CVE-2026-21435: webtransport-go is an implementation of the WebTransport protocol

Q: How do I fix CVE-2026-21435?

webtransport-go: upgrade to 0.10.0. Exact commands for each OS are on this page.

Q: Is CVE-2026-21435 being exploited?

CVE-2026-21435 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-21435 affect?

CVE-2026-21435 affects webtransport-go. Severity is medium (CVSS 5.3). 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

Q: How urgent is CVE-2026-21435?

Priority 34/100. Remotely exploitable, no login, no user action

Routine

MEDIUM 5.3REMOTEUPGRADE

CVE-2026-21435 is a medium-severity flaw (CVSS 5.3) affecting webtransport-go. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 60%)
CISA exploited list	No
Weakness	CWE-400
Published	2026-02-12
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: quic-go

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-02-12 19:15 UTCPublished CVE record published
2026-06-17 10:18 UTCUpdated Record last modified

quic-go webtransport-go UPGRADE

Affected: < 0.10.0

Fixed in: 0.10.0

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy webtransport-go || rpm -q webtransport-go || webtransport-go --version

2. UPGRADE to 0.10.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade webtransport-go

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade webtransport-go

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-21435?

webtransport-go: upgrade to 0.10.0. Exact commands for each OS are on this page.

Is CVE-2026-21435 being exploited?

CVE-2026-21435 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-21435 affect?

CVE-2026-21435 affects webtransport-go. Severity is medium (CVSS 5.3). 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

How urgent is CVE-2026-21435?

Priority 34/100. Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-21438 MEDIUM 5.3 UPGRADE webtransport-go is an implementation of the WebTransport protocol
CVE-2026-21434 MEDIUM 5.3 UPGRADE webtransport-go is an implementation of the WebTransport protocol

References

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