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.
The record classifies CVE-2025-53521 under weakness classes CWE-121, CWE-770. Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on confidentiality, integrity, availability. 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 379 days ago.
Severity and exploitation probability
- CVSS v3.1 base score
- 9.8 — CRITICAL
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H - CVSS v4.0 base score
- 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 - EPSS (FIRST)
- 2.2% probability of exploitation in the next 30 days, higher than 82.7% of all scored CVEs
- CISA KEV
- Listed since 2026-03-27, federal remediation deadline 2026-03-30
- Threadlinqs priority
- 9/10 — CISA KEV-listed, which Threadlinqs floors at 9
- Published
- 2025-10-15, last modified 2026-03-31
Is CVE-2025-53521 being exploited?
CISA added CVE-2025-53521 to the Known Exploited Vulnerabilities catalog on 2026-03-27, which means the agency holds evidence of exploitation in the wild; US federal civilian agencies had to remediate it by 2026-03-30 under BOD 22-01. It currently carries a trending score of 46 in the Threadlinqs vulnerability feed.
Affected products and versions
- F5: Big-ip Access Policy Manager
How to fix CVE-2025-53521
Vendor advisory: https://my.f5.com/manage/s/article/K000156741. Because CVE-2025-53521 is KEV-listed, US federal civilian agencies were required to apply the vendor fix, or stop using the product, by 2026-03-30. Apply the vendor fix referenced above to every affected product listed in this record, then confirm the running version against the vendor advisory.
Threat activity tracking CVE-2025-53521
4 tracked threats in the Threadlinqs corpus reference CVE-2025-53521, either in the campaign’s CVE list or as an indicator on the campaign record.
- Multi-Stage Linux Intrusion via End-of-Life F5 BIG-IP and Unpatched Confluence — SSH Foothold to NTLM Relay (CVE-2025-33073, CVE-2025-53521) — HIGH · 2026-05-26
- F5 BIG-IP APM Unauthenticated Remote Code Execution via Stack Buffer Overflow (CVE-2025-53521) — CISA KEV Active Exploitation by Chinese Nation-State Actor — CRITICAL · 2026-04-02
- CVE-2025-53521: F5 BIG-IP APM Unauthenticated Remote Code Execution via Stack-based Buffer Overflow — CRITICAL · 2026-04-01
- CVE-2025-53521: F5 BIG-IP APM Unauthenticated Remote Code Execution via apmd Process — Active Exploitation by UNC5221 (BRICKSTORM) — CRITICAL · 2026-03-30
Sources
Enriched from CVE.org, FIRST EPSS, CISA KEV. Last verified by Threadlinqs on .