CVE-2021-21409Network attack vector

Request smuggling via skipped end-stream header validation

Published Mar 30, 2021 · Updated Aug 3, 2024

HTTP request smuggling in Netty before 4.1.61.Final allows remote attackers to send concealed requests through an HTTP/2-to-HTTP/1.1 proxy. DefaultHttp2ConnectionDecoder updates a new end-stream request's stream state before deciding whether its initial headers are trailers, causing content-length validation to be skipped. Exploitation requires a single HTTP/2 HEADERS frame with endStream set and translation by a proxy to HTTP/1.1, where inconsistent parsing compromises request integrity.

CVSS severity5.9
Medium
EPSS probability4.93%
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 4.1.61.Final allows remote attackers to send concealed requests through an HTTP/2-to-HTTP/1.1 proxy. DefaultHttp2ConnectionDecoder updates a new end-stream request's stream state before deciding whether its initial headers are trailers, causing content-length validation to be skipped. Exploitation requires a single HTTP/2 HEADERS frame with endStream set and translation by a proxy to HTTP/1.1, where inconsistent parsing compromises request integrity.

The record

CVE
CVE-2021-21409
Published
Mar 30, 2021
Updated
Aug 3, 2024
Vendor
The Netty Project
Product
Netty
Classifications
CWE-444, CAPEC-33, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 30, 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=< 4.1.61.Final

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Netty Project · Nettyversion=< 4.1.61.Final

Published CVSS scores

5.9NISTCVSS:3.1/AV:N/AC:H/PR:N/UI:N/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.

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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.

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Technologies

Your stack

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Your stack

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