mediumMultiple Choice
ISC2 CC Practice Question: Refer to the exhibit
Exhibit
Refer to the exhibit. === syslog output === Jan 15 09:23:45 firewall01 %ASA-4-106023: Deny tcp src outside:192.0.2.10/3456 dst inside:10.0.0.5/22 by access-group "OUTSIDE_IN" [0x0, 0x0] Jan 15 09:23:46 firewall01 %ASA-4-106023: Deny tcp src outside:192.0.2.10/3457 dst inside:10.0.0.5/23 by access-group "OUTSIDE_IN" [0x0, 0x0] Jan 15 09:23:47 firewall01 %ASA-4-106023: Deny tcp src outside:192.0.2.10/3458 dst inside:10.0.0.5/80 by access-group "OUTSIDE_IN" [0x0, 0x0] Jan 15 09:23:48 firewall01 %ASA-4-106023: Deny tcp src outside:192.0.2.10/3459 dst inside:10.0.0.6/22 by access-group "OUTSIDE_IN" [0x0, 0x0] Jan 15 09:23:49 firewall01 %ASA-4-106023: Deny tcp src outside:192.0.2.10/3460 dst inside:10.0.0.6/23 by access-group "OUTSIDE_IN" [0x0, 0x0]
Refer to the exhibit. A security analyst is reviewing firewall logs and notices repeated denied TCP packets from 192.0.2.10 to internal hosts. The packets are being denied by the access-group "OUTSIDE_IN". What is the most likely reason for these denials?
⚠ Common exam trap
ISC2 often tests the distinction between a port scan and a DoS attack, where candidates mistakenly choose DoS because they see 'repeated denied packets' without recognizing the pattern of multiple destinations versus a single target flood.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
An external host is performing a port scan against internal systems.
The repeated denied TCP packets from 192.0.2.10 (an external IP) to multiple internal hosts indicate a port scan. The access-group 'OUTSIDE_IN' is applied to the outside interface, and the firewall is denying these packets because they match a deny ACE (access control entry) that blocks unsolicited inbound traffic. This pattern of multiple denied connections from a single external source to different internal destinations is characteristic of a reconnaissance scan, not a DoS attack or data exfiltration.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
An external host is performing a port scan against internal systems.
Why this is correct
Repeated denied TCP packets from a single external address to multiple internal hosts, blocked inbound by the OUTSIDE_IN access-group, indicate reconnaissance. Sequential connection attempts across ports and hosts match port-scanning behaviour rather than legitimate traffic or misconfiguration.
- ✗
The firewall is misconfigured and blocking legitimate traffic.
Why it's wrong here
The OUTSIDE_IN access-group denying traffic from an external address is expected perimeter behaviour, so no misconfiguration is evidenced; a misconfigured ACL would typically block legitimate internal-to-external or established flows. It tempts because ACL errors do cause false denials, but the exhibit shows deliberate inbound filtering working correctly.
- ✗
An internal host has been compromised and is exfiltrating data.
Why it's wrong here
Exfiltration would appear as outbound traffic from an internal host leaving the network, logged by an inside-to-outside ACL, not inbound packets from 192.0.2.10 denied by OUTSIDE_IN. It tempts because compromised hosts are a real concern, but the direction and source address here contradict that scenario.
- ✗
An external host is launching a denial-of-service (DoS) attack.
Why it's wrong here
Denied inbound TCP to internal hosts indicates the OUTSIDE_IN ACL is functioning as designed, not a DoS: a DoS floods accepted or half-open connections rather than triggering repeated ACL denies. It tempts because volumetric attacks do generate many packets, but those would be permitted or rate-limited, not denied by an inbound access-group.
Visual reference
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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