Actively exploited vulnerability
F5 BIG-IP: memory corruption flaw under attack
CVE-2025-53521 · F5 BIG-IP Stack-Based Buffer Overflow Vulnerability
What is affected
F5 BIG-IP is mostly run by companies, schools and public bodies, not on home computers. You are unlikely to have it yourself, but organisations that hold your data may. Breaches that start with flaws like this one are how personal data ends up leaked.
F5 BIG-IP APM contains a stack-based buffer overflow vulnerability that could allow a threat actor to achieve remote code execution.
What an attacker can do
This is a memory corruption flaw. It lets an attacker corrupt the program's memory by feeding it specially crafted content: a web page, a file or a network message. The result ranges from a crash to the attacker running their own code.
- How it is reached
- Over the internet or network
- Access the attacker needs
- No account needed
- Does the victim have to do something?
- No, works without the victim doing anything
What to do
- At home: nothing to install. Keep your own devices updated and use a different password for every service, so that a breach at one organisation does not open your other accounts.
- If you administer F5 BIG-IP at work: apply the vendor's fix or mitigation now and review logs for signs of compromise, because the flaw was being exploited before it was listed.
- If the product is past its support date and no fix exists, take it off the internet or retire it.
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Vendor advisories and fixes
How urgent is it
CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on March 27, 2026, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by March 30, 2026, a deadline that has already passed. That deadline does not bind anyone else, but it shows how seriously the agency rates it.
The EPSS model estimates a 2.3% probability that this flaw will be exploited somewhere in the next 30 days. That is higher than 83% of all scored vulnerabilities.
Its CVSS severity score is 9.3 out of 10 (critical), as recorded in the US National Vulnerability Database.
Technical description
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.