Quicly is vulnerable to connection state corruption (CVE-2026-44436) | HOL Guard CVE

High · CVSS 7.5CVE-2026-44436

Quicly is vulnerable to connection state corruption

Answer in brief

CVE-2026-44436 records a High severity (CVSS 7.5) vulnerability in Quicly is vulnerable to connection state corruption. The current sources do not mark it as known exploited. The current feed maps h2o/quicly (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Jul 16, 2026Updated Jul 17, 2026Source checked Sep 19, 2026First seen by HOL Jul 17, 2026Material review Jul 17, 2026

Upstream Advisory

Key facts

Risk: High · CVSS 7.5
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Not reported

Why this deserves its current priority

CVSS is 7.5. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps h2o/quicly (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
h2o/quiclygeneric	< 8b178e6	Not reported

Recommended response

Check inventory. Check lockfiles and deployed manifests for h2o/quicly.
Review the reported fix. Monitor this advisory for an available fix and review any installs of the affected package.

Evidence timeline and material changes

Published upstream
Jul 16, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Jul 17, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Jul 17, 2026

Sources and claim methodology

Upstream source description

Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server. Prior to commit 8b178e6, Quicly is vulnerable to a Denial of Service attack through connection state corruption. In QUIC Invariants, the maximum length of a Connection ID is 255 bytes, while QUIC version 1 further restricts the maximum to 20 bytes. Quicly implements QUIC version 1 and therefore its CID buffers are limited to 20 bytes. However, to be able to respond to unknown versions of QUIC, its packet decoder accepts Connection IDs of up to 255 bytes. As its CID buffers are merely 20 bytes long, Quicly must reject QUIC version 1 packets with Connection IDs longer than that. The command line tool bundled with Quicly has had that check, however the library itself lacked such enforcement. As a consequence, when used by applications that lack their own enforcement, the connection state becoming inconsistent to buffer overrun. Fortunately, the overflow stops within the allocated chunk of memory, but nevertheless, the bug leads to assertion failures. This issue has been fixed by commit 8b178e6.

Quoted source text, attributed separately from HOL analysis.