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CRV2 FrameworkSpecIssuesASPNetStrongAssembiles

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Overview Strongly Named assemblies

During the build process either QA or Developers are going to publish the code into executable formats. Usually this consists of an exe or and one or several DLL’s. During the build/publish process a decision needs to be made to sign or not sign the code. Signing your code is called creating “strong names” by Microsoft. If you create a project using Visual Studio and use Microsofts “Run code analysis” most likely your will encounter a Microsoft design error if the code is not strong named; “Warning 1 CA2210 : Microsoft.Design : Sign 'xxx.exe' with a strong name key.”

Code review needs to be aware if strong naming is being used, benefits and what threat vectors strong naming helps prevent or understand the reasons for not using strong naming.

A strong name is a method to sign an assembly’s identity using its text name, version number, culture information, a public key and a digital signature.(Solis, 2012)

  • Strong naming guarantees a unique name for that assembly.
  • Strong names protect the version lineage of an assembly. A strong name can ensure that no one can produce a subsequent version of your assembly. Users can be sure that a version of the assembly they are loading comes from the same publisher that created the version the application was built with.

The above two point are very important if you are going to use Global Assembly Cache (GAC).

  • Strong names provide a strong integrity check and prevent spoofing. Passing the .NET Framework security checks guarantees that the contents of the assembly have not been changed since it was built. Note, however, that strong names in and of themselves do not imply a level of trust like that provided, for example, by a digital signature and supporting certificate. If you use the GAC assemblies remember the assemblies are not verified each time they load since the GAC by design is a locked-down, admin-only store.

What strong names can’t prevent is a malicious user from stripping the strong name signature entirely, modifying the assembly, or re-signing it with the malicious user’s key.

The code reviewer needs to understand how the strong name private key will be kept secure and managed. This is crucible if you decide strong name signatures are a good fit for your organization.

If principle of least privilege is used so code is not or less susceptible to be access by the hacker and the GAC is not being used strong names provides less benefits or no benefits at all.

How to use Strong Naming

Signing tools

In order to create a Strongly name assembly there are a set of tools and steps that you need to follow

Using Visual Studio

In order to use Visual Studio to create a Strongly Named Assembly, it is necessary to have a copy of the public/private key pair file. Its is also possible to create this pair key in Visual Studio

In Visual Studio 2005, the C#, Visual Basic, and Visual J# integrated development environments (IDEs) allow you to generate key pairs and sign assemblies without the need to create a key pair using Sn.exe(Strong Name Tool). These IDEs have a Signing tab in the Project Designer. . The use of the AssemblyKeyFileAttribute to identify key file pairs has been made obsolete in Visual Studio 2005.

The following figure ilustrates the process done by the compiler StrongNameAssembly.png

Using Strong Name tool

The Sign Tool is a command-line tool that digitally signs files, verifies signatures in files, or time stamps files. The Sign Tool is not supported on Microsoft Windows NT, Windows Me, Windows 98, or Windows 95. In case you aren't using the "Visual Studio Command Prompt" (Start >> Microsoft Visual Studio 2010 >> Visual Studio Tools >> Visual Studio Command Prompt (2010)) you can locate sn.exe at %ProgramFiles%\Microsoft SDKs\Windows\v7.0A\bin\sn.exe

The following command creates a new, random key pair and stores it in keyPair.snk.

sn -k keyPair.snk

The following command stores the key in keyPair.snk in the container MyContainer in the strong name CSP.

sn -i keyPair.snk MyContainer

The following command extracts the public key from keyPair.snk and stores it in publicKey.snk.

sn -p keyPair.snk publicKey.snk

The following command displays the public key and the token for the public key contained in publicKey.snk.

sn -tp publicKey.snk

The following command verifies the assembly MyAsm.dll.

sn -v MyAsm.dll

The following command deletes MyContainer from the default CSP.

sn -d MyContainer

Using the Assembly Linker(AI.exe)

This tool is automatically installed with Visual Studio and with the Windows SDK. To run the tool, we recommend that you use the Visual Studio Command Prompt or the Windows SDK Command Prompt (CMD Shell). These utilities enable you to run the tool easily, without navigating to the installation folder. For more information, see Visual Studio and Windows SDK Command Prompts.

If you have Visual Studio installed on your computer: On the taskbar, click Start, click All Programs, click Visual Studio, click Visual Studio Tools, and then click Visual Studio Command Prompt. -or- If you have the Windows SDK installed on your computer: On the taskbar, click Start, click All Programs, click the folder for the Windows SDK, and then click Command Prompt (or CMD Shell).

At the command prompt, type the following:

al sources options


All Visual Studio compilers produce assemblies. However, if you have one or more modules (metadata without a manifest), you can use Al.exe to create an assembly with the manifest in a separate file. To install assemblies in the cache, remove assemblies from the cache, or list the contents of the cache, use the Global Assembly Cache Tool (Gacutil.exe).

The following command creates an executable file t2a.exe with an assembly from the t2.netmodule module. The entry point is the Main method in MyClass.

al t2.netmodule /target:exe /out:t2a.exe /main:MyClass.Main

Use Assembly attributes

You can insert the strong name information in the code directly. For this, depending on wherethe key file is located you can use AssemblyKeyFileAttribute or AssemblyKeyNameAttribute

Use Compiler options :use /keyfile or /delaysign

Safeguarding the key pair from developers is necessary to maintain and guarantee the integrity of the assemblies. The public key is should be accessible, but access to the private key is restricted to only a few individuals. When developing assemblies with strong names, each assembly that references the strong-named target assembly contains the token of the public key used to give the target assembly a strong name. This requires that the public key be available during the development process. You can use delayed or partial signing at build time to reserve space in the portable executable (PE) file for the strong name signature, but defer the actual signing until some later stage (typically just before shipping the assembly). You can use /keyfile or /delaysign in C# and VB.NET (MSDN)