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 ofCREATE 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 fixedpolicy 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.
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:
|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 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:
|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 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.
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:
|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.
|Code Access Security Permissions||Execute only||Execute + access to external resources||Unrestricted (including P/Invoke)|
|Programming model restrictions||Yes||Yes||No restrictions|
|Local data access||Yes||Yes||Yes|
|Ability to call native code||No||No||Yes|