CLR Integration Code Access Security

مقالة
04/01/2012

The common language runtime (CLR) supports a security model called code access security for managed code. In this model, permissions are granted to assemblies based on the identity of the code. For more information, see the "Code Access Security" section in the .NET Framework software development kit.

The security policy that determines the permissions granted to assemblies is defined in three different places:

Machine policy: This is the policy in effect for all managed code running in the machine on which SQL Server is installed.
User policy: This is the policy in effect for managed code hosted by a process. For SQL Server, the user policy is specific to the Windows account on which the SQL Server service is running.
Host policy: This is the policy set up by the host of the CLR (in this case, SQL Server) that is in effect for managed code running in that host.

The code access security mechanism supported by the CLR is based on the assumption that the runtime can host both fully trusted and partially trusted code. The resources that are protected by CLR code access security are typically wrapped by managed application programming interfaces that requirethe corresponding permission before allowing access to the resource. The demandfor the permission is satisfied only if all the callers (at the assembly level) in the call stack have the corresponding resource permission.

The set of code access security permissions that are granted to managed code when running inside SQL Server is the intersection of the set of permissions granted by the above three policy levels. Even if SQL Server grants a set of permissions to an assembly loaded in SQL Server, the eventual set of permissions given to user code may be restricted further by the user and machine-level policies.

SQL Server Host Policy Level Permission Sets

The set of code access security permissions granted to assemblies by the SQL Server host policy level is determined by the permission set specified when creating the assembly. There are three permission sets: SAFE, EXTERNAL_ACCESS and UNSAFE (specified using the PERMISSION_SET option of CREATE ASSEMBLY (Transact-SQL)).

SQL Server supplies a host-level security policy level to the CLR while hosting it; this policy is an additional policy level below the two policy levels that are always in effect. This policy is set for every application domain that is created by SQL Server. This policy is not meant for the default application domain that would be in effect when SQL Server creates an instance of the CLR.

The SQL Server host-level policy is a combination of SQL Server fixed* *policy for system assemblies and user-specified policy for user assemblies.

The fixed policy for CLR assemblies and SQL Server system assemblies grants them full trust.

The user-specified portion of the SQL Server host policy is based on the assembly owner specifying one of three permission buckets for each assembly. For more information about the security permissions listed below, see the .NET Framework SDK.

SAFE

Only internal computation and local data access are allowed. SAFE is the most restrictive permission set. Code executed by an assembly with SAFE permissions cannot access external system resources such as files, the network, environment variables, or the registry.

SAFE assemblies have the following permissions and values:

Permission	Value(s)/Description
SecurityPermission	Execution: Permission to execute managed code.
SqlClientPermission	Context connection = true , context connection = yes: Only the context-connection can be used and the connection string can only specify a value of "context connection=true" or "context connection=yes". AllowBlankPassword = false: Blank passwords are not permitted.

SecurityPermission

Execution: Permission to execute managed code.

SqlClientPermission

Context connection = true , context connection = yes: Only the context-connection can be used and the connection string can only specify a value of "context connection=true" or "context connection=yes".

AllowBlankPassword = false: Blank passwords are not permitted.

EXTERNAL_ACCESS

EXTERNAL_ACCESS assemblies have the same permissions as SAFE assemblies, with the additional ability to access external system resources such as files, networks, environmental variables, and the registry.

EXTERNAL_ACCESS assemblies also have the following permissions and values:

Permission	Value(s)/Description
DistributedTransactionPermission	Unrestricted: Distributed transactions are allowed.
DNSPermission	Unrestricted: Permission to request information from Domain Name Servers.
EnvironmentPermission	Unrestricted: Full access to system and user environment variables is allowed.
EventLogPermission	Administer: The following actions are allowed: creating an event source, reading existing logs, deleting event sources or logs, responding to entries, clearing an event log, listening to events, and accessing a collection of all event logs.
FileIOPermission	Unrestricted: Full access to files and folders is allowed.
KeyContainerPermission	Unrestricted: Full access to key containers is allowed.
NetworkInformationPermission	Access: Pinging is permitted.
RegistryPermission	Allows read rights to HKEY_CLASSES_ROOT, HKEY_LOCAL_MACHINE, HKEY_CURRENT_USER, HKEY_CURRENT_CONFIG, and HKEY_USERS.
SecurityPermission	Assertion: Ability to assert that all the callers of this code have the requisite permission for the operation. ControlPrincipal: Ability to manipulate the principal object. Execution: Permission to execute managed code. SerializationFormatter: Ability to provide serialization services.
SmtpPermission	Access: Outbound connections to SMTP host port 25 are allowed.
SocketPermission	Connect: Outbound connections (all ports, all protocols) on a transport address are allowed.
SqlClientPermission	Unrestricted: Full access to the datasource is allowed.
StorePermission	Unrestricted: Full access to X.509 certificate stores is allowed.
WebPermission	Connect: Outbound connections to web resources are allowed.

