CVE-2025-20333: Cisco Secure Firewall Adaptive Security Appliance (ASA) Software · Classic Buffer Overflow
Affects Cisco Cisco Secure Firewall Adaptive Security Appliance (ASA) SoftwareSource CVE Database V5
Exploited in the wild · KEV due 26 Sep 2025 · 71% EPSS : apply the vendor's fix or mitigations to Cisco Secure Firewall Adaptive Security Appliance (ASA) Software now.
Data as of 10/25/2026, 16:58:53 UTC
- KEV deadline
- 09/26/2025 US federal agencies must patch by this date; a sensible deadline for everyone else.
- EPSS
- 71.2% chance of exploitation in 30 days – higher than 99% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:HExploitation evidence
t = 0 is the CVE’s publication on cve.org
Disclosed: evidence found.
CVE published 25 Sep 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wild: evidence found.
Listed in CISA KEV the same day as disclosure
Observed by OffSeq Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors
Timeline
CVE ID reserved
by cisco
, 350 days before disclosure
CVE published
on cve.org
, t = 0
In Threat Radar
record ingested
, 3 minutes after disclosure
Added to CISA KEV (opens in a new tab)
exploitation confirmed
, the same day as disclosure
KEV remediation due
US federal deadline
, 1 day after disclosure
A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), CISA KEV (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 2026-10-25.
What to do
CISA 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). Agencies must follow the mitigation steps provided by CISA (URL listed below in Notes) and vendor’s instructions (URL listed below in Notes). Adhere to the applicable BOD 22-01 guidance for cloud services or discontinue use of the product if mitigations are not available.
- cisa.gov/news-events/directives/ed-25-03-identify-and-mitigate-potential-compromise-cisco-devices (opens in a new tab)
- cisa.gov/news-events/directives/supplemental-direction-ed-25-03-core-dump-and-hunt-instructions (opens in a new tab)
- cisa.gov/eviction-strategies-tool/create-from-template (opens in a new tab)
- sec.cloudapps.cisco.com/security/center/resources/asa_ftd_continued_attacks (opens in a new tab)
- sec.cloudapps.cisco.com/security/center/private/resources/asa_ftd_continued_attacks (opens in a new tab)
- sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-webvpn-z5xP8EUB (opens in a new tab)
- nvd.nist.gov/vuln/detail/CVE-2025-20333 (opens in a new tab)
Affected software
| Product | Affected | Fixed in |
|---|---|---|
| CiscoCisco Secure Firewall Adaptive Security Appliance (ASA) Software | Affected
| Fixed in– |
| CiscoCisco Secure Firewall Threat Defense (FTD) Software | Affected
| Fixed in– |
Check the firmware or software version for Cisco Secure Firewall Adaptive Security Appliance (ASA) Software in the device's admin interface against the vendor advisory.
Description
CVE-2025-20333 is a critical buffer overflow vulnerability in the VPN web server component of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software. It allows an authenticated remote attacker with valid VPN credentials to send crafted HTTP(S) requests that can lead to arbitrary code execution as root. Successful exploitation could result in full compromise of the affected device. The vulnerability arises from improper validation of user-supplied input in HTTP(S) requests. The CVSS v3.1 base score is 9.9, indicating critical severity. Multiple specific 9.8.x and 9.8.3.x versions are affected. Exploits are known to be in the wild. No patch information is provided in the input data.
Impact
An authenticated remote attacker with valid VPN credentials can exploit this vulnerability to execute arbitrary code as root on the affected device. This could lead to complete compromise of the Cisco Secure Firewall ASA or FTD device, impacting confidentiality, integrity, and availability. The vulnerability is critical with a CVSS score of 9.9. Known exploits are active in the wild, increasing the risk to affected environments.
Mitigation
Restrict VPN access to trusted users and networks, and monitor for suspicious HTTP(S) requests targeting the VPN web server. Follow Cisco's official security advisories for updates and apply patches promptly once released.
Weaknesses
- CWE-120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow') (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #11
The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer.
References
- sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-webvpn-z5xP8EUB (opens in a new tab)
- sec.cloudapps.cisco.com/security/center/resources/asa_ftd_continued_attacks (opens in a new tab)
- cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-20333 (opens in a new tab)