---
title: "CVE-2025-53521: F5 BIG-IP Stack-Based Buffer Overflow Vulnerability"
url: https://erroneo.us/security/cve/CVE-2025-53521/
description: "CVE-2025-53521 (F5 BIG-IP): Exploited in the wild (CISA KEV). When a BIG-IP APM access policy is configured on a virtual server, specific malicious traffic…"
type: vulnerability-record
section: "Security Digest"
date_published: 2025-10-15
date_modified: 2026-10-21
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2025-53521"
kev: true
cvss: 9.3
epss: 0.02295
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2025-53521: F5 BIG-IP Stack-Based Buffer Overflow Vulnerability

**Signal:** Exploited: patch now if you run it. CISA has confirmed exploitation in the wild. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Yes, if you run F5 BIG-IP. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2026-03-27, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-03-30 ; treat that as the outer limit, not the target.
- EPSS 2.3% , percentile 83: FIRST’s model estimates a 2.3% chance of exploitation activity in the next 30 days, higher than 83% of all scored CVEs (elevated). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Editorial verdict: Patch this week. F5 Breach: Nation-State Stole BIG-IP Source Code — The breach itself needs no new action a year on, but its aftermath does: two unauthenticated BIG-IP APM RCEs are now on CISA's exploited list, so every BIG-IP owner should be on current fixed builds with management interfaces off the internet.

## What it is

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.

## Affected products (CVE record)

- F5 BIG-IP: 17.5.0 < 17.5.1.3, 17.1.0 < 17.1.3, 16.1.0 < 16.1.6.1, 15.1.0 < 15.1.10.8

## Severity

- f5 CVSS 3.1: 9.8 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`
- f5 CVSS 4.0: 9.3 `CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N`

## Exploitation

- CISA KEV: added 2026-03-27; federal due date 2026-03-30. Known ransomware use: Unknown.
- CISA 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.
- CISA SSVC assessment: exploitation active , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 2.30% probability of exploitation activity in the next 30 days; percentile 83.

## Sources

- [my.f5.com](https://my.f5.com/manage/s/article/K000156741)
- [Listed in CISA KEV notes (my.f5.com)](https://my.f5.com/manage/s/article/K000160486)
- [Listed in CISA KEV notes (my.f5.com)](https://my.f5.com/manage/s/article/K11438344)
- [CVE.org record CVE-2025-53521](https://www.cve.org/CVERecord?id=CVE-2025-53521)
- [NVD entry CVE-2025-53521](https://nvd.nist.gov/vuln/detail/CVE-2025-53521)
- [CISA Known Exploited Vulnerabilities catalog entry](https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-53521)

---
Source page: https://erroneo.us/security/cve/CVE-2025-53521/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt