External risk intelligence

Cisco Secure Firewall DoS via VPN Reload

CVE advisoryKnown Exploit

CVE-2026-76418

The vulnerability affects SSL VPN services in Cisco Secure Firewall ASA and FTD software. These products are specifically designed as internet-facing edge gateways for remote access, making the affected interface a public-facing service by design in normal deployment scenarios.

Denial of Service

Cisco Secure Firewall Asa Software And Cisco Secure Ftd Software

Halo Surface Signal: 5 out of 5 — more likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A security issue has been identified in Cisco Secure Firewall devices affecting their Remote Access SSL VPN service. This vulnerability could allow an attacker to cause the device to unexpectedly reload, disrupting service for users. The main concern at this time is to confirm if our environment is impacted and to what extent.

An issue could disrupt VPN service availability.
It affects internet-facing remote access.
Confirm relevance and exposure for our network.

Attack Path

How an attacker could exploit the issue

An attacker could target the remote access SSL VPN service on Cisco Secure Firewall devices. By sending a specially crafted request while connecting to the VPN, an attacker could trigger a logic error within the secondary authentication configuration. This could cause the device to unexpectedly reload, leading to a denial of service.

No authentication required.
Triggered by a crafted VPN connection request.
Causes unexpected device reload (Denial of Service).

Live Threat

Current exploitation, exposure, and threat context

When supported by the advisory, an unauthenticated, remote attacker could cause a denial of service condition by sending a crafted request to the SSL remote access VPN. This could result in the affected device reloading unexpectedly.

Device availability and remote access.
Sending crafted VPN requests.
Service disruption.

Operational Fix

Recommended remediation, mitigation, and detection steps

This denial-of-service vulnerability in Cisco Secure Firewall VPN services primarily impacts infrastructure and network security teams responsible for the device's availability and configuration. The immediate first step is to confirm the presence of the affected Cisco devices, assess their external reachability and business criticality, and identify the accountable asset owner to prioritize and plan remediation.

Infrastructure and security teams own the issue.
Verify device exposure and critical business function.
Plan maintenance for remediation.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is Cisco Secure Firewall ASA and FTD software?

These are security platforms used to protect network perimeters. They include VPN services that allow remote users to connect securely to corporate networks. These software packages function as gateways that manage incoming traffic, making them essential infrastructure for maintaining secure, remote connectivity.

What does CWE-690 mean for CVE-2026-76418?

CWE-690 refers to an unchecked return value, which can lead to a NULL pointer dereference. In this CVE, a logic error in how the software processes data means it fails to handle certain inputs correctly. Instead of safely rejecting a bad request, the software encounters an unexpected state, causing the entire device to restart.

How can an attacker trigger this CVE-2026-76418 vulnerability?

An attacker triggers this by sending a specially crafted request during a connection attempt to the SSL remote access VPN. Crucially, the system is only vulnerable when secondary authentication is specifically configured for a tunnel group. If secondary authentication is not enabled, this specific logic error path is not triggered.

Why does Halo Surface Signal categorize this as internet-facing?

Halo Surface Signal identifies this as high-risk because the affected SSL VPN services are designed to be reachable from the internet to facilitate remote access. Because these services must be open to the public to function for their intended users, any device running this software with the specified configuration is naturally exposed.

Do I need to patch my Cisco Secure Firewall immediately?

The first step is to confirm if you are running the affected software version and if you have secondary authentication configured. Once you verify your exposure, coordinate with your infrastructure team to review official Cisco guidance. Prioritize these actions based on the criticality of your VPN services to business operations.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.