Unauthenticated remote DoS against Cisco Secure Firewall FTD — widely deployed enterprise firewall; can reload devices and disrupt network security infrastructure.
CVE-2026-20135
TLS 1.3 parsing bug in Cisco FTD allows crafted packets to crash device (DoS).
Is CVE-2026-20135 being exploited?
Not confirmed. CVE-2026-20135 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.67% probability of exploitation in the next 30 days.
How severe is CVE-2026-20135?
CVE-2026-20135 is rated High with a CVSS score of 8.6. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-20135?
Not known from our data. No patch reference has been recorded for CVE-2026-20135, which is not the same as no patch existing — a fix may have shipped without a tagged reference, or after this record was written. The vendor advisory is the only authority on whether a fix exists, and mitigations may be available regardless.
What does CVE-2026-20135 affect?
CVE-2026-20135 affects Cisco Secure Firewall Threat Defense (FTD) Software. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-20135?
Apply vendor advisory guidance; update ASAP or restrict/expose TLS1.3 listeners.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Remote unauthenticated attacker can send a crafted TLS1.3 packet to crash the LINA process and force device reload, causing network/firewall DoS.
Patch when possible
- affected7.0.0
- affected7.0.0.1
- affected7.0.1
- affected7.0.1.1
- affected7.0.2
- affected7.2.0
- affected7.0.2.1
- affected7.0.3
- affected7.2.0.1
- affected7.0.4
- affected7.2.1
- affected7.0.5
- affected7.3.0
- affected7.2.2
- affected7.2.3
- affected7.3.1
- affected7.2.4
- affected7.0.6
- affected7.2.5
- affected7.2.4.1
- affected7.3.1.1
- affected7.4.0
- affected7.0.6.1
- affected7.2.5.1
- affected7.4.1
- affected7.2.6
- affected7.0.6.2
- affected7.4.1.1
- affected7.2.7
- affected7.2.5.2
- affected7.3.1.2
- affected7.2.8
- affected7.6.0
- affected7.4.2
- affected7.2.8.1
- affected7.0.6.3
- affected7.4.2.1
- affected7.2.9
- affected7.0.7
- affected7.7.0
- affected7.4.2.2
- affected7.2.10
- affected7.6.1
- affected7.4.2.3
- affected7.0.8
- affected7.6.2
- affected7.7.10
- affected7.7.11
- affected7.0.8.1
- affected7.6.2.1
- affected7.7.10.1
- affected7.4.2.4
- affected7.2.10.2
- affected7.4.3
- affected7.6.4
- affected7.4.4
- affected7.0.9
- affected7.2.11
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.
cisa ssvc
- exploitation
- none
- automatable
- yes
- technical impact
- partial
Assessed by CISA 17 Sept 2026 (SSVC 2.0.3).
timeline
- 16 Sept 2026Published
- 17 Sept 2026CISA SSVC assessment: exploitation none · 1 day later
- 17 Sept 2026CVE record last updated · same day later
- 18 Sept 2026Analysed by vulneraibility.com · 1 day later
- Linksec.cloudapps.cisco.com /security/center/content/CiscoSecurityAdvisory/cisco-sa-ftd-tls1.3-dos-dLxwFWgF
sources
- CVE List (CVE.org): description, affected versions, references; CVE Terms of Use
- NVD: CVSS and CPE data
- CISA Vulnrichment (ADP): SSVC decision points
- FIRST EPSS: exploit probability and percentile
- Summary, impact and triage tags: written by our AI analysis pipeline from the sources above. Check them against the sources.