h2o: buffer overflow when using the %{error}x log directive

published 2026-11-04 4 d ago modified 2026-11-04 4 d ago also EUVD-2026-117178

cvss: 8.1; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.3%; EPSS 0.3% chance of exploitation in 30 days, percentile 25%
public exploits: 0; none known

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 48b7ae12107fd2b118e4641da49599e9aac8edd8, H2O's access-log formatter could write past its log-line buffer when an affected custom access-log format was configured. The allocation errors affected %{error}x, %q when the query string was empty, and numeric elements whose temporary terminating NUL reached the end of the buffer. A matching request could crash the server, while the effect of the one-byte NUL overwrite beyond a crash was not established. This vulnerability is fixed with commit 48b7ae12107fd2b118e4641da49599e9aac8edd8.

Patch / fix

links 2

github.com/h2o/h2o@48b7ae1 commit
github.com/h2o/h2o#3757 pull request

Exploit prediction

0.32% chance of exploitation in the next 30 days, higher than 25% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	8.1	CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected products

h2o h2o

Classification

Weakness

CWE-787 Out-of-bounds Write

References

Sources

CVE List CVE-2026-55368
NVD CVE-2026-55368

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Summary