CVE-2025-53521
F5 BIG-IP Stack-Based Buffer Overflow Vulnerability
F5 BIG-IP
When a BIG-IP APM access policy is configured on a virtual server, specific malicious traffic can lead to Remote Code Execution (RCE). Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.
Exploitation status
CISA Known Exploited Vulnerabilities
Listed by CISA on 27 March 2026. US federal agencies were required to remediate it by 30 March 2026.
EPSS — exploitation probability
FIRST puts the probability of exploitation activity in the next 30 days at 2.3%, which is higher than 82.9% of all scored vulnerabilities. EPSS is a forecast of activity, not a measure of severity, and it is rescored daily.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Frequently asked questions
Is CVE-2025-53521 being exploited by ransomware groups?
No ransomware group in this database is currently recorded as exploiting CVE-2025-53521.
What does CVE-2025-53521 affect?
CVE-2025-53521 affects F5 BIG-IP. When a BIG-IP APM access policy is configured on a virtual server, specific malicious traffic can lead to Remote Code Execution (RCE). Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.
How severe is CVE-2025-53521?
CVE-2025-53521 is rated CRITICAL with a CVSS base score of 9.8. EPSS puts the probability of exploitation in the next 30 days at 2.3%, higher than 82.9% of all scored vulnerabilities. It is listed in the CISA Known Exploited Vulnerabilities catalog, with a federal remediation deadline of 30 March 2026.
How should organisations respond to CVE-2025-53521?
Apply the vendor patch for F5 BIG-IP as the first priority, and treat any internet-facing instance as the most urgent. Because this vulnerability is associated with ransomware activity, also review whether the affected systems were reachable before patching, rather than assuming that patching alone closes the incident.