CVE-2021-21295Public exploitNetwork attack vector

Request smuggling via unchecked HTTP/2 content length

Published Mar 9, 2021 · Updated Aug 3, 2024

HTTP request smuggling in The Netty Project netty-codec-http2 before 4.1.60.Final allows remote attackers to inject backend requests. The HTTP/2 decoding path propagates Content-Length without verifying it against DATA-frame bytes before Http2StreamFrameToHttpObjectCodec creates HTTP/1.1 objects. Exposure requires HTTP2MultiplexCodec or Http2FrameCodec, that conversion codec, and forwarding of the resulting objects to a remote HTTP/1.1 peer.

CVSS severity5.9
Medium
EPSS probability18.89%
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 The Netty Project netty-codec-http2 before 4.1.60.Final allows remote attackers to inject backend requests. The HTTP/2 decoding path propagates Content-Length without verifying it against DATA-frame bytes before Http2StreamFrameToHttpObjectCodec creates HTTP/1.1 objects. Exposure requires HTTP2MultiplexCodec or Http2FrameCodec, that conversion codec, and forwarding of the resulting objects to a remote HTTP/1.1 peer.

The record

CVE
CVE-2021-21295
Published
Mar 9, 2021
Updated
Aug 3, 2024
Vendor
The Netty Project
Product
netty-codec-http2
Classifications
CWE-444, CAPEC-33, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 9, 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=< 4.1.60.Final

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Netty Project · netty-codec-http2version=< 4.1.60.Final

Published CVSS scores

5.9GitHubCVSS: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 exploitPublished

Weakness, pattern, technique

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

Public exploit references

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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