CA2101: Specify marshalling for P/Invoke string arguments

Article
09/05/2023

Property	Value
Rule ID	CA2101
Title	Specify marshalling for P/Invoke string arguments
Category	Globalization
Fix is breaking or non-breaking	Non-breaking
Enabled by default in .NET 9	As suggestion

Cause

A platform invoke member allows for partially trusted callers, has a string parameter, and does not explicitly marshal the string.

Rule description

When you convert from Unicode to ANSI, it is possible that not all Unicode characters can be represented in a specific ANSI code page. Best-fit mapping tries to solve this problem by substituting a character for the character that cannot be represented. The use of this feature can cause a potential security vulnerability because you cannot control the character that is chosen. For example, malicious code could intentionally create a Unicode string that contains characters that are not found in a particular code page, which are converted to file system special characters such as '..' or '/'. Note also that security checks for special characters frequently occur before the string is converted to ANSI.

Best-fit mapping is the default for the unmanaged conversion, WChar to MByte. Unless you explicitly disable best-fit mapping, your code might contain an exploitable security vulnerability because of this issue.

Caution

Code Access Security (CAS) should not be considered a security boundary.

How to fix violations

To fix a violation of this rule, explicitly marshal string data types.

When to suppress warnings

Do not suppress a warning from this rule.

Example

The following example shows a method that violates this rule, and then shows how to fix the violation.

class NativeMethods
{
    // Violates rule: SpecifyMarshalingForPInvokeStringArguments.
    [DllImport("advapi32.dll", CharSet = CharSet.Auto)]
    internal static extern int RegCreateKey(IntPtr key, String subKey, out IntPtr result);

    // Satisfies rule: SpecifyMarshalingForPInvokeStringArguments.
    [DllImport("advapi32.dll", CharSet = CharSet.Unicode)]
    internal static extern int RegCreateKey2(IntPtr key, String subKey, out IntPtr result);
}

Friend Class NativeMethods
    ' Violates rule: SpecifyMarshalingForPInvokeStringArguments.
    <DllImport("advapi32.dll", CharSet:=CharSet.Auto)>
    Friend Shared Function RegCreateKey(ByVal key As IntPtr, ByVal subKey As String, <Out> ByRef result As IntPtr) As Integer
    End Function

    ' Satisfies rule: SpecifyMarshalingForPInvokeStringArguments.
    <DllImport("advapi32.dll", CharSet:=CharSet.Unicode)>
    Friend Shared Function RegCreateKey2(ByVal key As IntPtr, ByVal subKey As String, <Out> ByRef result As IntPtr) As Integer
    End Function
End Class

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