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HIGH

CVE-2026-20135

CVSS v3

8.6

HIGH

EPSS Score

0.5 %

exploit probability, as of 2026-09-28

CISA KEV

No

known exploited

Exploitation

—

SSVC status

Description

A vulnerability in the TLS 1.3 implementation in Cisco Secure Firewall Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause an affected device to reload unexpectedly, resulting in a denial of service (DoS) condition. This vulnerability is due to improper buffer management during the TLS 1.3 connection. An attacker could exploit this vulnerability by sending a crafted TLS 1.3 packet to an affected system through a TLS 1.3-enabled listening socket. A successful

Technical details

Published
2026-09-16
Last Modified
2026-09-18

Frequently asked questions

What is CVE-2026-20135?

A vulnerability in the TLS 1.3 implementation in Cisco Secure Firewall Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause an affected device to reload unexpectedly, resulting in a denial of service (DoS) condition. This vulnerability is due to improper buffer management during the TLS 1.3 connection. An attacker could exploit this vulnerability by sending a crafted TLS 1.3 packet to an affected system through a TLS 1.3-enabled listening socket. A successful

Is CVE-2026-20135 actively exploited?

Active exploitation of CVE-2026-20135 has not been confirmed. Its EPSS score was 0.5% on 2026-09-28, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2026-20135?

CVE-2026-20135 has a CVSS v3 base score of 8.6 (HIGH severity).

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