# CVE-2026-44433 — h2o quicly

> As of 2026-07-16, CVE-2026-44433 is a MEDIUM-severity vulnerability in h2o quicly, CVSS v3.1 5.3. No Threadlinqs-tracked threat campaign has been attributed to CVE-2026-44433 as of 2026-07-16; the identifier is re-checked against the Threadlinqs threat corpus on every daily ingest.

**Last updated:** 2026-07-16

## What is CVE-2026-44433?

Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server. Prior to commit 8b178e6, an adversarial peer could send a STREAM frame carrying just one byte at the largest offset being permitted to obtain additional flow control credit, which under certain circumstances could lead to a Denial of Service. Assuming the application prepares a receive buffer for storing all data that arrive out-of-order, up to the largest offset being received, this behavior could lead to the application allocating large amount of memory with the peer sending only a handful of packets, resulting in memory exhaustion. In addition to the receive buffer allocation strategy, the severity of this vulnerability depends on how the application controls the stream concurrency. In case of the H2O HTTP server, under its default setting, this bug increases the maximum amount of memory allocated per connection by about 4 times. This issue has been fixed by commit 8b178e6.

The record classifies CVE-2026-44433 under weakness classes [CWE-770](https://intel.threadlinqs.com/cwe/CWE-770), [CWE-400](https://intel.threadlinqs.com/cwe/CWE-400). Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction. 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:** 5.3 — MEDIUM (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 2.7/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- **Published:** 2026-07-16

## Is CVE-2026-44433 being exploited?

It currently carries a trending score of 30 in the Threadlinqs vulnerability feed.

## Affected products and versions

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

## How to fix CVE-2026-44433

No vendor patch reference has been recorded for CVE-2026-44433 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-2026-44433

No threat campaign in the Threadlinqs corpus currently references CVE-2026-44433, 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. 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/quicly/security/advisories/GHSA-f7qr-4p37-9gx9)

**Other references**

- [github.com](https://github.com/h2o/quicly/commit/8b178e692c51a3b1031612ef89f03a53aac63c15)

Canonical: https://intel.threadlinqs.com/cve/CVE-2026-44433
Full detection coverage and IOCs for threats exploiting CVE-2026-44433 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp