[tomasr]

You need to be a local administrator to write to the GAC by default. See http://msdn.microsoft.com/en-us/library/yf1d93sz(v=vs.110).a...

So if you can modify the files in the GAC, you're already compromised at that point.

[bmm6o]

Writing to the GAC is easy, but assemblies in the GAC are strongly-named. Actually replacing one without the private key would require forging a signature, which is what parent was probably referring to. You can give an assembly the same name, but if it's signed with a different key it gets a different strong name (and therefore goes in a slightly different directory).