# What is CVE-2021-21295?

> CVE-2021-21295 is a medium-severity flaw (CVSS 5.9) in Netty, published on 9 March 2021. It is not on the list of flaws known to be exploited today. The latest forecast puts its chance of use in the next 30 days at 19%. If you run an affected version, plan in the vendor’s fix.

Canonical page: https://www.precursorintelligence.com/what-is/cve-2021-21295

## At a glance

- **Verdict:** Fix this soon. Not confirmed as exploited yet, but serious or likely enough that it should not wait for a quiet month.
- **Precursor Intelligence risk score:** 37 out of 100 (Low); action: Monitor. Kept up to date as the evidence changes.
- **Severity:** Medium (CVSS 5.9 out of 10)
- **Known to be exploited:** No, not on the list of flaws known to be exploited today
- **Chance of use in the next 30 days:** 19% (latest exploit-likelihood forecast)
- **Published:** 9 March 2021
- **Record last updated:** 7 November 2023
- **Main product:** Netty
- **Affected products:** 8

## What is CVE-2021-21295? In plain terms

It can be attacked from anywhere on the internet without logging in, and nobody has to click anything. If it works, the attacker can change data.

### The official description

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`.

### What kind of flaw is it?

- CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

### How an attack on it works

- Where an attacker can be: Anywhere on the internet
- How hard it is to pull off: Hard: conditions must line up
- Login needed first: No login needed
- Does someone have to click: No, nobody has to do anything
- What an attacker gets: Can change data

Severity vector: `CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N`

## Is CVE-2021-21295 being exploited? Not that we know of

It is not on the list of flaws known to be exploited today. The latest forecast puts the chance of it being used in the next 30 days at 19%. That can change at any time, which is why we keep watching.

Exploit likelihood over the last 7 days: 19%, 19%, 19%, 19%, 19%, 19%, 19%.

### Timeline

- 9 March 2021: Published
- 7 November 2023: Record last updated
- 19 November 2025: Exploit likelihood jumped
- 22 November 2025: Exploit likelihood fell

### How Precursor Intelligence ranks it

CVE-2021-21295 scores **37 out of 100 (Low)**, with the action "Monitor".

Every published flaw gets a risk score from 0 to 100, kept up to date as the evidence changes. Four things go into the score, from the one that counts most to the ones that count less:

- Evidence it is being exploited: biggest factor
- How severe it is: strong factor
- How widely exposed it is: smaller factor
- Threat activity around it: smaller factor

85 and above is Critical. Anything confirmed as exploited is always marked for an emergency patch, whatever its score. When this flaw matches software you track, it lands in your list with its score and the reasons behind it.

## Who is affected by CVE-2021-21295?

The published record lists 8 products. Versions are as published: "before 4.1.60" means every version up to, but not including, 4.1.60.

- [Netty](https://www.precursorintelligence.com/product/netty/netty) by [Netty](https://www.precursorintelligence.com/vendors/netty) (Application): before 4.1.60
- [Communications Cloud Native Core Policy](https://www.precursorintelligence.com/product/oracle/communications_cloud_native_core_policy) by [Oracle](https://www.precursorintelligence.com/vendors/oracle) (Application): 1.14.0
- [Debian Linux](https://www.precursorintelligence.com/product/debian/debian_linux) by [Debian](https://www.precursorintelligence.com/vendors/debian) (Operating system): 10.0
- [Kudu](https://www.precursorintelligence.com/product/apache/kudu) by [Apache](https://www.precursorintelligence.com/vendors/apache) (Application): before 1.16.0
- [Oncommand API Services](https://www.precursorintelligence.com/product/netapp/oncommand_api_services) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Application): version not specified
- [Oncommand Workflow Automation](https://www.precursorintelligence.com/product/netapp/oncommand_workflow_automation) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Application): version not specified
- [Quarkus](https://www.precursorintelligence.com/product/quarkus/quarkus) by [Quarkus](https://www.precursorintelligence.com/vendors/quarkus) (Application): up to 1.13.7
- [Zookeeper](https://www.precursorintelligence.com/product/apache/zookeeper) by [Apache](https://www.precursorintelligence.com/vendors/apache) (Application): 3.5.9

