Security Digest · Vulnerability record · Known exploited
CVE-2025-53521: F5 BIG-IP Stack-Based Buffer Overflow Vulnerability
Exploited in the wild (CISA KEV). When a BIG-IP APM access policy is configured on a virtual server, specific malicious traffic can lead to Remote Code Execution (RCE).
CISA KEV · added 2026-03-27 CVSS 9.3 Critical EPSS 2.3% · p83 Patch this week
Should you act?
Exploited: patch now if you run it
CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.
- Yes, if you run F5 BIG-IP. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2026-03-27, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-03-30; treat that as the outer limit, not the target.
- EPSS 2.3%, percentile 83: FIRST’s model estimates a 2.3% chance of exploitation activity in the next 30 days, higher than 83% of all scored CVEs (elevated). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Editorial verdict: Patch this week. F5 Breach: Nation-State Stole BIG-IP Source Code — The breach itself needs no new action a year on, but its aftermath does: two unauthenticated BIG-IP APM RCEs are now on CISA's exploited list, so every BIG-IP owner should be on current fixed builds with management interfaces off the internet.
What it is
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.
CISA’s summary
F5 BIG-IP APM contains a stack-based buffer overflow vulnerability that could allow a threat actor to achieve remote code execution.
Weakness type: CWE-121.
Product: F5 BIG-IP · Assigned by: f5 · Published: 2025-10-15
Affected and fixed versions
| Vendor | Product | Affected versions (per CVE record) |
|---|---|---|
| F5 | BIG-IP | 17.5.0 < 17.5.1.3, 17.1.0 < 17.1.3, 16.1.0 < 16.1.6.1, 15.1.0 < 15.1.10.8 |
Severity scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| f5 | 3.1 | 9.8 Critical | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H |
| f5 | 4.0 | 9.3 Critical | 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 |
- Attack vector: network (remotely reachable)
- Attack complexity: low
- Attack requirements: none
- Privileges required: none (no login)
- User interaction: none
CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.
Exploitation and urgency
- CISA KEV: added 2026-03-27; federal due date 2026-03-30. Known ransomware use: Unknown.
- 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 SSVC assessment: exploitation active, automatable yes, technical impact total.
- FIRST EPSS (model of 2026-10-21): 2.30% probability of exploitation activity in the next 30 days; percentile 83.
Timeline
- — CVE record published by f5
- — erroneo.us brief: F5 Breach: Nation-State Stole BIG-IP Source Code
- — Added to CISA’s Known Exploited Vulnerabilities catalog
- — CISA remediation deadline for US federal agencies
- — CVE record last updated
Sources and references
Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.
Vendor and project advisories, patches
- my.f5.com my.f5.com
- Listed in CISA KEV notes (my.f5.com) my.f5.com
- Listed in CISA KEV notes (my.f5.com) my.f5.com
Vulnerability records
- CVE.org record CVE-2025-53521 cve.org
- NVD entry CVE-2025-53521 nvd.nist.gov