UNSAFE

UNSAFE allows assemblies unrestricted access to resources, both within and outside SQL Server. Code executing from within an UNSAFE assembly can also call unmanaged code.

UNSAFE assemblies are given FullTrust.

Security Note
SAFE is the recommended permission setting for assemblies that perform computation and data management tasks without accessing resources outside SQL Server. EXTERNAL_ACCESS is recommended for assemblies that access resources outside SQL Server. EXTERNAL_ACCESS assemblies by default execute as the SQL Server service account. It is possible for EXTERNAL_ACCESS code to explicitly impersonate the caller's Windows Authentication security context. Since the default is to execute as the SQL Server service account, permission to execute EXTERNAL_ACCESS should only be given to logins trusted to run as the service account. From a security perspective, EXTERNAL_ACCESS and UNSAFE assemblies are identical. However, EXTERNAL_ACCESS assemblies provide various reliability and robustness protections that are not in UNSAFE assemblies. Specifying UNSAFE allows the code in the assembly to perform illegal operations against the SQL Server process space, and hence can potentially compromise the robustness and scalability of SQL Server. For more information about creating CLR assemblies in SQL Server, see Managing CLR Integration Assemblies.

SAFE is the recommended permission setting for assemblies that perform computation and data management tasks without accessing resources outside SQL Server. EXTERNAL_ACCESS is recommended for assemblies that access resources outside SQL Server. EXTERNAL_ACCESS assemblies by default execute as the SQL Server service account. It is possible for EXTERNAL_ACCESS code to explicitly impersonate the caller's Windows Authentication security context. Since the default is to execute as the SQL Server service account, permission to execute EXTERNAL_ACCESS should only be given to logins trusted to run as the service account. From a security perspective, EXTERNAL_ACCESS and UNSAFE assemblies are identical. However, EXTERNAL_ACCESS assemblies provide various reliability and robustness protections that are not in UNSAFE assemblies. Specifying UNSAFE allows the code in the assembly to perform illegal operations against the SQL Server process space, and hence can potentially compromise the robustness and scalability of SQL Server. For more information about creating CLR assemblies in SQL Server, see Managing CLR Integration Assemblies.

Accessing External Resources

If a user-defined type (UDT), stored procedure, or other type of construct assembly is registered with the SAFE permission set, then managed code executing in the construct is unable to access external resources. However, if either the EXTERNAL_ACCESS or UNSAFE permission sets are specified, and managed code attempts to access external resources, SQL Server applies the following rules:

If	Then
The execution context corresponds to a SQL Server login.	Attempts to access external resources are denied and a security exception is raised.
The execution context corresponds to a Windows login and the execution context is the original caller.	The external resource is accessed under the security context of the SQL Server service account.
The caller is not the original caller.	Access is denied and a security exception is raised.
The execution context corresponds to a Windows login and the execution context is the original caller and the caller has been impersonated.	Access uses the caller security context; not the service account.

Permission Set Summary

The following chart summarizes the restrictions and permissions granted to the SAFE, EXTERNAL_ACCESS, and UNSAFE permission sets.

	SAFE	EXTERNAL_ACCESS	UNSAFE
Code Access Security Permissions	Execute only	Execute + access to external resources	Unrestricted (including P/Invoke)
Programming model restrictions	Yes	Yes	No restrictions
Verifiability requirement	Yes	Yes	No
Local data access	Yes	Yes	Yes
Ability to call native code	No	No	Yes

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