webtransport-go: CloseWithError can block indefinitely

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-21435: 0.46% estimated exploitation activity in the next 30 days; percentile 40.30 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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 WTCLOSESESSION capsule and causing the close operation to hang. Details WebTransport over HTTP/3 signals session termination by sending a WTCLOSESESSION 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 WTCLOSESESSION capsule with a short deadline. If the capsule cannot be sent within this deadline, the CONNECT stream is reset instead. This closes the…

Published by the source

12 Feb 2026, 15:29 UTC

Last modified by the source

12 Feb 2026, 22:07 UTC

Source last checked

26 Oct 2026, 15:14 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-px4r-g4p3-hhqv

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# Patch Next advisory brief: CVE-2026-21435

Advisory: webtransport-go: CloseWithError can block indefinitely
IDs: CVE-2026-21435, GHSA-px4r-g4p3-hhqv
Record: https://patchnext.com/advisories/cve-2026-21435 (revision 1, source checked 26 Oct 2026, 15:14 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/quic-go/webtransport-go 0.10.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/quic-go/webtransport-go: affected <= 0.9.0; fixed in 0.10.0

## Suggested inspection (read-only)
- Go github.com/quic-go/webtransport-go: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/quic-go/webtransport-go.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-px4r-g4p3-hhqv (GitHub-reviewed advisory GHSA-px4r-g4p3-hhqv lists affected ranges: Go github.com/quic-go/webtransport-go >=0 <=0.9.0)

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