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Cisco Secure Firewall: Snort 3 Detection Engine DoS (CVE-2025-20217) By CyberDudeBivash – Ruthless, Engineering-Grade Threat Intel

 


1) What happened

Cisco disclosed a high-severity Denial-of-Service vulnerability in the Snort 3 Detection Engine used by Cisco Secure Firewall Threat Defense (FTD). Crafted traffic can push the Snort process into an infinite loop (CWE-835), halting inspection and triggering a watchdog restart—resulting in inspection downtime and potential traffic handling disruption. CVSS 8.6 (High). No workarounds; patched software is available. Cisco

2) Why you should care (risk at a glance)

  • Unauthenticated, remote trigger via crafted network traffic passing through the device. CiscoNVD

  • Affects FTD appliances with Intrusion Policy enabled and Snort 3 active (Snort 3 must be the running engine to be exploitable). Cisco

  • Impact: traffic inspection stalls while Snort loops/restarts → DoS on security services; repeated triggers can cause recurring outages. Cisco

3) Technical breakdown

Root cause: Incorrect processing of certain inspected packets in Snort 3 → unbounded loop in the detection engine. (Classified as CWE-835: loop with unreachable exit.) CiscoZeroPath

Exploit conditions:

  • Device is running a vulnerable FTD release.

  • Snort 3 engine is active in the intrusion policy.

  • Attacker can send crafted traffic through the device. Cisco

What you’ll observe:

  • Snort process restart by the system watchdog; gaps in IPS/IDS visibility; possible CPU spikes prior to restart. Cisco

4) Affected / not affected

  • Affected: Cisco Secure Firewall Threat Defense (FTD) releases called out in Cisco’s “Fixed Software” table; only when Snort 3 is active in the policy. Cisco

  • Not affected: ASA, FMC, Meraki/Umbrella, and open-source Snort packages (per Cisco advisory). Cisco

5) Detection & hunting (Blue-Team playbook)

Immediate checks

  1. Confirm Snort version in use on each FTD (Cisco provides steps to verify which engine is active). Cisco

  2. In FMC/Syslog, hunt for:

    • Repeated Snort restarts / crash-recovery messages (watchdog).

    • Sudden drops in intrusion event volume coincident with CPU spikes. (Inference based on Cisco’s watchdog restart behavior.) Cisco

  3. NetFlow/PCAP around outage windows—look for repeated crafted sequences from the same source (possible probing/fuzzing).

Ongoing monitoring

  • Create SIEM rules for multiple Snort restarts within N minutes on the same sensor.

  • Alert on inspection downtime or policy engine not running states after policy deploys.

6) Mitigation & fixes

There are no configuration workarounds. Patch. Cisco has released fixed FTD software—use the Cisco Software Checker to map your exact release to the first fixed version and plan upgrades. Cisco

Upgrade plan (practical):

  1. Inventory all FTD versions + confirm Snort3 active. Cisco

  2. Use Cisco Software Checker to get your First Fixed/“Combined First Fixed” versions; schedule rolling upgrades per HA pair/cluster. Cisco

  3. Pre-upgrade: export FMC backups; stage images; review release notes & compatibility. Cisco

  4. Post-upgrade validation: confirm Snort health, policy status, throughput, and event flow.

Interim risk-reduction (if patching window is constrained):

  • Rate-limit or geo-fence untrusted ingress segments most likely to carry crafted fuzz traffic (compensating control; not a fix).

  • Tighten edge ACLs/WAF where feasible to reduce exposure to malformed patterns (until patch is applied). (General compensating practice; Cisco provides no official workaround for this CVE.) Cisco

7) MITRE ATT&CK mapping

  • TA0040 Impact → T1499 DoS (Network/Service): Adversary degrades security service availability by forcing repeated Snort restarts.

8) Executive briefing (copy-ready)

  • What: High-severity DoS in Cisco FTD Snort 3 (CVE-2025-20217; CVSS 8.6). CiscoNVDCisco

  • Risk: Remote unauthenticated traffic can stall inspection; watchdog restarts create visibility gaps. Cisco

  • Fix: Upgrade to Cisco’s fixed releases; no workaround. Use Software Checker to find your first fixed version. Cisco

  • Action today: Verify Snort 3 usage, prioritize internet-facing sensors, schedule patches, and monitor for repeated Snort restarts.

9) References (read this first)

  • Cisco Security Advisory cisco-sa-ftd-dos-SvKhtjgt (summary, impact, fixed software, no workarounds). Cisco

  • NVD: CVE-2025-20217 (public CVE record & description). NVD

  • Cisco event response summary (bundle, CVSS 8.6). Cisco

  • Technical write-up on the infinite loop/CWE-835 behavior. ZeroPath

10) CyberDudeBivash recommendations

  • CISOs: Treat as patch-now for all internet-exposed FTDs running Snort 3.

  • Blue Teams: Build an “IPS health” dashboard (Snort restarts, inspection gaps, CPU) and enable SIEM alerts for recurrence.

  • Red/Purple Teams: Validate controls by replaying benign malformed sequences in a lab to confirm sensors no longer flap post-patch.

  • Ops: Lock in upgrade runbooks for FTD clusters/HA pairs; test failover before production windows.

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#Cisco #Snort3 #FTD #CVE202520217 #DoS #ThreatIntel #CyberDudeBivash

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