CA2119: Seal methods that satisfy private interfaces

Article
09/05/2023

Property	Value
Rule ID	CA2119
Title	Seal methods that satisfy private interfaces
Category	Security
Fix is breaking or non-breaking	Breaking
Enabled by default in .NET 8	No

Cause

An inheritable public type provides an overridable method implementation of an internal (Friend in Visual Basic) interface.

Rule description

Interface methods have public accessibility, which cannot be changed by the implementing type. An internal interface creates a contract that is not intended to be implemented outside the assembly that defines the interface. A public type that implements a method of an internal interface using the virtual (Overridable in Visual Basic) modifier allows the method to be overridden by a derived type that is outside the assembly. If a second type in the defining assembly calls the method and expects an internal-only contract, behavior might be compromised when, instead, the overridden method in the outside assembly is executed. This creates a security vulnerability.

How to fix violations

To fix a violation of this rule, prevent the method from being overridden outside the assembly by using one of the following:

Make the declaring type sealed (NotInheritable in Visual Basic).
Change the accessibility of the declaring type to internal (Friend in Visual Basic).
Remove all public constructors from the declaring type.
Implement the method without using the virtual modifier.
Implement the method explicitly.

When to suppress warnings

It is safe to suppress a warning from this rule if, after careful review, no security issues exist that might be exploitable if the method is overridden outside the assembly.

Suppress a warning

If you just want to suppress a single violation, add preprocessor directives to your source file to disable and then re-enable the rule.

#pragma warning disable CA2119
// The code that's violating the rule is on this line.
#pragma warning restore CA2119

To disable the rule for a file, folder, or project, set its severity to none in the configuration file.

[*.{cs,vb}]
dotnet_diagnostic.CA2119.severity = none

For more information, see How to suppress code analysis warnings.

Example 1

The following example shows a type, BaseImplementation, that violates this rule.

// Internal by default.
interface IValidate
{
    bool UserIsValidated();
}

public class BaseImplementation : IValidate
{
    public virtual bool UserIsValidated()
    {
        return false;
    }
}

public class UseBaseImplementation
{
    public void SecurityDecision(BaseImplementation someImplementation)
    {
        if (someImplementation.UserIsValidated() == true)
        {
            Console.WriteLine("Account number & balance.");
        }
        else
        {
            Console.WriteLine("Please login.");
        }
    }
}

Interface IValidate
    Function UserIsValidated() As Boolean
End Interface

Public Class BaseImplementation
    Implements IValidate

    Overridable Function UserIsValidated() As Boolean _
     Implements IValidate.UserIsValidated
        Return False
    End Function

End Class

Public Class UseBaseImplementation

    Sub SecurityDecision(someImplementation As BaseImplementation)

        If (someImplementation.UserIsValidated() = True) Then
            Console.WriteLine("Account number & balance.")
        Else
            Console.WriteLine("Please login.")
        End If

    End Sub

End Class

Example 2

The following example exploits the virtual method implementation of the previous example.

public class BaseImplementation
{
    public virtual bool UserIsValidated()
    {
        return false;
    }
}

public class UseBaseImplementation
{
    public void SecurityDecision(BaseImplementation someImplementation)
    {
        if (someImplementation.UserIsValidated() == true)
        {
            Console.WriteLine("Account number & balance.");
        }
        else
        {
            Console.WriteLine("Please login.");
        }
    }
}

Public Class BaseImplementation

    Overridable Function UserIsValidated() As Boolean
        Return False
    End Function

End Class

Public Class UseBaseImplementation

    Sub SecurityDecision(someImplementation As BaseImplementation)

        If (someImplementation.UserIsValidated() = True) Then
            Console.WriteLine("Account number & balance.")
        Else
            Console.WriteLine("Please login.")
        End If

    End Sub

End Class

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