7.4

CVE-2021-43848: Unititialized memory access in h2o

Modified
AliasesUBUNTU-CVE-2021-43848
Published 01-02-2022Last modified 23-04-2025

Vulnerability summary

CVE-2021-43848 is a high (CVSS 7.4) Use of Uninitialized Resource vulnerability in dena h2o. EPSS estimates a 2.67% chance of exploitation in the next 30 days, higher than 85.7% of scored vulnerabilities.

Priority
Prioritisesevere

Severe, CVSS 7.4 (CVE.org); no exploitation reported by the 5 checked sources; EPSS 3% (FIRST, 03-11-2026)

Source freshness unavailable

How is this decided?
CVSS score
7.4/10
010
High · v3.1 (CVE.org)
EPSS score
2.67%
0%100%
Low · 0.00 pp in 30 days
KEVNot listedNot listed by any tracked catalog.
RansomwareNot flaggedNo ransomware use recorded.
Public exploits0No public exploits recorded

Timeline

  1. 01-02-2022
    PublishedVulnerability first disclosed.
  2. 23-04-2025
    Last modifiedVulnerability information updated.

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.

CVSS metrics

4 scores · 3 sources · 2 versions

EPSS trend

Probability

2.67%

Chance of exploitation in the next 30 days

Percentile

86%

Versus all other CVEs

30-day change

No change 0.00 pp

From 2.67% on 04-10-2026

04-10-202603-11-2026

Known exploited

Compare KEV catalogs
CISA KEV

Not in the CISA KEV catalog.

ENISA CNW

Not in the ENISA CNW catalog.

CIRCL

Not in the CIRCL catalog.

Public exploits

0 found

No public exploits indexed.

Weaknesses

CWE · root cause

Affected systems

Version check

Known conditions: network-reachable, no authentication, no user interaction (CVSS vector). Other conditions: not stated in public data.

Dena1 product
  • h2o< 2021-12-20
H2o1 product
  • h2oCommits prior to 8c0eca3
Ubuntu1 product
  • h2oAll versions (No fix recorded)

3 vendors · 3 products

References

4 links