CVE-2026-20250: Cisco Secure Firewall ASA and FTD for Firepower 3100 and 4200 Series DTLS Denial of Service Vulnerability
Tech GuruQuick Answer
CVE-2026-20250 can let crafted unauthenticated DTLS traffic reload Cisco Secure Firewall 3100 or 4200 Series devices running vulnerable ASA or FTD software with DTLS flow offload enabled. Offload is enabled by default. Disabling it is Cisco's workaround, but shifts processing to software and may raise CPU use or reduce throughput; a platform-specific fixed release is still required.
Fix Or Immediate Action
What to do now: Confirm the affected product, release, and configuration. Apply Cisco's documented workaround as temporary risk reduction. Enter the exact installed product, platform, and release in Cisco Software Checker and upgrade to the returned First Fixed release. The CVE-specific live advisory and Cisco Software Checker result control the First Fixed release; any combined hardening baseline is supplemental only.
Last verified: 2026-09-17 UTC from Cisco's advisory and CSAF record. Recheck both the affected-products and fixed-software sections before a production change.
Context
Scope And Impact In Plain Language
Who should check: Operators of Secure Firewall 3100 and 4200 Series devices running ASA or FTD. Confirm the actual hardware series before applying this DTLS flow-offload notice.
When this applies: This vulnerability affects Cisco Secure Firewall 3100 Series and 4200 Series devices if they are running a vulnerable release of Cisco Secure Firewall ASA or Secure FTD Software and have DTLS flow offload enabled. DTLS flow offload is enabled by default. For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory. Determine Whether DTLS Flow Offload Is Enabled To determine whether DTLS flow offload is enabled on a Cisco Secure Firewall 3100 Series or 4200 Series device, use the show flow-offload-dtls info CLI command, as shown in the following example: firewall# show flow-offload-dtls info DTLS offload : Enabled Egress Optimization: Enabled
What could happen: On a vulnerable device with DTLS flow offload enabled, an unauthenticated remote attacker may send crafted DTLS traffic that causes a reload. Offload is enabled by default, but the installed release and hardware scope must also match.
Not sure? Record the hardware series, software build, and DTLS offload state. Before using the retained disable-offload workaround, assess session volume and CPU headroom: Cisco warns that moving DTLS processing into software can reduce throughput and increase CPU use.
Status At A Glance
| Product scope | Cisco Secure Firewall ASA / FTD |
|---|---|
| CVE | CVE-2026-20250 |
| TechGeeks response tier | P1 — urgent |
| Publication | New final Cisco advisory |
| First published | 2026-09-16T16:00:00+00:00 |
| Last updated | 2026-09-16T16:00:00+00:00 |
| Cisco advisory SIR | High |
| CVE-specific Cisco SIR | Not separately stated |
| CVSS base score / severity | 8.6 — High |
| CVSS vector | CVSS v3.1 — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H |
| CISA KEV at last check | No — this is separate from Cisco severity and is not proof of no exploitation. |
| Cisco exploitation statement | The Cisco Product Security Incident Response Team (PSIRT) is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory. |
What Happened
ASA and FTD mishandle resources while processing certain DTLS messages on affected 3100 and 4200 Series appliances. A crafted stream of DTLS traffic can force a reload and interrupt service when DTLS flow offload is enabled. Cisco provides software fixes and a disable-offload workaround. The retained procedure explains the FTD FlexConfig option and warns that software processing can be especially costly at high DTLS session volumes.
Affected Scope
This vulnerability affects Cisco Secure Firewall 3100 Series and 4200 Series devices if they are running a vulnerable release of Cisco Secure Firewall ASA or Secure FTD Software and have DTLS flow offload enabled. DTLS flow offload is enabled by default. For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory. Determine Whether DTLS Flow Offload Is Enabled To determine whether DTLS flow offload is enabled on a Cisco Secure Firewall 3100 Series or 4200 Series device, use the show flow-offload-dtls info CLI command, as shown in the following example: firewall# show flow-offload-dtls info DTLS offload : Enabled Egress Optimization: Enabled
Match the exact hardware or virtual platform, installed train, patch level, enabled service, role, and management or data-plane exposure. A Cisco product-family name alone does not establish applicability.
