CVE-2021-21295

What it is

Netty is an open-source, asynchronous event-driven network application framework for rapid development of maintainable high performance protocol servers & clients. In Netty (io.netty:netty-codec-http2) before version 4.1.60.Final there is a vulnerability that enables request smuggling. If a Content-Length header is present in the original HTTP/2 request, the field is not validated by `Http2MultiplexHandler` as it is propagated up. This is fine as long as the request is not proxied through as HTTP/1.1. If the request comes in as an HTTP/2 stream, gets converted into the HTTP/1.1 domain objects (`HttpRequest`, `HttpContent`, etc.) via `Http2StreamFrameToHttpObjectCodec `and then sent up to the child channel's pipeline and proxied through a remote peer as HTTP/1.1 this may result in request smuggling. In a proxy case, users may assume the content-length is validated somehow, which is not the case. If the request is forwarded to a backend channel that is a HTTP/1.1 connection, the Content-Length now has meaning and needs to be checked. An attacker can smuggle requests inside the body as it gets downgraded from HTTP/2 to HTTP/1.1. For an example attack refer to the linked GitHub Advisory. Users are only affected if all of this is true: `HTTP2MultiplexCodec` or `Http2FrameCodec` is used, `Http2StreamFrameToHttpObjectCodec` is used to convert to HTTP/1.1 objects, and these HTTP/1.1 objects are forwarded to another remote peer. This has been patched in 4.1.60.Final As a workaround, the user can do the validation by themselves by implementing a custom `ChannelInboundHandler` that is put in the `ChannelPipeline` behind `Http2StreamFrameToHttpObjectCodec`.

At a glance

Exploitability

Can be exploited over the network without logging in. High risk of data tampering.

Exploit odds

18.9% chance of exploitation in the next 30 days (EPSS). Higher than most.

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2021-03-09 19:15 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-10-08 22:17 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 18.9% in the next 30 days (EPSS).
• Severity: 5.9 MEDIUM (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2021-03-09 19:15 UTC

Last modified

2026-10-08 22:17 UTC

Exploit probability

18.9% · EPSS, next 30 days

Weakness

CWE-444

Record status

Modified

Affected

• Netty < 4.1.60
• Netapp Oncommand Api Services
• Netapp Oncommand Workflow Automation
• Debian Linux 10.0
• Quarkus ≤ 1.13.7
• Apache Kudu < 1.16.0
• Apache Zookeeper 3.5.9
• Oracle Communications Cloud Native Core Policy 1.14.0

Attack profile CVSS 3.1 · 5.9

Attack vector

Network

Complexity

High

Privileges needed

None

User interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

High

Availability

None

Is it in your code?

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Read https://cve-list.com/agent/cve/CVE-2021-21295.md and follow it. Check whether this project is affected by CVE-2021-21295. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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