h2o Access-Log Formatter buffer overflow

Summaryinfo

A vulnerability has been found in h2o and classified as critical. This issue affects some unknown processing of the component Access-Log Formatter. Performing a manipulation results in buffer overflow. This vulnerability is cataloged as CVE-2026-55368. It is possible to initiate the attack remotely. There is no exploit available. To fix this issue, it is recommended to deploy a patch.

Detailsinfo

A vulnerability was found in h2o (the affected version unknown). It has been classified as critical. This affects an unknown part of the component Access-Log Formatter. The manipulation with an unknown input leads to a buffer overflow vulnerability. CWE is classifying the issue as CWE-120. The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer, leading to a buffer overflow. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-55368 since 06/16/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. The technical details are unknown and an exploit is not publicly available.

Applying the patch 48b7ae12107fd2b118e4641da49599e9aac8edd8 is able to eliminate this problem.

Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Name

• h2o

Website

• Product: https://github.com/h2o/h2o/

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