CVE-2026-21434: webtransport-go is an implementation of the WebTransport protocol

Routine

MEDIUM 5.3REMOTEUPGRADE

CVE-2026-21434 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. From 0.3.0 to 0.9.0, an attacker can cause excessive memory consumption in webtransport-go's session implementation by sending a WT_CLOSE_SESSION capsule containing an excessively large Application Error Message. The implementation does not enforce the draft-mandated limit of 1024 bytes on this field, allowing a peer to send an arbitrarily large message payload that is fully read

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-770
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: >= 0.3.0 and < 0.10.0

Fixed in: 0.10.0

Safe to downgrade to: a release before 0.3.0

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.

3. If no update is available yet, DOWNGRADE to a release before 0.3.0 (Debian / Ubuntu)

apt-cache madison webtransport-go sudo apt install webtransport-go=<VERSION_BEFORE_0.3.0> sudo apt-mark hold webtransport-go

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list webtransport-go sudo dnf downgrade webtransport-go-<VERSION_BEFORE_0.3.0> sudo dnf versionlock add webtransport-go

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

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-2026-21434?

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

Is CVE-2026-21434 being exploited?

CVE-2026-21434 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-2026-21434 affect?

CVE-2026-21434 affects webtransport-go. Severity is medium (CVSS 5.3). webtransport-go is an implementation of the WebTransport protocol. From 0.3.0 to 0.9.0, an attacker can cause excessive memory consumption in webtransport-go's session implementation by sending a WT_CLOSE_SESSION capsule containing an exces

How urgent is CVE-2026-21434?

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-2026-21435 MEDIUM 5.3 UPGRADE webtransport-go is an implementation of the WebTransport protocol

References

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