High severity · published Jul 10, 2026
CVE-2026-55213
h2o: musl libc stack overflow (QPACK)
7.5
CVSS
0.5%
EPSS
No
On KEV
Description
h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1, when h2o processes a QPACK instruction sent from the peer over HTTP/3, lib/http3/qpack.c might allocate an on-stack buffer as large as approximately 800 KB by calling alloca, which exceeds the default pthread stack size used by musl libc and causes the h2o server to crash with a segmentation fault while touching the guard page. This issue is fixed in commit edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
None
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.5% · higher than 42% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
H2
h2o · h2o
< edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1 · source: vendor
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-55213
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-55213
github.com
https://github.com/h2o/h2o/commit/edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1
github.com
https://github.com/h2o/h2o/security/advisories/GHSA-432c-8xmj-frmq
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