## How do I fix CVE-2021-21295?

1. **Check whether you run it.** Compare your versions with the affected products above. Not sure what you run? Precursor Intelligence can work it out from your public website, or you can pick it from a list.
2. **Apply the vendor’s fix.** Install the update named in the vendor’s advisory. We’ve put the advisories and patch notes first in the list.
3. **If you can’t patch today.** Follow any mitigation the vendor publishes, and limit who can reach the affected system from the internet until the update is in.
4. **Keep an eye on it.** Track the product and we tell you as soon as we spot this flaw on the list of exploited flaws, or a jump in its exploit likelihood.

### References

- [github.com](https://github.com/Netflix/zuul/pull/980) (Patch, Third-party advisory)
- [github.com](https://github.com/netty/netty/commit/89c241e3b1795ff257af4ad6eadc616cb2fb3dc4) (Patch, Vendor advisory)
- [github.com](https://github.com/netty/netty/security/advisories/GHSA-wm47-8v5p-wjpj) (Third-party advisory)
- [lists.apache.org](https://lists.apache.org/thread.html/r02e467123d45006a1dda20a38349e9c74c3a4b53e2e07be0939ecb3f%40%3Cdev.ranger.apache.org%3E)
- [lists.apache.org](https://lists.apache.org/thread.html/r040a5e4d9cca2f98354b58a70b27099672276f66995c4e2e39545d0b%40%3Cissues.hbase.apache.org%3E)
- [lists.apache.org](https://lists.apache.org/thread.html/r04a3e0d9f53421fb946c60cc54762b7151dc692eb4e39970a7579052%40%3Ccommits.servicecomb.apache.org%3E)
- [lists.apache.org](https://lists.apache.org/thread.html/r0b09f3e31e004fe583f677f7afa46bd30110904576c13c5ac818ac2c%40%3Cissues.flink.apache.org%3E)
- [lists.apache.org](https://lists.apache.org/thread.html/r15f66ada9a5faf4bac69d9e7c4521cedfefa62df9509881603791969%40%3Cjira.kafka.apache.org%3E)
- [lists.apache.org](https://lists.apache.org/thread.html/r16c4b55ac82be72f28adad4f8061477e5f978199d5725691dcc82c24%40%3Ccommits.servicecomb.apache.org%3E)
- [lists.apache.org](https://lists.apache.org/thread.html/r1908a34b9cc7120e5c19968a116ddbcffea5e9deb76c2be4fa461904%40%3Cdev.zookeeper.apache.org%3E)
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- [lists.apache.org](https://lists.apache.org/thread.html/r584cf871f188c406d8bd447ff4e2fd9817fca862436c064d0951a071%40%3Ccommits.pulsar.apache.org%3E)
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## Questions

### What is CVE-2021-21295?

CVE-2021-21295 is a medium-severity flaw in Netty, published on 9 March 2021. It can be attacked from anywhere on the internet without logging in, and nobody has to click anything. If it works, the attacker can change data.

### How serious is CVE-2021-21295?

It scores 5.9 out of 10 on the CVSS severity scale, which is medium. Our risk score for CVE-2021-21295 is 37 out of 100, which is Low. Its action is "Monitor". Not confirmed as exploited yet, but serious or likely enough that it should not wait for a quiet month.

### Is CVE-2021-21295 being exploited?

It is not on the list of flaws known to be exploited today. The latest forecast puts the chance of it being used in the next 30 days at 19%. That can change at any time, which is why we keep watching.

### Which versions does CVE-2021-21295 affect?

For Netty: before 4.1.60. The record lists 8 products in all, each with its versions, under "Who is affected" on this page.

### How do I fix CVE-2021-21295?

Install the update the vendor names in its advisory, linked first under "How do I fix" on this page. If you can't patch today, follow the vendor's mitigation and limit who can reach the affected system from the internet.

## Does CVE-2021-21295 affect you?

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