CVE-2021-43848 Unititialized memory access in h2o

High 7.4 Published 1 Feb 2022 Updated 23 Apr 2025

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

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: None listed

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

Which versions are affected by CVE-2021-43848?

It affects 1 product.

Products include 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: 1 proofs of concept on GitHub.

Sources

Also known as GHSA-f9xw-j925-m4m4, UBUNTU-CVE-2021-43848.