CVE-2026-21435

webtransport-go CloseWithError can block indefinitely

CVSS 3.1 5.3 MEDIUMEUVD CVSS 3.1 5.3 CVSS 3.1 · [email protected] · Secondary 5.3 MEDIUMCVSS 3.1 · [email protected] · Primary 7.5 HIGHCVSS_V3 Publisher rating MODERATEFixed by github.com/quic-go/webtransport-go 0.10.0osv · severity unknownFixed by github.com/quic-go/webtransport-go 0.10.0KEV status unknown

Original advisory

cve-program · CVE-2026-21435

Published

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.

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euvd · EUVD-2026-6717

Published

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nvd · CVE-2026-21435

Published

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osv · GHSA-px4r-g4p3-hhqv

Published

## Summary 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. ## Details WebTransport over HTTP/3 signals session termination by sending a WT_CLOSE_SESSION capsule on the CONNECT stream. The capsule is only needed to transmit a reason phrase and an error code to the peer. After the capsule is sent, the CONNECT stream is closed. In affected versions, the closure procedure blocked indefinitely while waiting for sufficient QUIC flow control credit from the peer. A malicious peer can withhold this credit, preventing the capsule from being sent. ## The Fix webtransport-go now attempts to send the WT_CLOSE_SESSION capsule with a short deadline. If the capsule cannot be sent within this deadline, the CONNECT stream is reset instead. This closes the WebTransport session promptly without transmitting the optional error details. This prevents indefinite blocking on session closure.

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osv · GO-2026-4488

Published

webtransport-go: CloseWithError can block indefinitely in github.com/quic-go/webtransport-go

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Affected versions, risk and source history

Exploitation and risk

Source status

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cve-program · CVE-2026-21435

Published

webtransport-go CloseWithError can block indefinitely

Source published: 12 Feb 2026 18:22:58 UTC
Source modified: 17 Feb 2026 15:36:08 UTC
Saved here: 23 Oct 2026 15:48:27 UTC
Source terms: CVE Program Terms of Use

Original source record · Cite this exact observation · Complete original publisher JSON

Source: cve-program · CVE Program Terms of Use. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

CVSS 3.1 · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

euvd · EUVD-2026-6717

Published

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 with…

Additional publisher context

EUVD source date text has no stated timezone and is retained without inventing a UTC instant. Related bulletin membership is not an alias, common cause or applicability claim. A missing vector with a zero score is treated as unrated. Complete vendor/product and underlying source evidence remains in the original JSON.

Original publisher date fields

datePublished: Feb 12, 2026, 6:22:58 PM

dateUpdated: Feb 17, 2026, 3:36:08 PM

Source published: Feb 12, 2026, 6:22:58 PM (timezone not stated)
Source modified: Feb 17, 2026, 3:36:08 PM (timezone not stated)
Saved here: 15 Nov 2026 07:33:49 UTC
Source terms: ENISA EUVD · source terms and attribution apply

Original source record · Cite this exact observation · Complete original publisher JSON

Source: euvd · ENISA EUVD · source terms and attribution apply. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

EUVD CVSS 3.1 · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

nvd · CVE-2026-21435

Published

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 with…

Additional publisher context

NVD analysis and CPE criteria are source context, not proof that a package or your environment is affected. Complete affected data, logical configurations and SSVC are retained in the original JSON.

Source analysis status: Analyzed

Weakness classifications: CWE-400

Original publisher date fields

published: 2026-02-12T19:15:51.503

lastModified: 2026-06-17T10:18:40.043

1 source CPE criteria · context only

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

The original JSON retains the logical configuration operators, exclusions and version bounds.

Source published: 12 Feb 2026 19:15:51 UTC
Source modified: 17 Jun 2026 10:18:40 UTC
Saved here: 25 Oct 2026 05:17:32 UTC
Source terms: NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

Source: nvd · NIST NVD usage terms; underlying source attribution retained. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

CVSS 3.1 · [email protected] · Secondary · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

CVSS 3.1 · [email protected] · Primary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

osv · GHSA-px4r-g4p3-hhqv

Published

webtransport-go: CloseWithError can block indefinitely

Source published: 12 Feb 2026 15:29:01 UTC
Source modified: 10 Sep 2026 03:50:59 UTC
Saved here: 24 Oct 2026 13:19:41 UTC
Source terms: GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

Source: osv · GitHub Advisory Database · CC-BY-4.0. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Publisher rating · MODERATE

osv · GO-2026-4488

Published

webtransport-go: CloseWithError can block indefinitely in github.com/quic-go/webtransport-go

Source published: 17 Feb 2026 18:09:06 UTC
Source modified: 23 Feb 2026 12:43:59 UTC
Saved here: 24 Oct 2026 13:19:41 UTC
Source terms: OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

Source: osv · OSV · upstream source license applies. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.