What’s Vulnerable › Vulnerabilities

CVE-2026-21435: webtransport-go CloseWithError can block indefinitely

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-02-12 · updated 2026-06-17
• Weakness: CWE-400

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.

Affected products

• quic-go webtransport-go: before 0.10.0 — fixed in 0.10.0

How to fix

Upgrade to a fixed version: quic-go webtransport-go 0.10.0. Check the vendor advisory for mitigations.

References

• https://github.com/quic-go/webtransport-go/releases/tag/v0.10.0
• https://github.com/quic-go/webtransport-go/security/advisories/GHSA-px4r-g4p3-hhqv

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