Skip to content

Response, New York. Noah Park: Check Point Patches Actively Exploited Management Server Path Traversal. Detection, Dallas. Ava Okello: Report Finds Nearly Half of Deployed Detections Need Attention Before SOCs Can Trust Them.

SOCtember

Always First. Fast SOC News.

Header of Cisco's security advisory cisco-sa-asaftd-tunnel-dos-s6mgkb75, showing the title, a High severity rating and the advisory details.
Image: Cisco (fair use). The advisory for CVE-2026-76418, first published Oct. 28, 2026.

Response

Cisco Says Attackers Are Exploiting CVE-2026-76418 to Crash ASA and FTD Firewalls When Secondary Authentication Is On the SSL VPN Tunnel Group

The advisory rates the unauthenticated denial-of-service flaw 8.6 and says PSIRT became aware of active exploitation in October. Only devices with a reachable SSL remote access VPN and secondary authentication on a tunnel group are exposed, and Cisco published fixed releases plus a mitigation that strips those secondary checks.

Noah Park, ResponseNew York5 min read

NEW YORK - Cisco said Wednesday that attackers are exploiting a newly disclosed flaw in its Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense software that lets an unauthenticated, remote attacker force a firewall to reload, knocking its remote access VPN offline.

The flaw, CVE-2026-76418, sits in the Remote Access SSL VPN service and carries a CVSS base score of 8.6. "In October 2026, the Cisco PSIRT became aware of active exploitation of this vulnerability," the company's advisory says. Cisco has released fixed software, says there are no workarounds, and has published no indicators of compromise or details of who is behind the attacks or whom they have hit.

Which firewalls are exposed

The bug is narrower than a typical VPN flaw. Cisco says it is caused by "logic errors when secondary authentication is configured on a tunnel group," and that an attacker can trigger it by sending a crafted request while connecting to the SSL remote access VPN. A device is affected only if three things are true: it runs a vulnerable ASA or FTD release, its SSL remote access VPN login service is reachable, and secondary authentication is configured in the tunnel group scope.

The first check is simple. If the running configuration contains the webvpn keyword and SSL VPN is enabled on at least one interface, the VPN side of the condition is met. The second check is where teams can go wrong.

Section of Cisco's advisory explaining how to check tunnel groups, including the default DefaultWEBVPNGroup, for secondary-authentication-server-group.
Image: Cisco (fair use).The tunnel group checks that decide whether a firewall is exposed.

If tunnel-group-list enable is configured, Cisco says administrators must check every tunnel group, including the default DefaultWEBVPNGroup, for secondary-authentication-server-group, and it notes that the check "is not limited to profiles that are displayed in the group list on the login page." If the group list is disabled, the default group still has to be checked, "because disabling the group list does not remove exposure when that default group uses secondary authentication." On FTD, the advisory tells teams to review the deployed remote access VPN policy and the resulting running configuration in Secure Firewall Management Center or Device Manager, and adds that "a successful VPN login is not required to reach the affected processing path."

Cisco lists Cloud-delivered Firewall, Cloud-delivered Firewall Management Center, Secure Access and Secure FMC Software as not affected. The flaw was found by Eric Fernald while resolving a Cisco Technical Assistance Center support case, the advisory says.

Patch, or strip secondary authentication

Fixed release tables from Cisco's advisory for Secure Firewall ASA and Secure FTD software.
Image: Cisco (fair use).First fixed releases for each ASA and FTD train.

The first fixed ASA releases are 9.16.4.104 for 9.16 and earlier, 9.18.4.96, 9.20.4.50, 9.22.3.200, 9.23.1.48, and 9.24.1.27 or 9.24.10.1 for the 9.24 train. For FTD they are 7.0.11 for 7.0 and earlier, 7.2.13, 7.4.9, 7.6.7, 7.7.14, 10.0.3 and 10.1.1. Cisco points customers to its Software Checker to find the combined fixed release across all advisories that apply to a given build.

Where an upgrade has to wait, Cisco describes a mitigation rather than a workaround. If DefaultWEBVPNGroup does not use secondary authentication, administrators can disable the tunnel group list with no tunnel-group-list enable, after which users must know the correct connection profile instead of picking it from a list. If the default group does use secondary authentication, the only mitigation is to remove it with no secondary-authentication-server-group, which Cisco says lowers the security posture of the remote access VPN.

Workarounds section of Cisco's advisory describing two mitigation cases for DefaultWEBVPNGroup.
Image: Cisco (fair use).The two mitigation cases, each with a tradeoff Cisco spells out.

That tradeoff is the decision most SOC and network teams will face this week: keep secondary authentication on the VPN and accept the outage risk until the upgrade lands, or remove them briefly to keep the headend up. Because a successful attack shows up as an unexpected reload rather than a login, teams should also pull reload and crash history for every ASA and FTD VPN headend for October, treat unexplained reloads on devices that match Cisco's configuration conditions as possible exploitation, and record which tunnel groups had secondary authentication at the time. Cisco has not said whether the reloads it investigated were part of a broader intrusion.

Sources:


Noah Park covers incident response and blue-team operations for SOCtember from New York.

Related stories

Response desk