CVE-2021-43848

mediumpriority · track · 35EPSS 0.027 · 86 %ileexploit · PoCEUUS
grimoire priorityP1 · Urgent
Exploitable?
Public exploit
1 exploit · PoC
Likely / severe?
2.67% (86th) EPSS
CVSS 4.3 · medium
Is it mine?
2 products
Check your exposure
Who says so?
3 sources
cnnvd · enisa_euvd · nvd

EPSS · scored 2026-10-28

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.

01

cvss

Technical severity, 0–10. One row per scoring source.

7.4
010

base score 0–10 · higher = more severe · hover a vector to decode⚠ sources disagree · medium–high · 4.3–7.4

v2.04.3mediumAV:N/AC:M/Au:N/C:P/I:N/A:Nprimary · NVD / NIST USexploitability 8.6 · impact 2.9
v3.15.9mediumCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:Nprimary · NVD / NIST USexploitability 2.2 · impact 3.6
v3.17.4highCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:Hsecondary · ENISA EU
v3.17.4highCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:Hsecondary · security-advisories@github.comexploitability 2.2 · impact 5.2
v3.15.6mediumCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N/E:Psecondary · CVSS-BT (t0sche)
02

ssvc

A stakeholder decision — act, attend, or track — from exploitation and impact.

Trackcoordinatorcisa_ssvcUS
03

weaknesses

The CWE weakness classes this flaw belongs to.

CWE-908US
04

exploits (1)

Public proof-of-concept and weaponised exploit code.

05

affected products (2)

Products and version ranges known to be affected.

06

also known as (2)

Other names and identifiers for this entity.

07

sources

Which sources assert this record, and when.

cnnvdenisa_euvdnvdNVDCVE.orgMITRE
published 2022-02-01modified 2026-06-17
08

references (7)

External citations — patches, advisories, analysis.

scry

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