---
id: "CVE-2021-22991"
kind: "vulnerability"
aliases: ["CVE-2021-22991"]
title: "F5 BIG-IP Traffic Management Microkernel: F5 BIG-IP Traffic Management Microkernel Buffer Overflow"
severity: "critical"
cvss: 9.8
epss: 0.61064
exploited: true
status: "open"
category: "infrastructure"
products: [{"name": "F5 BIG-IP Traffic Management Microkernel", "fixed": [], "affected": "", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["big-ip", "exploited", "f5", "infrastructure", "load-balancer", "waf"]
not_affected_if: ["no-big-ip-deployment"]
published_at: "2021-03-31T18:15:14+00:00"
first_seen_at: "2026-10-23T19:11:53+00:00"
updated_at: "2026-10-23T21:57:35+00:00"
sources: [{"source": "kev", "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-22991", "first_seen_at": "2026-10-23T19:11:53+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2021-22991", "first_seen_at": "2026-10-23T19:13:44+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-22991", "first_seen_at": "2026-10-23T21:57:35+00:00"}]
---

# F5 BIG-IP Traffic Management Microkernel: F5 BIG-IP Traffic Management Microkernel Buffer Overflow

F5 BIG-IP's Traffic Management Microkernel has a buffer overflow that lets a remote attacker bypass URL-based access controls. CISA lists it as actively exploited in the wild.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- vendor:F5 BIG-IP Traffic Management Microkernel affected ? fixed ?

## Fix

Apply the F5 BIG-IP update per the vendor advisory (the sources do not name the fixed version).

## Timeline

- 2026-10-23T19:11:53+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-22991
- 2026-10-23T19:13:44+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2021-22991
- 2026-10-23T21:57:35+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2021-22991

## From the sources

On BIG-IP versions 16.0.x before 16.0.1.1, 15.1.x before 15.1.2.1, 14.1.x before 14.1.4, 13.1.x before 13.1.3.6, and 12.1.x before 12.1.5.3, undisclosed requests to a virtual server may be incorrectly handled by the Traffic Management Microkernel (TMM) URI normalization, which may trigger a buffer overflow, resulting in a DoS attack. In certain situations, it may theoretically allow bypass of URL based access control or remote code execution (RCE). Note: Software versions which have reached End of Software Development (EoSD) are not evaluated.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_