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Note: due to the circular nature of the definitions you’ll need some prior knowledge of the CoreCLR security model or reading it twice to make sense out of this ;-)

Security levels

The CoreCLR security model divide all code into three distinct levels: transparent, safe-critical and critical. This model is much simpler to understand (and implement) than CAS (e.g. no stack-walk). Only a few rules can describe much of it.


By default all code is transparent under the CoreCLR model.

  • Transparent code is limited to what it can do. Specifically it cannot directly call critical code (e.g. p/invokes, unsafe code) but it can call safe-critical and other transparent code.

  • Since it’s default there’s no need to add a [SecurityTransparent] attribute everywhere.

Safe Critical

Safe critical code is a gateway between transparent and critical code. As such it represent the riskiest code (the less, the better).

  • Code needed to bridge a (safe) transparent API to (not necessarily safe) critical code needs to be decorated with [SecuritySafeCritical] attributes.

  • Safe critical code needs to make extra (pre and/or post) validations between the transparent and critical in order to make the call earn its safe prefix.


Critical code can do anything, like p/invoking into native code and having access to everything outside the host browser. The plugin itself could not work without critical code. However applications can (and must) do without direct access to it.

  • Critical code, including every visible API that does things that application code should not do (e.g. file IO), must be marked with a [SecurityCritical] attribute.

  • All unsafe code, p/invoke declarations are critical (but still needs to be marked as such).

Code categories

To make this even easier to understand, from an application developer point of view, all assemblies are split into two categories: the application (or user) code and the platform code.

Application Code

Application code runs with limited trust. This makes it possible, along with other features, to safely run untrusted code inside your browser. Application code is bound by the following rules:

  • All application code is transparent. Using attributes to (try to) change this will compile but will be ignored at execution time.

  • As transparent it can call other application code (all transparent) and platform code that is transparent (default) or decorated with the [SecuritySafeCritical] attribute.

  • Application code cannot directly call [SecurityCritical] decorated methods/types present in platform code.

Platform Code

Platform code is a subset of the managed code provided with the plugin. This code is fully-trusted. As such it cannot expose all of its API to application code, instead it expose them using three different security levels (see next section).

  • Platform code is, by default, transparent. This means that most of it can be called directly from application code.

  • Platform code contains critical code - i.e. code that do anything (like p/invoking, unsafe code). Such code is decorated with [SecurityCritical] attributes and cannot be called from application code.

  • Access from transparent to critical code (e.g. using the safe, and transparent, IsolatedStorage that itself use the p/invoke, and critical, System.IO code) is possible thru code decorated with a [SecuritySafeCritical] attribute.

Platform Code Assemblies includes:

  • mscorlib
  • Microsoft.VisualBasic [1][2]
  • System
  • System.Core
  • System.Net
  • System.Runtime.Serialization
  • System.ServiceModel [1][2]
  • System.ServiceModel.Web [1]
  • System.Windows
  • System.Windows.Browser
  • System.Xml

[1] does not contain any [SecurityCritical] or [SecuritySafeCritical] attribute [2] has a different public key than the other assemblies

Both [1] and [2] may be considered platform code - but since they don’t (but, I guess, eventually could) use [SecurityCritical] nor [SecuritySafeCritical] they are in effect totally transparent (like application code).

Where ? On Windows / Silverlight 2 the platform files can be found in:

  • C:\Program Files\Microsoft Silverlight\2.0.31005.0
  • C:\Program Files\Microsoft SDKs\Silverlight\v2.0\Reference Assemblies

The later only exists if you have the Silverlight 2 SDK installed.

Application Developer Shortcut

Just remember this: under the CoreCLR an application can call anything (i.e. transparent or safe-critical) as long as it is not critical.

Special Considerations


Special care is needed wrt internal code since most of the platform assemblies include [InternalsVisibleTo] attributes - and yes, the internals are open to some non-platform (i.e. application code) assemblies.


Reflection is possible but has some limitations.


Code generation is possible (e.g. DLR) but also has some limitations.


The CoreCLR security model does not deal with policies - its decisions are a boolean CAN or CANNOT do. While this works for the most basic parts it does not solve cases where more elaborate access rules are required, e.g.:

  • downloader policies
  • sockets policies

Special care (outside the scope of CoreCLR) are needed to cover them.

Implementation details

Security Attributes Compatibility

Calls from application code to platform code (that Moonlight provides) either succeed (e.g. calling transparent or [SecuritySafeCritical] code) or fails (e.g. calling [SecurityCritical] code).

Since there is no distinction from calling transparent or safe critical code our (Moonlight) [SecuritySafeCritical] attributes do not need to match Silverlight implementation. However we do have to match [SecurityCritical] attributes on the visible (public and protected) API.