How severe is CVE-2026-21435?

CVE-2026-21435 has a CVSS 3.x base score of 5.3, rated medium 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 low.

Is CVE-2026-21435 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (33rd percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-21435?

CVE-2026-21435 affects Quic-go Webtransport-go. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-21435?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

Does CVE-2026-21435 have an EU (EUVD) identifier?

Yes. CVE-2026-21435 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-6717.

When was CVE-2026-21435 published?

CVE-2026-21435 was published on 2026-02-12 and last updated on 2026-06-17.

CVE-2026-21435

CVE-2026-21435 is a medium-severity vulnerability in Quic-go Webtransport-go with a CVSS 3.x base score of 5.3. 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-400.

Key facts

Severity: Medium (CVSS 3.x base score 5.3)
EPSS exploit prediction: 0% (33rd percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-6717
Weakness: CWE-400
Affected product: Quic-go Webtransport-go
Published: 2026-02-12
Last modified: 2026-06-17

Description

webtransport-go is an implementation of the WebTransport protocol. Prior to v0.10.0, an attacker can cause a denial of service in webtransport-go by preventing or indefinitely delaying WebTransport session closure. A malicious peer can withhold QUIC flow control credit on the CONNECT stream, blocking transmission of the WT_CLOSE_SESSION capsule and causing the close operation to hang. This vulnerability is fixed in v0.10.0.

Frequently asked questions

What is CVE-2026-21435?: webtransport-go is an implementation of the WebTransport protocol. Prior to v0.10.0, an attacker can cause a denial of service in webtransport-go by preventing or indefinitely delaying WebTransport session closure. A malicious peer can withhold QUIC flow control credit on the CONNECT stream, blocking transmission of the WT_CLOSE_SESSION capsule and causing the close operation to hang. This vulnerability is fixed in v0.10.0.
How severe is CVE-2026-21435?: CVE-2026-21435 has a CVSS 3.x base score of 5.3, rated medium 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 low.
Is CVE-2026-21435 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (33rd percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-21435?: CVE-2026-21435 affects Quic-go Webtransport-go. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-21435?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-21435 have an EU (EUVD) identifier?: Yes. CVE-2026-21435 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-6717.
When was CVE-2026-21435 published?: CVE-2026-21435 was published on 2026-02-12 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:quic-go:webtransport-go:*:*:*:*:*:go:*:*

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