Professional Microsoft IIS 8

A Background of Website Separation

Back in the days of IIS 5.0, applications could be placed in one of three isolation modes: low, medium, or high. Low and medium isolation placed all websites in a shared pool that utilized a common user identity and security context. Failures that affected one site often would break other sites that were set to low or medium isolation, requiring a reset of IIS to fix the troubled sites. Those sites in high isolation faired better by partially protecting some sites from one another, but each high-isolation application had an extra memory footprint, and the user identities were shared just as they were in low and medium isolation. This usually meant that a shared Windows identity needed to be granted Read permissions to all sites on the server, creating potential security vulnerabilities.

IIS 6.0 introduced an excellent solution for this by implementing application pools. This allowed the system administrator to create pools of applications bundled together into groups as the administrator saw fit. With this enhancement, it was possible to completely separate sites from one another so that a serious failure in one application wouldn't compromise sites or applications in other pools. In addition, each application pool defined the Windows user identity under which the applications would run, allowing complete separation from a security point of view between application pools.

IIS 7.0, IIS 7.5, and IIS 8.0 build on the strong foundation of application ...