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MEDIUM severity

CVE-2026-21435 — webtransport-go

MEDIUM

CVE-2026-21435 is a medium-severity (CVSS 5.3) Uncontrolled Resource Consumption vulnerability in github.com/quic-go/webtransport-go. A fix is available for github.com/quic-go/webtransport-go — see the affected versions and patch details below.

webtransport-go CloseWithError can block indefinitely

Also known asGHSA-px4r-g4p3-hhqvGO-2026-4488
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Nov 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-21435.

EPSS Exploitation Probability

via FIRST.org ↗
0.5%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs40th percentile — riskier than 40% of all scored CVEsHighest risk
0.00%0.32%0.64%0.96%0.0%0.5%May 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected
🐹github.com/quic-go/webtransport-go

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

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.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐹Gogithub.com/quic-go/webtransport-goall versions0.10.0go get github.com/quic-go/webtransport-go@v0.10.0

Affected Products

1 product · 1 configurations
Application
webtransport-goquic-go
< 0.10.0
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/quic-go/webtransport-go, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update github.com/quic-go/webtransport-go to 0.10.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-21435 is resolved across your whole dependency graph.

  3. Workarounds

    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

## 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 versi
O3 Security · Impact-Aware SCA

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CVE-2026-21435: webtransport-go DoS — Fixed in 0.10.0