Threat Intelligence / CVE / CVE-2025-53521

CVE-2025-53521 — F5 Big-ip Access Policy Manager

CISA KEV

CVSS v3.1 9.8 (CRITICAL) · CVSS v4.0 9.3 · EPSS 41.4% (higher than 97.3% of all scored CVEs) · Priority 9/10 · Published 2025-10-15

As of 2026-04-02, CVE-2025-53521 is a CRITICAL-severity vulnerability in F5 Big-ip Access Policy Manager, CVSS v3.1 9.8, EPSS 41.4% (97.3th percentile). It is listed in the CISA Known Exploited Vulnerabilities catalog (added 2026-03-27), with a US federal remediation deadline of 2026-03-30. Threadlinqs Intelligence links 4 tracked threat campaigns to CVE-2025-53521, most recently “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)”.

Last updated: 2026-04-02

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 332 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
EPSS (FIRST): 41.4% probability of exploitation in the next 30 days, higher than 97.3% 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 2026-04-02

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 49 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

Seeded from threat-derived and not yet processed by the Threadlinqs enrichment pipeline, so blank CVSS, EPSS or KEV fields above mean NOT MEASURED rather than measured-absent.

Full detection coverage & IOCs for threats exploiting CVE-2025-53521 are available via the Threadlinqs MCP server (Purple tier). View plans →

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