CVE-2021-33037Network attack vector

Request smuggling via inconsistent Transfer-Encoding parsing

Published Jul 12, 2021 · Updated Aug 3, 2024

HTTP request smuggling in affected Apache Tomcat 8.5, 9.0, and 10.0 releases allows remote attackers to desynchronize proxied requests. Tomcat's HTTP parser ignored Transfer-Encoding for clients requesting HTTP/1.0 responses, accepted the invalid identity encoding, and failed to require chunked as the final encoding. Exploitation requires a reverse proxy that interprets the crafted request differently, allowing a smuggled request to alter subsequent back-end request boundaries.

CVSS severity5.3
Medium
EPSS probability75.35%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP request smuggling in affected Apache Tomcat 8.5, 9.0, and 10.0 releases allows remote attackers to desynchronize proxied requests. Tomcat's HTTP parser ignored Transfer-Encoding for clients requesting HTTP/1.0 responses, accepted the invalid identity encoding, and failed to require chunked as the final encoding. Exploitation requires a reverse proxy that interprets the crafted request differently, allowing a smuggled request to alter subsequent back-end request boundaries.

The record

CVE
CVE-2021-33037
Published
Jul 12, 2021
Updated
Aug 3, 2024
Vendor
The Apache Software Foundation
Product
Apache Tomcat
Classifications
CWE-444, CAPEC-33, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 12, 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=Apache Tomcat 10 10.0.0-M1 to 10.0.6
  2. Affected versionversion=Apache Tomcat 8 8.5.0 to 8.5.66
  3. Affected versionversion=Apache Tomcat 9 9.0.0.M1 to 9.0.46

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Apache Software Foundation · Apache Tomcatversion=Apache Tomcat 10 10.0.0-M1 to 10.0.6; version=Apache Tomcat 8 8.5.0 to 8.5.66; version=Apache Tomcat 9 9.0.0.M1 to 9.0.46

Published CVSS scores

5.3GitHub Advisory DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

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-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
CAPEC-33HTTP Request Smuggling
T1190Exploit Public-Facing Application

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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