Expect Regressions

Published: 2022-01-01
Last Updated: 2022-01-01 22:10:34 UTC
by Didier Stevens (Version: 1)
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Last year, my professional relationship with computers entered its 30th year (in other words: for 30 years now, I'm getting paid to work with computers).

One of the things I learned over this period is: "in IT, expect regression".

With regression, I mean this definition: "the process of going back to an earlier or less advanced form or state".

I've seen this happen several times with IT security systems.

For example, a proxy that is configured to block certain web site categories, no longer blocks these web sites, but grants access. It happens for various reasons, but typically, it will happen when a configuration change is made. For example, a new category is supposed to be blocked, but this new catagory is added to an onbsolete configuration file, that is then pushed to the proxies. Result: previous categories that were blocked, are no longer blocked.

Another example: a firewall that is supposed to block all egress traffic, except for typical web traffic ports like 80, 443, ..., no longer drops this traffic. This too happened with a configuration change, this time under the assumption that the egress blocking would be done by another network device.

What is typical about such regressions: you don't notice them immediately, and staff will not create helpdesk tickets for regressions that don't hinder them. If users are all of a sudden granted access to a web site that used to be blocked, they will not contact the helpdesk to report this ...

Such regressions should be catched by proper release management, but in many cases that I observed, solid release management was in place at that organisation.

Over the years, this has thaught me one thing: "expect unannounced regressions to happen".

This has changed my behavior in 2 ways:

1) I strive to conduct regular regression tests: check that security policies that are supposed to be enforced, are still enforced

2) When performing incident response, when in doubt that a certain security policy is truly enforced, test it or gather evidence to the contrary.


Please post a comment if you have examples of unexpected regressions that you've seen happen during your job.


Best wishes for 2022 from all of us at the SANS Internet Storm Center!


Didier Stevens
Senior handler
Microsoft MVP

Keywords: regressions
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