CVE-2024-20353: Cisco ASA and FTD Denial of Service Vulnerability

Should you act?

Exploited: patch now if you run it

CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Yes, if you run Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD). CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2024-04-24, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2024-05-01; treat that as the outer limit, not the target.
• EPSS 70.7%, percentile 99: FIRST’s model estimates a 70.7% chance of exploitation activity in the next 30 days, higher than 99% of all scored CVEs (very high). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.

What it is

A vulnerability in the management and VPN web servers for Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause the device to reload unexpectedly, resulting in a denial of service (DoS) condition. This vulnerability is due to incomplete error checking when parsing an HTTP header. An attacker could exploit this vulnerability by sending a crafted HTTP request to a targeted web server on a device. A successful exploit could allow the attacker to cause a DoS condition when the device reloads.

CISA’s summary

Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) contain an infinite loop vulnerability that can lead to remote denial of service condition.

Weakness type: CWE-835.

Product: Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) · Assigned by: cisco · Published: 2024-04-24

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: changed (can affect other components)

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: added 2024-04-24; federal due date 2024-05-01. Known ransomware use: Unknown.
• CISA required action: Apply mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable.
• CISA SSVC assessment: exploitation active, automatable yes, technical impact partial.
• FIRST EPSS (model of 2026-10-21): 70.69% probability of exploitation activity in the next 30 days; percentile 99.

Timeline

• 2024-04-24 — Added to CISA’s Known Exploited Vulnerabilities catalog
• 2024-04-24 — CVE record published by cisco
• 2024-05-01 — CISA remediation deadline for US federal agencies
• 2025-10-21 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

1. Listed in CISA KEV notes (sec.cloudapps.cisco.com) sec.cloudapps.cisco.com

Vulnerability records

1. CVE.org record CVE-2024-20353 cve.org
2. NVD entry CVE-2024-20353 nvd.nist.gov

Government

1. CISA Known Exploited Vulnerabilities catalog entry cisa.gov

Other references from the CVE record and advisory

1. blog.talosintelligence.com blog.talosintelligence.com

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