CVE-2021-3629Network attack vector

Service interruption via HTTP/2 flow control handling

Published May 24, 2022 · Updated Aug 3, 2024

Uncontrolled resource consumption in Undertow allows remote attackers to interrupt service by manipulating HTTP/2 flow control. Red Hat identifies browser-controlled HTTP/2 flow-control handling as the locus but has not disclosed the incorrect operation or resource that grows. The target must expose an HTTP/2 listener; no authentication or user interaction is required, and the reported consequence is server overhead or denial of service.

CVSS severity5.9
Medium
EPSS probability1.30%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Uncontrolled resource consumption in Undertow allows remote attackers to interrupt service by manipulating HTTP/2 flow control. Red Hat identifies browser-controlled HTTP/2 flow-control handling as the locus but has not disclosed the incorrect operation or resource that grows. The target must expose an HTTP/2 listener; no authentication or user interaction is required, and the reported consequence is server overhead or denial of service.

The record

CVE
CVE-2021-3629
Published
May 24, 2022
Updated
Aug 3, 2024
Vendor
Red Hat
Product
Undertow
Classifications
CWE-400, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 24, 2022CVE 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=undertow 2.0.40.Final, undertow 2.2.11.Final

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Red Hat · Undertowversion=undertow 2.0.40.Final, undertow 2.2.11.Final

Published CVSS scores

7.5GitHub Advisory DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
5.9NISTCVSS:3.1/AV:N/AC:H/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 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
T1499Endpoint Denial of Service

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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