CVE-2021-43848Network attack vector

Internal-state disclosure via premature HTTP/3 stream shutdown

Published Feb 1, 2022 · Updated Apr 23, 2025

Information disclosure in unreleased h2o HTTP/3 server builds allows remote attackers to read internal state via crafted QUIC frames. HTTP/3 frame parsers prematurely shut down a request stream while its generator is active, allowing buffered input handling to interpret uninitialized memory as received frames. Exposure requires HTTP/3 plus a reverse-proxy backend or reflecting endpoint; disclosed state includes plaintext traffic from other connections and TLS session tickets.

CVSS severity5.9
Medium
EPSS probability2.67%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Information disclosure in unreleased h2o HTTP/3 server builds allows remote attackers to read internal state via crafted QUIC frames. HTTP/3 frame parsers prematurely shut down a request stream while its generator is active, allowing buffered input handling to interpret uninitialized memory as received frames. Exposure requires HTTP/3 plus a reverse-proxy backend or reflecting endpoint; disclosed state includes plaintext traffic from other connections and TLS session tickets.

The record

CVE
CVE-2021-43848
Published
Feb 1, 2022
Updated
Apr 23, 2025
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-908
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 1, 2022CVE publishedPublication date reported by the CVE source.
  2. Apr 23, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

7.4UbuntuCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:H
5.9NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
4.3NVDAV:N/AC:M/Au:N/C:P/I:N/A:N
5.9GitHub Security AdvisoryCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-908Use of Uninitialized Resource

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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