The previous section described the relationship between Flask and a variety of efforts that involved capability-based systems or the interception of requests. This section describes the relationship between Flask and other efforts not previously mentioned. We focus on the research most directly related to Flask, although there are many other efforts with some relation to our work.
The security architecture of the Flask system is derived from the architecture of our previous prototype system DTOS , which had similar goals. However, while the DTOS security mechanisms were independent of any particular security policy, the mechanisms were not sufficiently rich to support some policies , especially dynamic security policies.
At the highest level of abstraction, the flexible security model for Flask is consistent with the Generalized Framework for Access Control (GFAC) . However, the GFAC model assumes that all controlled operations in the system are performed in the same atomic operation in which the policy is consulted, which is very difficult to achieve in a practical system and is the primary obstacle that the Flask system has had to overcome.
The specific issue of revocation is not a new issue in operating system design, although it has received surprisingly little recognition. Multics  effectively provided immediate revocation of all memory permissions by invalidating segment descriptors. Redell and Fabry , Karger  and Gong  all describe approaches for revoking previously granted capabilities, though none were actually implemented. Spring  implemented a capability revocation technique, though only the capabilities were revoked, not migrated permissions. Revocation of memory permissions is naturally provided by microkernel-based systems with external paging support, such as Mach , though revocation is not extended to other permissions. DTOS provided the security server with the ability to remove permissions previously granted and stored in the microkernel's permission cache. However, except for memory permissions where Mach's mechanisms could be used, DTOS did not provide for revocation of migrated permissions .
The Flask prototype is implemented within a microkernel-based operating system with hardware-enforced address space separation between processes. Several recent efforts (e.g., SPIN , VINO  and the Java protection models in ) have presented software-enforced process separation. The distinction is essentially irrelevant for the Flask architecture. It is essential that some form of separation between processes be provided, but the particular mechanism is not mandated by the Flask architecture. The general applicability of key aspects of the Flask architecture to other systems was concretely demonstrated by the adoption of the DTOS architecture in the security framework of SPIN . Indeed, we believe the abstract Flask architecture, and the lessons it teaches, can be applied to software other than operating systems, such as middleware or distributed systems, although of course vulnerability to insecurities in the underlying operating systems would remain.