CVE-2026-40170: ngtcp2 is a C implementation of the IETF QUIC protocol

Q: How do I fix CVE-2026-40170?

ngtcp2: upgrade to 1.22.1. Exact commands for each OS are on this page.

Q: Is CVE-2026-40170 being exploited?

CVE-2026-40170 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-40170 affect?

CVE-2026-40170 affects ngtcp2. Severity is high (CVSS 7.5). 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 i

Q: How urgent is CVE-2026-40170?

Priority 50/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

This week

HIGH 7.5REMOTEUPGRADE

CVE-2026-40170 is a high-severity flaw (CVSS 7.5) affecting ngtcp2. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 tha

Why this priority

High severity
Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.8% (top 44%)
CISA exploited list	No
Weakness	CWE-121, CWE-120
Published	2026-04-16
Last modified	2026-07-15

Publisher (CNA): GitHub, Inc. · Vendor: tatsuhiro-t

Last updated 2026-07-15 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-16 22:16 UTCPublished CVE record published
2026-07-15 02:21 UTCUpdated Record last modified

tatsuhiro-t ngtcp2 UPGRADE

Affected: < 1.22.1

Fixed in: 1.22.1

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy ngtcp2 || rpm -q ngtcp2 || ngtcp2 --version

2. UPGRADE to 1.22.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade ngtcp2

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade ngtcp2

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Advisories and coverage

Official advisories

AlmaLinux Errata: [ALSA-2026:25049] Critical: samba security update 2026-06-11

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-40170?

ngtcp2: upgrade to 1.22.1. Exact commands for each OS are on this page.

Is CVE-2026-40170 being exploited?

CVE-2026-40170 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-40170 affect?

CVE-2026-40170 affects ngtcp2. Severity is high (CVSS 7.5). 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 i

How urgent is CVE-2026-40170?

Priority 50/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

References

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