Exploited

CVE-2025-53521

F5 BIG-IP · Load balancers and access gateways (ADC)

F5 BIG-IP Stack-Based Buffer Overflow Vulnerability

At a glance

Exploited. Added to CISA’s catalog of exploited vulnerabilities on March 27, 2026.

Affected product: F5 BIG-IP (category Load balancers and access gateways (ADC), indicative classification). Name in CISA’s catalog: F5 BIG-IP.

What should I do?

Apply the patches or mitigations from F5; if none are available, stop using the product. For cloud services, follow the guidance from F5.

Deadline set by CISA for US federal agencies: 3 days (due March 30, 2026). It only binds those agencies, but it is a useful measure of urgency.

F5 security advisories page (general page, not the advisory for this vulnerability)
The vendor’s specific advisory is listed in the NVD references: CVE-2025-53521 on the NVD website.

Original text of the required action (CISA)

Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

Timeline

October 15, 2025 Vulnerability published Publication date (NVD)
March 27, 2026 Exploitation confirmed Added to CISA’s KEV catalog
March 30, 2026 CISA deadline Remediation deadline set for US federal agencies

The facts, with their sources

Exploitation confirmed: Yes, on March 27, 2026 Date added to the catalog of exploited vulnerabilities. Exploitation itself may have started earlier. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
Ransomware: No known use to date That does not guarantee it is not being used: the information is not public. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
Hunt for compromise: Not requested by CISA Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
CISA required action: Apply the patches or mitigations from F5; if none are available, stop using the product. For cloud services, follow the guidance from F5. Rewritten by this site from CISA’s original text. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
CISA deadline: March 30, 2026, 3 days Set for US federal agencies: a measure of urgency, not an obligation elsewhere. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
Vulnerability published: October 15, 2025 Source: National Vulnerability Database (NVD), collected September 29, 2026 at 15:00 UTC
Product according to CISA: F5 BIG-IP Filed by this site under “Load balancers and access gateways (ADC)” (indicative classification). Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H Technical detail of the score: attack vector, complexity, impact. Source: National Vulnerability Database (NVD), collected September 29, 2026 at 15:00 UTC

Severity and activity

9.8 / 10 CVSS severity: critical

Score from the vendor or CISA, relayed by the NVD. Collected Sep 29, 2026.

— Probability of exploitation in the next 30 days (EPSS)

FIRST’s EPSS model. Since exploitation is already confirmed, this figure is mostly useful to compare activity. Collected Sep 29, 2026.

Description

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.

F5 BIG-IP APM contains a stack-based buffer overflow vulnerability that could allow a threat actor to achieve remote code execution.

Summary from CISA (KEV catalog).

CERT-FR advisories

CERTFR-2026-ALE-004: Vulnérabilité dans F5 BIG-IP Access Policy Manager (updated July 28, 2026)

Information reused under the Open Licence 2.0 (Etalab); the date of last update is shown for each advisory. This site is neither affiliated with nor endorsed by ANSSI.

Sources for this page

CISA KEV catalog: exploitation, date added, required action, deadline, ransomware.
NVD (NIST): CVSS severity, publication date, description, vendor references.
EPSS (FIRST): probability of exploitation.
CERT-FR: advisories and alerts (in French).
Brand and category: indicative classification by this site (method).