CVE-2021-23009Network attack vector

Data-plane service interruption via HTTP/2 infinite loop

Published May 10, 2021 · Updated Aug 3, 2024

Denial of service in F5 Networks BIG-IP 15.1.x before 15.1.3 and 16.0.x before 16.0.1.1 allows remote attackers to interrupt data-plane traffic. TMM loops indefinitely while processing a malformed HTTP/2 request, then aborts and takes the configured high-availability action. No authentication or user interaction is required; the consequence is confined to data-plane traffic, with no control-plane exposure.

CVSS severity7.5
High
EPSS probability0.99%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in F5 Networks BIG-IP 15.1.x before 15.1.3 and 16.0.x before 16.0.1.1 allows remote attackers to interrupt data-plane traffic. TMM loops indefinitely while processing a malformed HTTP/2 request, then aborts and takes the configured high-availability action. No authentication or user interaction is required; the consequence is confined to data-plane traffic, with no control-plane exposure.

The record

CVE
CVE-2021-23009
Published
May 10, 2021
Updated
Aug 3, 2024
Vendor
F5 Networks
Product
BIG-IP
Classifications
CWE-835, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 10, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=16.0.x before 16.0.1.1 and 15.1.x before 15.1.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

F5 Networks · BIG-IPversion=16.0.x before 16.0.1.1 and 15.1.x before 15.1.3

Published CVSS scores

5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:P
7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-835Loop with Unreachable Exit Condition ('Infinite Loop')
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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