{"id":1783,"date":"2014-01-09T11:00:30","date_gmt":"2014-01-09T16:00:30","guid":{"rendered":"http:\/\/sqlity.net\/en\/?p=1783"},"modified":"2014-11-13T13:21:49","modified_gmt":"2014-11-13T18:21:49","slug":"changing-security-context-execute-revert","status":"publish","type":"post","link":"https:\/\/sqlity.net\/en\/1783\/changing-security-context-execute-revert\/","title":{"rendered":"Changing the Security Context with EXECUTE AS and REVERT"},"content":{"rendered":"
\n

Introduction<\/h3>\n

\nWhen testing SQL Server security, it is often necessary to login as a different user. However, repeated logins and logouts can become quite cumbersome quickly.\n<\/p>\n

\nBut there is help: Instead of establishing a new connection every time you need to test under a different login, you can instead just switch the current security context to a new login and switch it back when you are done. To execute a security context switch just use the EXECUTE AS<\/span> statement. The REVERT<\/span> statement lets you switch back to your original security context after you are finished.\n<\/p>\n

EXECUTE AS<\/h3>\n

\nLet's look at an example. For that we first need two logins and two users:\n<\/p>\n

\n[sql]\nCREATE LOGIN TestLogin1 WITH PASSWORD='********', CHECK_POLICY = OFF;
\nCREATE LOGIN TestLogin2 WITH PASSWORD='********', CHECK_POLICY = OFF;
\nCREATE USER TestUser1 FOR LOGIN TestLogin1;
\nCREATE USER TestUser2 FOR LOGIN TestLogin2;
\n[\/sql]\n<\/div>\n

\nNow, to be able to execute the EXECUTE AS<\/span> statement, you need to have impersonation permission on the account you want to switch to. So we need to grant that permission:\n<\/p>\n

\n[sql]\nGRANT IMPERSONATE ON LOGIN::TestLogin2 TO TestLogin1;
\n[\/sql]\n<\/div>\n

\nAs any grant or deny of server level permissions, this statement needs to be executed in master.\n<\/p>\n

\nNow that we have the permissions out of the way, let's try the EXECUTE AS<\/span> statement. Login to SQL Server using the TestLogin1 login and then execute this statement block:\n<\/p>\n

\n[sql]\nSELECT 'login' AS token_type,* FROM sys.login_token AS LT
\nUNION ALL
\nSELECT 'user' AS token_type,* FROM sys.user_token AS UT;
\nGO
\nEXECUTE AS LOGIN='TestLogin2';
\nGO
\nSELECT 'login' AS token_type,* FROM sys.login_token AS LT
\nUNION ALL
\nSELECT 'user' AS token_type,* FROM sys.user_token AS UT;
\n[\/sql]\n<\/div>\n

\nI introduced the two DMVs sys.login_token<\/span> and sys.user_token<\/span> yesterday in The Secret of the Security Token<\/a>.\n<\/p>\n

\nWhen you execute the above statements you will get a result like this:\n<\/p>\n

\n\"EXECUTE<\/a>\n<\/p>\n

\nAs you can see, the entire security context was switched out. Only the login TestLogin2 and the user TestUser2 shows up in the token list, after the EXECUTE AS<\/span> was executed. No trace of the original login or user remains.\n<\/p>\n

REVERT<\/h3>\n

\nTo undo the context switch, you can just call REVERT<\/span> anytime:\n<\/p>\n

\n[sql]\nSELECT 'login' AS token_type,* FROM sys.login_token AS LT
\nUNION ALL
\nSELECT 'user' AS token_type,* FROM sys.user_token AS UT;
\nGO
\nREVERT;
\nGO
\nSELECT 'login' AS token_type,* FROM sys.login_token AS LT
\nUNION ALL
\nSELECT 'user' AS token_type,* FROM sys.user_token AS UT;
\n[\/sql]\n<\/div>\n

\nThese switch the security context back using the REVERT<\/span> statement. Before and after the current security tokens are displayed:\n<\/p>\n

\n\"REVERT<\/a>\n<\/p>\n

\nThe new security context after using EXECUTE AS<\/span> stays until either REVERT<\/span> is used or the connection is dropped. If you however executed EXECUTE AS<\/span> inside a procedure, the context is reset automatically at the end of the procedure.\n<\/p>\n

\nMultiple EXECUTE AS<\/span> executions can be nested. Each execution of the REVERT<\/span> statement goes one level back up the stack. To be able to execute EXECUTE AS<\/span> the current execution context needs to have IMPERSONATE<\/span> permission on the target login. It does not matter if the original account had those permissions if another security context is currently active.\n<\/p>\n

Summary<\/h3>\n

\nEXECUTE AS<\/span> and REVERT<\/span> allow you to switch the security context from the login you used to connect to SQL Server to any other login that you have IMPERSONATE<\/span> permission on. This can simplify testing of security settings and permissions.\n<\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

Constantly logging out and back in to test security settings and permissions can be cumbersome. Learn how EXECUTE AS can ease your life.<\/p>\n

[more…]<\/a><\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[29,5,34],"tags":[54,50,38,51,56,15],"class_list":["post-1783","post","type-post","status-publish","format-standard","hentry","category-fundamentals","category-general","category-security","tag-connection","tag-permission","tag-security-2","tag-security-tokens","tag-server-principals","tag-sql-server"],"acf":[],"yoast_head":"\nChanging the Security Context with EXECUTE AS and REVERT - sqlity.net<\/title>\n<meta name=\"description\" content=\"Constantly logging out and back in to test security settings and permissions can be cumbersome. 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