We recently upgradedf from SPS 2003 to MOSS 2007. In the old portal, I had a couple of user controls running via the SmartPart. After installing Return of SmartPart and adding the controls, I got an error every time I tried to search. Way back in the dusty cobwebs, I remember troubleshooting this, and it was a permissions issue. Checking in the Applications log, I found the following error: "sharepoint Request for the permission of type 'System.Data.SqlClient.SqlClientPermission, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.".
Here's the fix:
Cause: Anything that access database from SP requires at least the WSS_Medium security policy in the web.config file. If you receive a security message from the web part, it's usually the trust element in the web.config file.
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Open wss_mediumtrust.config & wss_minimaltrust.config usually (C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\config\) look in your web.config file for the exact path.
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Find in wss_mediumtrust.config: <SecurityClass Name="SqlClientPermission" Description="System.Data.SqlClient.SqlClientPermis sion, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
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Copy and paste it in to the <SecurityClasses> node of wss_minimaltrust.config.
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In the PermissionSet section of this configuration file, add the following: (Find in wss_mediumtrust.config) <IPermission class="SqlClientPermission" version="1" Unrestricted="true"/>. Copy and paste it in to the a <PermissionSet> node of wss_minimaltrust.config.
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Save your wss_minimaltrust.config
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Reload your site, and try again
Presto! Original source is http://objectmix.com/sharepoint/297402-request-permission-type-system-data-sqlclient-sqlclientpermission-system-data-version-2-0-0-0-culture-neutral-publickeytoken-b77a5c561934e089-failed.html.