CSharp readonly modifier is not inforced by the CLR (when in Full Trust)

From OWASP

From MSDN "...The readonly keyword is a modifier that you can use on fields. When a field declaration includes a readonly modifier, assignments to the fields introduced by the declaration can only occur as part of the declaration or in a constructor in the same class...." (see also What's the equivalent of C#'s "readonly" in C++/CLI? , C#'s const vs. readonly and 10.4.2 Readonly fields)

As with my previous finding (Possible Type Confusion issue in .Net 1.1 (only works in Full Trust)) this feature is not enforeced in Full Trust environments, only in partial trust (when type safety verification occours)

To test this:

1) compile this sample

using System;

namespace Owasp
{
   class ReadOnlyTest
   {
       public static void Main()
       {
           Console.WriteLine("ReadOnly test\n\n");
           readOnly ro = new readOnly();
           Console.WriteLine(ro.sReadOnly);    
           ro.sReadWrite = "changed";
           Console.WriteLine(ro.sReadOnly);
       }
   }
   class readOnly
   {
       public readonly string sReadOnly;
       public string sReadWrite;
       public readOnly()
       {
           sReadOnly = "Read Only String";
       }   
   }
}

2) ILDASM it with

ildasm readOnly.exe /out:_readOnly.il

3) Edit the IL code in Notepad replacing

     IL_001c:  ldstr      "changed"
     IL_0021:  stfld      string Owasp.readOnly::sReadWrite

with      
     IL_001c:  ldstr      "changed"
     IL_0021:  stfld      string Owasp.readOnly::sReadOnly

4) ILASM it with

ilasm _readonly.il

5) execute _readonly.exe and you should get this result

ReadOnly test

Read Only String
changed

6) now change to a partial trust environment (for example a mapped network drive to the same folder (in my case the z:\ drive is mapped to \\127.0.0.1\{folder name} ) and note that when executing _readonly.exe you will get the following JIT exception:

Z:\readOnlytest>_readOnly.EXE

Unhandled Exception: System.Security.VerificationException: Operation could deabilize the runtime.   at Owasp.ReadOnlyTest.Main()

7) this shows that in partial trust the verification occours and in Full Trust it doesn't

8) you can also use PeVerify to get more details about the problems with _readOnly.exe