CVE-2026-55213Network attack vector

Unauthenticated server crash via oversized stack allocation

Published Jul 10, 2026 · Updated Jul 10, 2026

Stack exhaustion in H2O allows remote attackers to crash affected HTTP/3 servers by sending a crafted QPACK instruction. The QPACK decoder in lib/http3/qpack.c uses alloca for a peer-controlled Huffman-encoded header name, reserving up to roughly 800 KB on each worker thread stack. Exploitation requires HTTP/3 to be enabled and a musl libc build using its default 128 KB pthread stack, where the allocation reaches the guard page and terminates the server with SIGSEGV.

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

Stack exhaustion in H2O allows remote attackers to crash affected HTTP/3 servers by sending a crafted QPACK instruction. The QPACK decoder in lib/http3/qpack.c uses alloca for a peer-controlled Huffman-encoded header name, reserving up to roughly 800 KB on each worker thread stack. Exploitation requires HTTP/3 to be enabled and a musl libc build using its default 128 KB pthread stack, where the allocation reaches the guard page and terminates the server with SIGSEGV.

The record

CVE
CVE-2026-55213
Published
Jul 10, 2026
Updated
Jul 10, 2026
Vendor
H2O Project
Product
H2O
Classifications
CWE-789, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 10, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 10, 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=< edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

H2O Project · H2Oversion=< edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1

Published CVSS scores

7.5GitHub, Inc.CVSS: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 exploitUnconfirmed

Weakness, pattern, technique

CWE-789Memory Allocation with Excessive Size Value
T1499.004Application or System Exploitation

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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