Steps
What To Do Now
- Confirm. Inventory Cisco Secure Firewall ASA / FTD and record platform, exact release, patch level, enabled feature, role, and exposure.
- Preserve. Save UTC context plus off-device authentication, management, firewall, network, endpoint, and SIEM evidence before logs rotate or a disruptive change removes context.
- Scope. Compare each asset with the live Cisco advisory, including configuration prerequisites and exclusions.
- Reduce exposure. Evaluate and apply Cisco's temporary workaround in the highlighted panel below; it does not replace the permanent fix.
- Remediate. Follow the CVE-specific method and release guidance under Permanent Fix below.
- Validate. Confirm the resulting version, service health, logging, expected access, an expected denial, failover or cluster state, and representative dependent workflows.
- Escalate. If evidence or exposure creates concern, open incident response and Cisco TAC recovery work rather than treating patch completion as proof of trust.
Fixed Releases And Mitigation
Permanent Fix
Cisco method: Use Cisco Software Checker with the exact installed product, platform, and release to obtain the advisory-specific First Fixed release; the combined September hardening baseline is also recorded.
No compact per-CVE fixed-release table is reproduced here because Cisco may use different First Fixed values by CVE and software train. Use this CVE's live advisory table or Cisco Software Checker; do not infer a fixed release from a combined hardening baseline or another Cisco notice.
Workaround Or Temporary Mitigation
There is a workaround that addresses this vulnerability. Use the no flow-offload-dtls CLI command to disable DTLS flow offload. Note: For devices that are running Cisco FTD Software, the no flow-offload-dtls command can be pushed using FlexConfig. Disabling DTLS flow offload will cause DTLS-encrypted traffic to be processed in software rather than hardware, which may reduce throughput and increase CPU utilization on the affected device. This impact will be more significant in deployments with a high volume of DTLS sessions. While this workaround has been deployed and was proven successful in a test environment, customers should determine the applicability and effectiveness in their own environment and under their own use conditions. Customers should be aware that any workaround or mitigation that is implemented may negatively impact the functionality or performance of their network based on intrinsic customer deployment scenarios and limitations. Customers should not deploy any workarounds or mitigations before first evaluating the applicability to their own environment and any impact to such environment.
Important boundary: A workaround, ACL, exposure reduction, or service restriction can reduce risk while the update is prepared, but it does not patch the vulnerable code, prove every attack path is closed, or erase an earlier compromise.
Investigation And Recovery
Review management authentication, account and privilege changes, API and web requests, configuration changes, crashes or restarts, resource pressure, and relevant external network telemetry for the vulnerable period. Preserve original evidence and follow Cisco TAC or incident-response guidance for containment, credential rotation, trusted rebuild, or service restoration when findings warrant it.
Notes
Exploitation Evidence
Cisco's exact status at the last check: The Cisco Product Security Incident Response Team (PSIRT) is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory.
This CVE was not present in the TechGeeks CISA KEV snapshot at the last check. Cisco's 'not aware' statement and absence from KEV are time-bounded evidence, not claims that exploitation is impossible or has never happened. Severity and exploitation status remain separate signals.
Validation And Boundaries
This notice is documentation-backed. TechGeeks did not reproduce the exploit or independently test every fixed build. It does not prove that all members of the product family are affected, that the activity is widespread, that a clean indicator search excludes compromise, or that an update restored trust. Cisco's live advisory controls affected and fixed release information.
Related TechGeeks Resources
- TechGeeks Security Notice Center
- All Security Notices
- What to Do When a Device Hits CISA KEV
- Router CVE Exposure, Version, and Mitigation Checklist
- IoT Firmware Update and End-of-Support Checklist
- Live Protect and Runtime Vulnerability Shielding
References
Wrap-Up
Record whether CVE-2026-20250 applies. If it does, assign an owner and deadline appropriate to the High Cisco advisory SIR, preserve the evidence, complete Cisco's remediation, and document the validated result.
Correction policy: A material Cisco revision to scope, severity, exploitation status, workarounds, fixed software, or recovery guidance triggers an update and a new verification date.
