CVE-2021-43797Network attack vector

HTTP request smuggling via stripped header control characters

Published Dec 9, 2021 · Updated Aug 4, 2024

HTTP request smuggling in Netty before 4.1.71.Final allows remote attackers to alter backend requests through malformed header names. HttpObjectDecoder.splitHeader treats non-OWS control characters as ignorable whitespace and strips them instead of rejecting the header. The flaw is reachable when a Netty-based proxy forwards the sanitized request to a backend, enabling inconsistent request interpretation without authentication.

CVSS severity6.5
Medium
EPSS probability2.68%
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 Netty before 4.1.71.Final allows remote attackers to alter backend requests through malformed header names. HttpObjectDecoder.splitHeader treats non-OWS control characters as ignorable whitespace and strips them instead of rejecting the header. The flaw is reachable when a Netty-based proxy forwards the sanitized request to a backend, enabling inconsistent request interpretation without authentication.

The record

CVE
CVE-2021-43797
Published
Dec 9, 2021
Updated
Aug 4, 2024
Vendor
The Netty Project
Product
Netty
Classifications
CWE-444, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 9, 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=<= 4.1.7.0.Final

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Netty Project · Nettyversion=<= 4.1.7.0.Final

Published CVSS scores

4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:P/A:N
6.5GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/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')
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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