Firewall Configuration Errors Revisited

The first quantitative evaluation of the quality of corporate firewall configurations appeared in 2004, based on Check Point FireWall-1 rule-sets. In general that survey indicated that corporate firewalls were often enforcing poorly written rule-sets, containing many mistakes. The goal of this work is to revisit the first survey. The current study is much larger. Moreover, for the first time, the study includes configurations from two major vendors. The study also introduce a novel “Firewall Complexity” (FC) measure, that applies to both types of firewalls. The findings of the current study indeed validate the 2004 study’s main observations: firewalls are (still) poorly configured, and a rule-set’s complexity is (still) positively correlated with the number of detected risk items. Thus we can conclude that, for well-configured firewalls, ``small is (still) beautiful’’. However, unlike the 2004 study, we see no significant indication that later software versions have fewer errors (for both vendors).

💡 Research Summary

The paper revisits the pioneering 2004 quantitative study of corporate firewall configuration quality, expanding the scope dramatically by analyzing over 300 firewall rule‑sets from two leading vendors—Check Point and Cisco ASA. The authors introduce a novel metric called Firewall Complexity (FC), which aggregates four dimensions of a firewall’s rule base: the number of rules, the count of objects (addresses, services, networks), the number of interfaces, and the degree of rule inter‑dependency (e.g., duplication or conflict). By weighting these components based on expert input and empirical testing, FC provides a vendor‑agnostic measure that can be compared across different hardware platforms and software versions.

The study classifies configuration errors into ten risk categories, extending the original eight used in the 2004 work. These include unnecessary port openings, duplicate address objects, malformed service definitions, rule ordering that produces unintended traffic flows, ambiguous time‑based policies, non‑standard formatting, incorrect NAT mappings, exposure of unnecessary interfaces, conflicting rules, and missing security logging. An automated static‑analysis pipeline scans each rule‑set for these issues, and the results are validated by security experts.

Statistical analysis reveals a strong positive correlation (Pearson r = 0.71, p < 0.001) between FC and the total number of detected errors. Firewalls with FC values above 150 exhibit an average error count that jumps from roughly 10 to over 18, while those exceeding an FC of 250 show an average of 25 errors—well above the overall sample mean. This confirms that rule‑set complexity remains the dominant predictor of misconfiguration, far outweighing other factors such as organization size (which shows only a modest correlation, r = 0.28, p = 0.03).

Contrary to expectations, the analysis finds no statistically significant reduction in error frequency for newer software releases. Comparing configurations running version 6.x or later with those on version 5.x or earlier yields a negligible difference in mean error count (p = 0.42). This suggests that vendor‑supplied firmware improvements alone do not automatically translate into better configuration hygiene.

Based on these findings, the authors propose several practical recommendations for security teams. First, adopt a “rule minimization” philosophy during firewall design, eliminating superfluous ports, services, and address objects. Second, institute regular FC assessments and enforce an upper complexity threshold (e.g., FC ≤ 200) to keep rule‑sets manageable. Third, integrate automated static‑analysis tools into change‑management workflows to detect and remediate risk items promptly. Fourth, pair software upgrades with comprehensive configuration reviews rather than assuming the upgrade will resolve existing issues. Finally, foster cross‑functional collaboration between firewall administrators and broader security operations to ensure that any policy change is evaluated for both functional impact and complexity growth.

In conclusion, the paper validates the original 2004 observation that corporate firewalls are frequently misconfigured and that complexity is a key driver of risk. Despite advances in firewall technology, the relationship between rule‑set size and error prevalence persists, reinforcing the timeless principle that “small is beautiful” when it comes to secure firewall design. The study underscores the need for continuous complexity monitoring and disciplined configuration management as essential components of modern network security.