Firewalls and Security Groups
A rule is source, destination, port, protocol and action; every broad wildcard is an explicit expansion of attack surface.
Frame the problem
Security starts with a concrete asset, attacker capability and trust crossing.
Why the system fails
Rules allow the internet to reach a database or use broad source groups that silently grow over time.
The important question is not “what is Firewalls and Security Groups?” but “which assumption let untrusted data or an over-scoped identity cross network source → destination socket?” Trace the decision at the boundary, then constrain what can happen after the first control fails.
Design the control in layers
Start with the control closest to the interpretation or privilege boundary: Default deny and allow exact workload identities or narrow sources Then add a control that reduces blast radius and telemetry that proves the decision was enforced.
The resulting design is not labelled secure. Record the identified controls, the known failure paths, the remaining exposure, and the evidence you would need during an incident.
| Prevent | Detect | Recover |
|---|---|---|
| Default deny and allow exact workload identities or narrow sources · Review rule ownership and expiry · Keep databases private | Flow logs for denied and unusual allowed traffic | Contain the affected identity or component, scope impact from audit evidence, and preserve a regression test. |
Key points
- Asset: Service reachability and the paths into sensitive networks.
- Boundary: Network source → destination socket
- Primary control: Default deny and allow exact workload identities or narrow sources
- Detection signal: Flow logs for denied and unusual allowed traffic
- Always ask what limits damage when the primary control fails.
Boundary control exercise
This lesson uses the shared boundary-control exercise.
Follow the attack
Safe conceptual simulation: capability → missing control → crossed boundary → asset impact.
- 1Attacker starts with: Any host in an allowed source range, including a compromised one.
- 2Rules allow the internet to reach a database or use broad source groups that silently grow over time.
- 3The weak or missing boundary control is crossed: Network source → destination socket
- 4Impact: Direct service attack or lateral access to sensitive ports.
- Direct service attack or lateral access to sensitive ports.
Defend, detect, recover
One prevention is a single point of security failure. Layer it and make failure observable.
- • Default deny and allow exact workload identities or narrow sources
- • Review rule ownership and expiry
- • Keep databases private
- • Flow logs for denied and unusual allowed traffic
- • Contain the affected identity or component.
- • Scope access from audit evidence.
- • Fix the boundary and add a regression test.
- • Misconfiguration and new access paths can bypass the intended control.
- • A privileged insider or compromised control plane may still reach the asset.