CVE-2024-20353
CVE-2024-20353 is a security flaw in Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD), Cisco’s firewall. Attackers can crash Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) or stop it working from anywhere, without logging in. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 24 Apr 2024.[1]
Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed in version 9.14.4.23 of asa.
Description
CISA calls it an ASA and FTD denial of service in Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD). In its words: “Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) contain an infinite loop vulnerability that can lead to remote denial of service condition.”[1]
The CVE record adds: “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.”[2]
The underlying weakness is CWE-835, in MITRE’s list of common software weaknesses.
It scores 8.6 out of 10 on the Common Vulnerability Scoring System, which rates it high, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record lists one fixed version: everything before it is affected.[2]
| Product | Fixed in |
|---|---|
| asa | 9.14.4.23 |
Most families won’t have Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD); it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 24 Apr 2024, within a day of it being made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 1 May 2024 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 70%. That’s in the top 1% of the flaws it scores.[4]
CISA assesses it as automatable: attackers can run it against many targets at once.
CISA doesn’t say who was attacked, or how.
Fix
Cisco has published a fix or advice.[3] CISA’s required action: “Apply mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable.”[1]
- Check whether you use it.Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed (fixed in version 9.14.4.23 of asa), or not in use. CISA gave US government agencies until 1 May 2024 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 8.6.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 70%.[4] |
See also
Other Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) flaws
- CVE-2024-20481: ASA and FTD denial-of-service ·
- CVE-2024-20359: ASA and FTD privilege escalation ·
- CVE-2020-3259: ASA and FTD information disclosure ·
- CVE-2020-3580: ASA and FTD cross-site scripting (XSS) ·
- CVE-2020-3452: ASA and FTD read-only path traversal ·
Other Cisco flaws
- CVE-2026-76418, Cisco Secure Firewall ASA Software and Cisco Secure FTD Software: unchecked return value to NULL pointer dereference ·
- CVE-2026-76519, Cisco Secure Email Gateway: relative path traversal ·
- CVE-2026-76504, Cisco Catalyst SD-WAN Manager: hex encoding ·
- CVE-2026-76460, Cisco Identity Services Engine: incorrect use of privileged APIs ·
- CVE-2026-76461, Cisco Secure Email Gateway: SQL injection ·
Added the same day
- CVE-2024-4040, CrushFTP: VFS sandbox escape ·
- CVE-2024-20359, Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD): ASA and FTD privilege escalation ·
Every Cisco flaw on CISA’s list · Everything added in April 2024
References
- ^ Cybersecurity and Infrastructure Security Agency. “Cisco ASA and FTD Denial of Service Vulnerability”. Known Exploited Vulnerabilities Catalog, added 24 Apr 2024.
- ^ National Institute of Standards and Technology. “CVE-2024-20353 Detail”. National Vulnerability Database, published 24 Apr 2024.
- ^ Cisco. Advisories and fixes: sec.cloudapps.cisco.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.698, percentile 99.4, 31 Oct 2026.
- CVE Program. “CVE-2024-20353”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.