What is CVE-2025-53521?

CVE-2025-53521 is a critical-severity vulnerability in F5 Big-ip, classified under Stack-based Buffer Overflow. CVSS score: 9.8/10. Published 2025-10-15.

How severe is CVE-2025-53521?

Critical severity. CVSS v3 base score is 9.8 out of 10.

Is CVE-2025-53521 known to be exploited?

Yes. CVE-2025-53521 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2026-03-27), indicating it is being actively exploited.

Buffer overflow in F5 Big-ip

CVE-2025-53521

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.

Vulnerability class: Buffer Overflow

EPSS: 0.088 (92.7th percentile) — read the EPSS interpretation.

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

Affected products

F5 Big-ip — versions 17.5.0, 17.1.0, 16.1.0

Weakness classification (CWE)

CWE-121 — Stack-based Buffer Overflow

CISA KEV (Known Exploited Vulnerabilities)

This CVE is on the CISA KEV catalog, added on 2026-03-27. CISA KEV inclusion means CISA has confirmed in-the-wild exploitation; US federal agencies are required to remediate within a published due date.

BOD 22-01 due date: 2026-03-30.

Required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

References

my.f5.com/manage/s/article/K000156741 (vendor-advisory)

Frequently asked questions

What is CVE-2025-53521?: CVE-2025-53521 is a critical-severity vulnerability in F5 Big-ip, classified under Stack-based Buffer Overflow. CVSS score: 9.8/10. Published 2025-10-15.
How severe is CVE-2025-53521?: Critical severity. CVSS v3 base score is 9.8 out of 10.
Is CVE-2025-53521 known to be exploited?: Yes. CVE-2025-53521 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2026-03-27), indicating it is being actively exploited.