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

In short

F5 BIG-IP has a weakness classified as stack-based buffer overflow (CWE-121). CISA added it to its Known Exploited Vulnerabilities catalog on 2026-03-27. NVD lists CVSS 4.0 at 9.3 CRITICAL.

What it is

“F5 BIG-IP APM contains a stack-based buffer overflow vulnerability that could allow a threat actor to achieve remote code execution.” — CISA KEV, retrieved 2026-11-01

“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.” — NVD, retrieved 2026-11-01

Who is affected

F5 BIG-IP. NVD lists the following versions as affected:

• Versions from 15.1.0 inclusive, before 15.1.10.8
• Versions from 16.1.0 inclusive, before 16.1.6.1
• Versions from 17.1.0 inclusive, before 17.1.3
• Versions from 17.5.0 inclusive, before 17.5.1.3

Exploitation status

CISA KEV added: 2026-03-27. Known ransomware campaign use: Not reported by CISA. Due date CISA set for US federal civilian agencies: 2026-03-30.

Scores

NVD CVSS 4.0: 9.3 CRITICAL; vector 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X; source [email protected].

EPSS by FIRST: 2.30%; percentile 83.26; score date 2026-10-31. EPSS estimates the probability of exploitation in the next 30 days.

CWE: CWE-121.

Timeline

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 KEV

What to do

• Confirm whether you run F5 BIG-IP and which version.
• Apply the vendor update or mitigation in the advisory.
• Follow CISA’s required action.
• Review the vendor guidance on signs of compromise.
• Track the fix through completion.

References

• Vendor Advisory: https://my.f5.com/manage/s/article/K000156741
• NVD detail
• CVE.org record
• CISA KEV catalog

Sources and dates

CISA Known Exploited Vulnerabilities Catalog, version 2026.10.30; retrieved 2026-11-01. CISA KEV data: CC0 1.0. No endorsement by CISA.

NVD retrieved: 2026-11-01. This product uses the NVD API but is not endorsed or certified by the NVD.

EPSS scores by FIRST; score date 2026-10-31.