CVE-2026-44436Public exploitNetwork attack vector

Service crash via oversized QUIC connection IDs

Published Jul 16, 2026 · Updated Jul 16, 2026

Out-of-bounds write in H2O quicly before commit 8b178e6 allows remote attackers to crash dependent QUIC services via oversized CIDs. quicly_decode_packet accepts QUIC v1 source or destination CIDs longer than 20 bytes before fixed-size quicly_cid_t storage copies them, corrupting connection state. Applications without their own CID-length enforcement are exposed; the overwrite remains inside its allocated chunk but triggers assertions that interrupt service.

CVSS severity7.5
High
EPSS probability0.49%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Out-of-bounds write in H2O quicly before commit 8b178e6 allows remote attackers to crash dependent QUIC services via oversized CIDs. quicly_decode_packet accepts QUIC v1 source or destination CIDs longer than 20 bytes before fixed-size quicly_cid_t storage copies them, corrupting connection state. Applications without their own CID-length enforcement are exposed; the overwrite remains inside its allocated chunk but triggers assertions that interrupt service.

The record

CVE
CVE-2026-44436
Published
Jul 16, 2026
Updated
Jul 16, 2026
Vendor
H2O Project
Product
quicly
Classifications
CWE-787, CWE-120, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 16, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 16, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 8b178e6

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

H2O Project · quiclyversion=< 8b178e6

Published CVSS scores

7.5GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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.

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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 exploitPublished

Weakness, pattern, technique

CWE-787Out-of-bounds Write
CWE-120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
T1499.004Application or System Exploitation

Public exploit references

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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