# CVE-2026-44452 — h2o

> As of 2026-07-17, CVE-2026-44452 is a MEDIUM-severity vulnerability in h2o, CVSS v3.1 5.9, EPSS 0.2% (16.2th percentile). No Threadlinqs-tracked threat campaign has been attributed to CVE-2026-44452 as of 2026-07-17; the identifier is re-checked against the Threadlinqs threat corpus on every daily ingest.

**Last updated:** 2026-07-17

## What is CVE-2026-44452?

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 8dc37cb, when h2o receives a ClientHello message over TLS or QUIC and it contains a zero-length SNI extension, the h2o server runs over the zero-length hostname while trying to copy the hostname, assuming that it is NULL-terminated. This is a potential denial-of-service attack vector in sense that it might trigger segmentation violation. This issue has been fixed by commit 8dc37cb.

The record classifies CVE-2026-44452 under weakness classes [CWE-125](https://intel.threadlinqs.com/cwe/CWE-125), [CWE-170](https://cwe.mitre.org/data/definitions/170.html). Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on availability. 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 81 days ago.

## Severity and exploitation probability

- **CVSS v3.1 base score:** 5.9 — MEDIUM (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **EPSS (FIRST):** 0.2% probability of exploitation in the next 30 days, higher than 16.2% of all scored CVEs
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 3.6/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- **Published:** 2026-07-16, last modified 2026-07-17

## Is CVE-2026-44452 being exploited?

It currently carries a trending score of 34 in the Threadlinqs vulnerability feed.

## Affected products and versions

- [h2o](https://intel.threadlinqs.com/vendors/h2o): h2o

## How to fix CVE-2026-44452

No vendor patch reference has been recorded for CVE-2026-44452 in the tracked sources. Follow the references below for a fix, and treat the products listed above as exposed until the vendor states otherwise.

## Threat activity tracking CVE-2026-44452

No threat campaign in the Threadlinqs corpus currently references CVE-2026-44452, in its CVE list or as an indicator. The linkage is recomputed from the live corpus on every daily ingest, so this section fills in as soon as a tracked campaign cites the identifier.

## Sources

Enriched from CVE.org, NVD, FIRST EPSS. Last verified by Threadlinqs on 2026-07-18. This product uses the NVD API but is not endorsed or certified by the NVD.

**Vendor advisory and patch**

- [github.com](https://github.com/h2o/h2o/security/advisories/GHSA-w68q-rqwx-7wvq)

**Other references**

- [github.com](https://github.com/h2o/h2o/commit/8dc37cb1e6171f7f772667618ea440696fed82c3)

Canonical: https://intel.threadlinqs.com/cve/CVE-2026-44452
Full detection coverage and IOCs for threats exploiting CVE-2026-44452 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp