Quick answers
- Is CVE-2026-55368 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-55368 being exploited in the wild.
- How severe is CVE-2026-55368?
- High severity, with a CVSS score of 8.1, published 4 November 2026.
- Which products and versions are affected?
- H2o prior to commit 48b7ae12107fd2b118e4641da49599e9aac8edd8.
- Is there a fix for CVE-2026-55368?
- Yes: fixed in commit 48b7ae12107fd2b118e4641da49599e9aac8edd8. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
- How is CVE-2026-55368 attacked?
- It is attacked over the network, in a web application.
- Am I affected by CVE-2026-55368?
- Check whether you run H2o at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
How to fix
- Fixed in: commit 48b7ae12107fd2b118e4641da49599e9aac8edd8
Technical details
The access-log formatter in H2O's logconf.c failed to reserve sufficient buffer capacity when formatting log lines with specific format specifiers (%{error}x, %q with empty query string, or numeric elements). This allows a single-byte NUL overwrite past the buffer, causing an out-of-bounds write that crashes the server. A network attacker can trigger this with a matching request requiring no authentication.
Affected products
- h2o H2O prior to commit 48b7ae12107fd2b118e4641da49599e9aac8edd8
Timeline
- 2026-11-04: disclosed
- 2026-10-23: patched: commit 48b7ae1 merged