Introduction to Code Access Security

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Introduction to Code Access Security

• Phil Denoncourt MCSD, MCDBA, MCAD, MCSE, MCSA

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Security in Todays Businesses

• Business are under more pressure to assure that
  their systems are secure
• Stockholders
• Regulatory Requirements
• More hackers / computers / information about how
  to hack now available.
• Information is a valuable company asset and needs
  to be protected.
• System downtime affects productivity

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Security in .NET

• Security is traditionally not thought about until
  deployment phase.
• Security is usually applied on resources files
  printers.
• .NET secures the application, not resources
• .NET security is two fold
• Developers must specify what privileges their app
  will use.
• Adminstrators must specify what applications can
  run on workstations.

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What is Code Access Security (CAS)

• Enables admins to control access to programs not
  files
• Assembly informs runtime of privileges it needs,
  and runtime determines if it can run.
• Allows you to be confident about the code you are
  executing
• Prevents someone from inserting a shadow
  assembly, and executing bad code.
• By default you can do nothing without privileges
• OS security will override CAS security
• If you dont have OS permission to a file, .NET
  wont be able to read it.

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Concepts

• Evidence
• Prove to me who you are
• CAS Policies are matched to your code based on
  various attributes (evidence) in your code.
• Application Directory , Hash, Publisher, Site,
  URL, Zone
• Roll your own
• Permissions
• Hall pass metaphor.
• If you have the pass, you can go to the bathroom.
  If you dont have the pass, you cant
• A task that your code is allowed to perform
• EventLogPermission, FileIOPermission,
  OleDbPermission, RegistryPermission, UIPermission
• Roll your own

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.NET Configuration 1.1
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Policy Levels

• Three Levels Enterprise, Machine and User
• Permission set for any given assembly is
  evaluated for each level.
• Code is granted the minimum permission among all
  the levels.

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Permission set

• Straightforward a set of permissions
• Predefined sets FullTrust, Execution, Internet,
  Nothing, Everything
• For example Internet has following permissions
• File Dialog, Isolated File Storage, Security,
  User Interface, Printing
• Create your own set of permissions

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Code groups

• Defines a set of evidence that will contain a set
  of permissions.
• To Create a code group
• Give a name
• Choose type of evidence
• Choose permission set to associate with code group

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Permissions

• Define the privilege being requested
• Without permissions, assembly can do nothing.
• Most Permissions can control full access to
  privilege or limited access
• Request access to all files on the filesystem or
  just c\myappssettings.ini

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Specifying Permissions in your Code

• Declarative Security
• Static, compiled into assembly
• Implemented as attributes
• Can be applied to Assemblies, Classes or Methods
• Imperative Security
• Can be dynamic
• Permission objects created in code
• Before privilege is needed, demand method is
  called

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Requesting Permissions

• Allows you to specify what privileges your
  assembly needs to run.
• RequestMinimum
• Your app wont function without this privilege
• RequestOptional
• Your app can function without this privilege, but
  doesnt like to
• RequestRefuse
• My app is dangerous if you give me this privilege

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Demand vs. Assert

• Different methods of making sure privilege is
  held.
• Demand makes sure that your method, and all other
  methods in the call stack have that privilege
• Walks up the call stack making sure no methods
  refuse permission.
• Assert just makes sure your method has the
  desired privilege

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Why Request Permissions

• Prevents your app from receiving too much
  privileges
• Reduces amount of errorhandling you need
• Allows admins to see what permissions your app
  needs using the Permview tool.

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Demanding Permissions Declaratively
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Demanding Permissions Imperatively
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Requesting Permissions at Assembly Level
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References

• An Overview of Security in the .NET Framework -
  Dr. Demien Watkins
• MSDN
• The Security Infrastructure of the CLR Provides
  Evidence, Policy, Permissions, and Enforcement
  Services Don Box
• MSDN
• Managing .NET Code Access Security (CAS) Policy -
  Joe Mayo
• CoDE Magazine