CVE-2025-53521: F5 BIG-IP | defend.network

What is 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.

CVSS9.8 NVD 3.1

SeverityCRITICAL

Exploitation In the wild In CISA KEV

EPSS2% · P81

Triage statusActive Exploit

ActionPatch immediately

CVSS vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

NVD published2025-10-15

NVD last modified2026-06-17

Timeline

2025-10-15Published to the U.S. National Vulnerability Database (NVD)
2026-03-27Added to the CISA Known Exploited Vulnerabilities (KEV) catalog
2026-03-29First covered in a defend.network daily briefing
2026-03-30CISA federal remediation deadline (BOD 22-01)
2026-06-17NVD record last updated

CISA Known Exploited Vulnerability

F5 BIG-IP Stack-Based Buffer Overflow Vulnerability

Added to KEV2026-03-27

Federal patch deadline2026-03-30

Known ransomware useUnknown

Affected product

F5 BIG-IP

Remediation Steps

Verify current BIG-IP APM version against F5 security advisory
Apply critical patch from F5 as released to CISA KEV catalog
Implement network segmentation to restrict APM access if patching delayed
Monitor access logs for exploitation attempts (POST requests with unusual parameters)
Test patch in staging environment before production deployment

References

Referenced in our briefings & reports

Vulnerability Priority Report – Week 5 of March 2026 (March 30 – April 5)
FBI Director email breached; Citrix & F5 zero-days exploited (2026-03-30)
Iran breaches FBI Director email; Citrix & F5 zero-days unpatched (2026-03-29)

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🤖 This CVE page is generated by defend.network from NVD, CISA KEV, EPSS, and our verified daily briefings. Severity and exploitation data come from official sources; always verify remediation steps against the official vendor advisory before acting in production.