CVE intelligence

CVE-2026-40170

Vulnerability intelligence

Published Apr 16, 2026Sources checked Nov 13, 2026

7.5HIGHCVSS out of 10

What this means

Review the available evidence

CVE-2026-40170 and is rated High severity with a CVSS score of 7.5. It is not in the current CISA KEV record we collected. That does not prove exploitation has not occurred.

What to do next

Remediation and response

Microsoft Security Response Center guidance

Vendor revision: Nov 12, 2026

Release Notes

Vendor-listed releases

2:4.23.13-3.azl4

CISA KEVNot listedBased on the latest collected catalog

EPSS0.8%Estimated 30-day exploitation probability

Ransomware useNot markedCISA KEV ransomware field

Threat actors0Source-linked actor relationships

Overview

What is CVE-2026-40170?

ngtcp2 is a C implementation of the IETF QUIC protocol. In versions prior to 1.22.1, ngtcp2_qlog_parameters_set_transport_params() serializes peer transport parameters into a fixed 1024-byte stack buffer without bounds checking.

When qlog is enabled, a remote peer can send sufficiently large transport parameters during the QUIC handshake to cause writes beyond the buffer boundary, resulting in a stack buffer overflow. This affects deployments that enable the qlog callback and process untrusted peer transport parameters.

This issue has been fixed in version 1.22.1. If developers are unable to immediately upgrade, they can disable the qlog on client.

Source: NVD