J. Limit accessibility of fields by declaring the array private. But, in declaring the array private, this code example violates OBJ05-J. Do not return references to private mutable class members.

Suppose that SomeType is immutable.

private static final SomeType [] SOMETHINGS = { ... };
public static final getSomethings() {return SOMETHINGS;} 

Even though SomeType is immutable, the public getter method enables untrusted clients to modify the SOMETHINGS array. Any element of the array can be assigned a new value, namely a reference to a new SomeType object.

Compliant Solution (clone)

Continuing with the assumption that SomeType is immutable, one One approach is to have a private array and a public method that returns a copy of the array:

private static final SomeType [] SOMETHINGS = { ... };
public static final SomeType [] somethings() {
  return SOMETHINGS.clone();
}

Now, the original array values cannot be modified by a clientany client.  If SomeType were mutable, this approach would not be effective because the array clone references the same SomeType objects as the SOMETHINGS array. If the client modified the clone SomeType objects directly, the SomeType objects referenced by the SOMETHINGS array would also change.

Compliant Solution

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(Unmodifiable List)

Continuing with the assumption that SomeType is immutable, an An alternative approach is to have a private array from which a public immutable list is contructedconstructed:

private static final SomeType [] THE_THINGS = { ... };
public static final List&amp;lt;SomeType&amp;gt;List<SomeType> SOMETHINGS =
  Collections.unmodifiableList(Arrays.asList(THE_THINGS));

Now, neither the original array values nor the public list can be modified by a client. If SomeType were mutable, this would not be effective because the list references the same SomeType objects as the SOMETHINGS array. The unmodifiabileList prevents the list from being modified, not the elements in the list. If the client modified the list's SomeType objects directly, the SomeType objects referenced by the SOMETHINGS array would also change.

Risk Assessment

Having a public static final array is a potential security risk , as because the array elements may be modified by a client.

Automated Detection

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Tool	Version	Checker	Description
Parasoft Jtest	Include Page Parasoft_V Parasoft_V	CERT.OBJ13.RMO	Avoid referencing mutable fields
SonarQube	Include Page SonarQube_V SonarQube_V	S2386 S2384	Mutable fields should not be "public static" Mutable members should not be stored or returned directly
SpotBugs	Include Page SpotBugs_V SpotBugs_V	MS_EXPOSE_REP EI_EXPOSE_REP EI_EXPOSE_STATIC_REP2 EI_EXPOSE_STATIC_REP2	Implemented (since 4.3.0)

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule guideline on the CERT website.

References

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[Bloch 2008]	Item 13, "Minimize the Accessibility of Classes and Members"
[JLS 2015]	§6.6, "Access Control"

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Image Added Image Added Image AddedJLS 06|AA. Java References#JLS 06]\] Section 6.6, Access Control \[[Bloch 08|AA. Java References#Bloch 08]\] Item 13: Minimize the accessibility of classes and membersSEC36-J. Ensure that the bytecode verifier is applied to all involved code upon any modification      02. Platform Security (SEC)      03. Declarations and Initialization (DCL)