CVE-2026-55368: high-severity vulnerability in h2o
h2o: buffer overflow when using the %{error}x log directive
Published · Updated
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21Vexday Risk Score
No sign of exploitation. No public exploitation artifact known so far.
ssvc Trackcvss 8.1epss 0.3%
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.
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
Affected products
h2o · h2oRelated CVEs — h2o
In the same product, most dangerous first.
CVE-2021-43848HIGHUnititialized memory access in h2oEPSS 2.7%CVE-2023-30847HIGHH2O vulnerable to read from uninitialized pointer in the reverse proxy handlerEPSS 0.9%CVE-2023-50247LOWh2o QUIC state exhaustion DoSEPSS 0.9%CVE-2024-45403LOWH2O assertion failure when HTTP/3 requests are cancelledEPSS 0.7%CVE-2026-54340HIGHh2o has HTTP/2 state amplificationEPSS 0.5%CVE-2026-44453HIGHh2o is vulnerable to musl libc stack overflowEPSS 0.5%