# CVE-2021-43848: Unititialized memory access in h2o

> Attackers can leak internal server state, including unencrypted traffic and TLS tickets, from h2o HTTP/3 servers via uninitialized memory access.

- **Severity:** High 7.4 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 1 Feb 2022
- **Updated:** 23 Apr 2025
- **Also known as:** GHSA-f9xw-j925-m4m4, UBUNTU-CVE-2021-43848

## What is CVE-2021-43848?

CVE-2021-43848 is a high-severity vulnerability in h2o, published on 1 Feb 2022. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

h2o is an open source http server. In code prior to the `8c0eca3` commit h2o may attempt to access uninitialized memory. When receiving QUIC frames in certain order, HTTP/3 server-side implementation of h2o can be misguided to treat uninitialized memory as HTTP/3 frames that have been received. When h2o is used as a reverse proxy, an attacker can abuse this vulnerability to send internal state of h2o to backend servers controlled by the attacker or third party. Also, if there is an HTTP endpoint that reflects the traffic sent from the client, an attacker can use that reflector to obtain internal state of h2o. This internal state includes traffic of other connections in unencrypted form and TLS session tickets. This vulnerability exists in h2o server with HTTP/3 support, between commit 93af138 and d1f0f65. None of the released versions of h2o are affected by this vulnerability. There are no known workarounds. Users of unreleased versions of h2o using HTTP/3 are advised to upgrade immediately.

Weakness: CWE-908 (Use of Uninitialized Resource)

## Which versions are affected by CVE-2021-43848?

It affects 1 product.


Products: Dena H2o.

## How do I fix CVE-2021-43848?

No fix has been published for CVE-2021-43848 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-43848 being exploited?

There is no known exploitation of CVE-2021-43848 in the wild and no public exploit code.

Public exploit code:

- exploit for CVE-2021-43848 (github_poc): https://github.com/neex/hui2ochko

## Other vulnerabilities in h2o

- [CVE-2018-0608](https://insights.sbomify.com/CVE-2018-0608/): Memory corruption in H2O (critical 9.8)
- [CVE-2016-7835](https://insights.sbomify.com/CVE-2016-7835/): Use after free in H2O (critical 9.1)
- [CVE-2023-30847](https://insights.sbomify.com/CVE-2023-30847/): H2O vulnerable to read from uninitialized pointer in the reverse proxy handler (high 8.2)
- [CVE-2017-10868](https://insights.sbomify.com/CVE-2017-10868/): Denial of service in H2O (high 7.5)
- [CVE-2017-10869](https://insights.sbomify.com/CVE-2017-10869/): Memory corruption in H2O (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-43848 and fix it if it is affected.

What it is: CVE-2021-43848 is a high-severity vulnerability in h2o, published on 1 Feb 2022. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-43848 in the wild and no public exploit code.

Affected product: h2o. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-43848/ (Markdown: https://insights.sbomify.com/CVE-2021-43848/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-43848
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-43848
- OSV: https://osv.dev/vulnerability/CVE-2021-43848

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-43848/
Summaries are generated and may contain mistakes; the official description above is the record.