Is CVE-2021-43848 actively exploited?

No active exploitation of CVE-2021-43848 has been confirmed so far. Continue monitoring, as exploit status can change.

Can CVE-2021-43848 be exploited remotely?

Yes. CVE-2021-43848 is exploitable remotely over the network (CVSS attack vector: Network).

What is the CVSS score of CVE-2021-43848?

CVE-2021-43848 has a CVSS score of 5.9 (MEDIUM).

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MEDIUM

CVE-2021-43848

CWE-908Published: February 1, 2022· Updated: Jun 17, 2026

5.9

CVSS v3.1

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.

NVD Source

Technical Analysis

CVE-2021-43848 can be exploited remotely over the network without requiring physical or adjacent access, significantly expanding the attack surface for threat actors.

The vulnerability requires no privileges and no user interaction, making it a prime target for automated exploitation campaigns and worm-like propagation.

A successful exploit results in complete confidentiality breach (data exposure), with a CVSS base score of 5.9.