Threats / F5 / CVE-2021-22991
CVE-2021-22991
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-06-22
F5 BIG-IP Traffic Management Microkernel vulnerability
Buffer overflow in F5 BIG-IP Traffic Management Microkernel allows bypass of URL-based access controls.
Verdict
Today item — known-exploited.
A buffer overflow in the Traffic Management Microkernel component enables attackers to circumvent URL-based access control policies, potentially exposing protected resources. Active exploitation has been observed in the wild.
01
Is it exploitable?
— the evidence, ranked above the scoreReported exploitation
3 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 2022-01-18).
Probability (EPSS)
EPSS 0.61064 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
Affected: F5, BIG-IP Traffic Management Microkernel. Confirm exact fixed builds in the vendor advisory.
Weakness (CWE)
Mapped to CWE-119 Memory Buffer Bounds Error — weakness family: Memory safety.CWE assignment from the public NVD record; the weakness class drives how the flaw is exploited.
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 request designed to overflow a buffer in the microkernel's URL processing logic.
Business
Access control policies that should restrict traffic to specific URLs are rendered ineffective.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I send the crafted payload through the BIG-IP appliance to reach protected backend resources.
Business
Unauthorized users gain access to sensitive applications and data that should be restricted by security policies.
3
Lateral reach — past segmentation narrative 3
Attacker
I bypass authentication and authorization checks by exploiting the memory corruption to alter traffic routing decisions.
Business
Compliance violations occur as access logs show policy violations and unauthorized data exposure incidents.
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