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CVE-2026-20349 – Unauthenticated Remote DoS – Cisco Secure Firewall ASA & FTD SSL VPN

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Summary

CVE-2026-20349 is a high-severity denial-of-service vulnerability in the Remote Access SSL VPN service of Cisco Secure Firewall ASA and Threat Defense (FTD) software. An unauthenticated, remote attacker can send a crafted HTTP request to the affected service to force the device to reload, cutting off both perimeter enforcement and remote access. Cisco has confirmed active exploitation of this vulnerability in the wild, and it carries a CVSS score of 8.6.

Technical details

  • Root cause: insufficient error checking when the SSL VPN service processes HTTP requests (tracked as CWE-244, Improper Clearing of Heap Memory Before Release).
  • Trigger condition: a device is vulnerable when it runs an affected software release and has IKEv2 Remote Access VPN (with client services), SSL VPN (WebVPN), or Zero Trust Network Access enabled, meaning its SSL listen sockets are active.
  • Attack vector: network-reachable, no authentication or user interaction required; the attacker sends a specially crafted HTTP request directly to the exposed VPN service.
  • Impact: forces the device to reload, producing a denial-of-service condition that removes both the firewall’s perimeter enforcement and the remote-access VPN path.
  • Cisco identified exploitation attempts in August 2026 and states the issue was found both during internal testing and independently reported by a security researcher.

Affected software

  • Cisco Secure Firewall ASA Software: releases in the 9.16, 9.18, 9.20, 9.22, 9.23, and 9.24 trains (approximately 9.16.1 through 9.20.4.46 and later trains) with an affected VPN feature enabled.
  • Cisco Secure Firewall Threat Defense (FTD) Software: releases in the 7.0, 7.2, 7.4, 7.6, 7.7, and 10.0 trains with an affected VPN feature enabled.
  • Only devices with IKEv2 Remote Access VPN (client services), SSL VPN, or ZTNA enabled are affected.

Severity

CVSS v3.1 Base Score: 8.6 (High)
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H

Mitigation and recommended actions

  • Immediate: Apply the Cisco-provided hotfixes for the affected ASA (9.16, 9.18, 9.20, 9.22, 9.23, 9.24) and FTD (7.0, 7.2, 7.4, 7.6, 7.7, 10.0) trains, obtained through the Cisco Software Center, and upgrade to a fixed release as advised in Cisco’s security advisory (cisco-sa-asaftd-vpn-dos-dzv4mQFF).
  • No workarounds are available; Cisco has not published a configuration-based mitigation for this vulnerability.
  • If patching cannot be performed immediately, consider restricting or monitoring exposure of the Remote Access SSL VPN, IKEv2 VPN, and ZTNA listening interfaces from the internet until the hotfix is applied.
  • This CVE has been added to CISA’s Known Exploited Vulnerabilities catalog; prioritize remediation accordingly.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • Asset URL/path: /+CSCOE+/logon.html, /+CSCOE+/logon.html#form_title_text, /+CSCOE+/saml/sp/metadata, /+CSCOE+/logout.html
  • Raw response body: /+CSCOU+/portal.css, /+CSCOU+/csco_logo.gif, /+CSCOU+/login-header-middle.jpg together with "SSL VPN Service", and document.location.replace("/+CSCOE+/logon.html"
  • Rendered page: an <img src="/+CSCOU+/..."> element with align="absmiddle", alt="Logo", title="Logo"

References

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