17th Annual Computer Security Applications Conference
December 10-14, 2001
New Orleans, Louisiana

Practical Automated Filter Generation to Explicitly Enforce Implicit Input Assumptions

Valentin Razmov
University of Washington

Daniel Simon
Microsoft Research

Vulnerabilities in distributed applications are being uncovered and exploited faster than software engineers can patch the security holes. All too often these weaknesses result from implicit assumptions made by an application about its inputs. One approach to defending against their exploitation is to interpose a filter between the input source and the application that verifies that the application’s assumptions about its inputs actually hold. However, ad hoc design of such filters is nearly as tedious and error-prone as patching the original application itself. We have automated the filter generation process based on a simple formal description of a broad class of assumptions about the inputs to an application. Focusing on the back-end server application case, we have prototyped an easy-to-use tool that generates server-side filtering scripts. These can then be quickly installed on a front-end web server (either in concert with the application or when a vulnerability is uncovered), thus shielding the server application from a variety of existing and exploited attacks, as solutions requiring changes to the application are developed and tested. Our measurements suggest that input filtering can be done efficiently and should not be a performance concern for moderately loaded web servers. The overall approach may be generalizable to other domains, such as firewall filter generation and API wrapper filter generation.

Keywords: input validation, filtering script, usability security

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