CVE-2021-42340Network attack vector

Service interruption via unreleased WebSocket metrics objects

Published Oct 14, 2021 · Updated Aug 4, 2024

Resource leak in Apache Tomcat allows remote attackers to exhaust heap memory and interrupt service through WebSocket connections. The HTTP upgrade metrics object remains retained after each WebSocket closes, causing connection statistics to accumulate until an OutOfMemoryError occurs. Repeated unauthenticated WebSocket connections are required; the bounded consequence is exhaustion-driven service interruption.

CVSS severity7.5
High
EPSS probability11.80%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Resource leak in Apache Tomcat allows remote attackers to exhaust heap memory and interrupt service through WebSocket connections. The HTTP upgrade metrics object remains retained after each WebSocket closes, causing connection statistics to accumulate until an OutOfMemoryError occurs. Repeated unauthenticated WebSocket connections are required; the bounded consequence is exhaustion-driven service interruption.

The record

CVE
CVE-2021-42340
Published
Oct 14, 2021
Updated
Aug 4, 2024
Vendor
The Apache Software Foundation
Product
Apache Tomcat
Classifications
CWE-772, T1499.003
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 14, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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=Apache Tomcat 10 10.0.0-M10 to 10.0.11
  2. Affected versionversion=Apache Tomcat 10 10.1.0-M1 to 10.1.0-M5
  3. Affected versionversion=Apache Tomcat 8 8.5.60 to 8.5.71
  4. Affected versionversion=Apache Tomcat 9 9.0.40 to 9.0.53

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Apache Software Foundation · Apache Tomcatversion=Apache Tomcat 10 10.0.0-M10 to 10.0.11; version=Apache Tomcat 10 10.1.0-M1 to 10.1.0-M5; version=Apache Tomcat 8 8.5.60 to 8.5.71; version=Apache Tomcat 9 9.0.40 to 9.0.53

Published CVSS scores

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 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-772Missing Release of Resource after Effective Lifetime
T1499.003Application Exhaustion Flood

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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