Threats / F5 / CVE-2025-53521
CVE-2025-53521
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-04
F5 BIG-IP vulnerability
F5 BIG-IP APM contains a stack-based buffer overflow vulnerability enabling remote code execution. The flaw is actively exploited in the wild.
Verdict
Today item — known-exploited.
A remotely exploitable stack buffer overflow in F5 BIG-IP APM permits unauthenticated attackers to execute arbitrary code with system privileges. Active exploitation confirms operational risk despite low EPSS probability.
CISA KEV Yes · 2026-03-275EPSS 0.02213 (verify live)6CVSS 9.33CISA SSVC Exploitation: active · Automatable: yes · Impact: total4
01
Is it exploitable?
— the evidence, ranked above the scoreReported exploitation
7 independent public reports of in-the-wild exploitation are cataloged.Distinct reporting sources (vendor, incident response, government); open them for the underlying claims.
Exploited in the wild
Listed in the CISA Known Exploited Vulnerabilities catalog (added 2026-03-27).
Probability (EPSS)
EPSS 0.02213 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 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) · Affected: F5, BIG-IP. Confirm exact fixed builds in the vendor advisory.CVSS — CNA-assigned (CVE.org), version 4.0, set at disclosure.
Weakness (CWE)
Mapped to CWE-121 Stack-based Buffer Overflow — weakness family: Memory safety.CWE — CNA-assigned (CVE.org); the weakness class drives how the flaw is exploited.
CISA SSVC (triage disposition)
Exploitation: active · Automatable: yes · Technical Impact: totalCISA's own stakeholder-specific triage inputs, published in the CVE record (CISA Coordinator role). NOTE: this is CISA's three decision points; the final Act/Attend/Track decision also requires a Mission & Well-being input not published here, so no overall decision is asserted.
Currency of the record
CISA added this to its KEV catalog 163 days after the CVE record was published.KEV lag = CISA-KEV dateAdded − CVE.org datePublished; a floor on how long the known-exploited flaw sat in the public record before the federal catalog listed it. Severity/weakness here is CNA- or CISA-assigned (current at disclosure), independent of NVD's analysis backlog.
02
Who’s exploiting it?
— attribution turns risk into urgencyAttribution not established
No confirmed (advisory-backed) threat-actor attribution is established for this record. Absence of a named actor is not absence of compromise — see Coverage & confidence.
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
I craft a malicious input that overflows the stack buffer in BIG-IP APM to overwrite return addresses and inject shellcode.
Business
Attackers gain code execution on critical load balancing infrastructure, enabling lateral movement into protected networks and data exfiltration.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I execute arbitrary commands with the privileges of the BIG-IP process to establish persistence and maintain access.
Business
Compromised BIG-IP instances become persistent footholds for ongoing reconnaissance and attacks against downstream systems.
3
Lateral reach — past segmentation narrative 3
Attacker
I pivot from the compromised BIG-IP to intercept, modify, or redirect traffic flowing through the load balancer.
Business
Business continuity is disrupted; customer data and transactions passing through BIG-IP are exposed to interception and manipulation.
04
What to do
— defensible action- Remediate per the vendor advisory — confirm the fixed build for your version and verify exposure.1
Say it to the boardA vulnerability with this evidence profile is a defensible budget line, not a backlog ticket — fund the change against the proof above.
05