CVE-2026-21434 webtransport-go affected by Memory Exhaustion Attack due to Missing Length Check in WT_CLOSE_SESSION Capsule

Medium 5.3 Published 12 Feb 2026 Updated 12 Feb 2026

An attacker can cause memory exhaustion in webtransport-go by sending oversized close session capsules, leading to denial of service.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-21434?

CVE-2026-21434 is a medium-severity denial of service vulnerability in quic-go webtransport-go, published on 12 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

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 and stored in memory. This allows an attacker to consume an arbitrary amount of memory. The attacker must transmit the full payload to achieve the memory consumption, but the lack of any upper bound makes large-scale attacks feasible given sufficient bandwidth. This vulnerability is fixed in 0.10.0.

Weakness: CWE-770

Which versions are affected by CVE-2026-21434?

4 open source packages ship an affected version, including github.com/quic-go/webtransport-go and github.com/marten-seemann/webtransport-go. It affects 1 product.

Package	Affected	Fixed in
`github.com/quic-go/webtransport-go` Go	All versions	`0.10.0`
`github.com/quic-go/webtransport-go` Go	From `0.3.0`	`0.10.0`
`github.com/quic-go/webtransport-go` Go	From `0.3.0`	`0.10.0`
`github.com/marten-seemann/webtransport-go` Go	From `0.3.0`	None

Products include Quic-Go Webtransport-Go.

How do I fix CVE-2026-21434?

Upgrade github.com/quic-go/webtransport-go to 0.10.0 or later.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-21434 being exploited?

There is no known exploitation of CVE-2026-21434 in the wild and no public exploit code.

Sources

Also known as GHSA-g6x7-jq8p-6q9q, GO-2026-4485, MINI-9gp9-7mfp-q9gc, MINI-cjvx-rf94-qwwf.