CVE-2025-20333
CVE-2025-20333 is a security flaw in Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense, Cisco’s firewall. Attackers can run their own code on Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 25 Sep 2025.[1]
Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense 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. Cisco has published a fix or advice.
Description
CISA calls it a secure firewall adaptive security appliance (ASA) and secure firewall threat defense (FTD) buffer overflow in Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense. In its words: “Cisco Secure Firewall Adaptive Security (ASA) Appliance and Secure Firewall Threat Defense (FTD) Software VPN Web Server contain a buffer overflow vulnerability that allows for remote code execution. This vulnerability could be chained with CVE-2025-20362.”[1]
The CVE record adds: “A vulnerability in the VPN web server of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software could allow an authenticated, remote attacker to execute arbitrary code on an affected device. This vulnerability is due to improper validation of user-supplied input in HTTP(S) requests. An attacker with valid VPN user credentials could exploit this vulnerability by sending crafted HTTP requests to an affected device. A successful exploit could allow the attacker to execute arbitrary code as root, possibly resulting in the complete compromise of the affected device.”[2]
The underlying weakness is buffer overflow (CWE-120), in MITRE’s list of common software weaknesses.
It scores 9.9 out of 10 on the Common Vulnerability Scoring System, which rates it critical, scored by the organisation that published the CVE using CVSS 3.1.[2]
Affected versions
NVD’s record doesn’t list version ranges for it; Cisco’s advisory has them[3]. CISA names the product as Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense.
Most families won’t have Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 25 Sep 2025, 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 26 Sep 2025 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 71%. That’s in the top 1% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
Cisco has published a fix or advice.[3] CISA’s required action: “The KEV due date refers to the deadline by which FCEB agencies are expected to review and begin implementing the guidance outlined in Emergency Directive (ED) 25-03 (URL listed below in Notes).”[1]
- Check whether you use it.Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense 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, or not in use. CISA gave US government agencies until 26 Sep 2025 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 9.9.[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 71%.[4] |
See also
Other Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense flaws
- CVE-2025-20362: secure firewall adaptive security (ASA) appliance and secure firewall threat defense (FTD) missing authorization ·
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 ·
Other buffer overflow flaws
- CVE-2025-43520, Apple devices: classic buffer overflow ·
- CVE-2022-37055, D-Link Routers: buffer overflow ·
- CVE-2020-15069, Sophos XG Firewall: buffer overflow ·
- CVE-2023-41064, Apple iOS, iPadOS, and macOS: imageIO buffer overflow ·
- CVE-2023-33010, Zyxel Multiple Firewalls: buffer overflow ·
Added the same day
- CVE-2025-20362, Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense: secure firewall adaptive security (ASA) appliance and secure firewall threat defense (FTD) missing authorization ·
Every Cisco flaw on CISA’s list · Everything added in September 2025
References
- ^ Cybersecurity and Infrastructure Security Agency. “Cisco Secure Firewall Adaptive Security Appliance (ASA) and Secure Firewall Threat Defense (FTD) Buffer Overflow Vulnerability”. Known Exploited Vulnerabilities Catalog, added 25 Sep 2025.
- ^ National Institute of Standards and Technology. “CVE-2025-20333 Detail”. National Vulnerability Database, published 25 Sep 2025.
- ^ Cisco. Advisories and fixes: sec.cloudapps.cisco.com, sec.cloudapps.cisco.com, sec.cloudapps.cisco.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.712, percentile 99.4, 31 Oct 2026.
- CVE Program. “CVE-2025-20333”. 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.