Error message in WSS 3.0: "The security validation for this page is invalid"

Source: Microsoft Support

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Calling BreakRoleInheritance(false)on a List, internally flips the AllowUnsafeUpdates flag to false, and this may lead to below mentioned error, if we do not explicitly set the AllowUnsafeUpdates to true again.

The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again.

When any object that implements ISecurable (those are SPWeb, SPList and SPListItem) breaks or reverts their role definition inheritance. This means every time you call SPRoleDefinitionCollection.BreakInheritance(), BreakRoleInheritance(), ResetRoleInheritance() or set the value of HasUniquePerm the AllowUnsafeUpdates property of the parent web will reset to its default value and you may need to set it back to true in order to do further updates to the same objects.

If we intend to do SPWeb and SPSite modifications after a call to BreakRoleInheritance for a List, a recommended usage is to couple it with an explicit setting of AllowUnsafeUpdates property to true for the web & site again.

BreakRoleInheritence API implements ISecurable interface, a call to this API internallu invalidates the context of the thread and reinitiates it.This is for Security Measures and to prevent Cross-site scripting.

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