How severe is CVE-2026-40170?

CVE-2026-40170 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2026-40170 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (51st percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-40170?

CVE-2026-40170 affects Tatsuhiro-t Ngtcp2. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-40170?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

Does CVE-2026-40170 have an EU (EUVD) identifier?

Yes. CVE-2026-40170 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-23302.

When was CVE-2026-40170 published?

CVE-2026-40170 was published on 2026-04-16 and last updated on 2026-06-30.

CVE-2026-40170

CVE-2026-40170 is a high-severity vulnerability in Tatsuhiro-t Ngtcp2 with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-121.

Key facts

Severity: High (CVSS 3.x base score 7.5)
EPSS exploit prediction: 1% (51st percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-23302
Weakness: CWE-121
Affected product: Tatsuhiro-t Ngtcp2
Published: 2026-04-16
Last modified: 2026-06-30

Description

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.

Frequently asked questions

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.
How severe is CVE-2026-40170?: CVE-2026-40170 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2026-40170 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (51st percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-40170?: CVE-2026-40170 affects Tatsuhiro-t Ngtcp2. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-40170?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2026-40170 have an EU (EUVD) identifier?: Yes. CVE-2026-40170 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-23302.
When was CVE-2026-40170 published?: CVE-2026-40170 was published on 2026-04-16 and last updated on 2026-06-30.

References

Affected products (1)

cpe:2.3:a:tatsuhiro-t:ngtcp2:*:*:*:*:*:*:*:*

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