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

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

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 12 Feb 2026
- **Updated:** 12 Feb 2026
- **Also known as:** GHSA-g6x7-jq8p-6q9q, GO-2026-4485, MINI-9gp9-7mfp-q9gc, MINI-cjvx-rf94-qwwf

## 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 (Allocation of Resources Without Limits or Throttling)

## 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 | Ecosystem | 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: Quic-Go Webtransport-Go.

## How do I fix CVE-2026-21434?

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

## Is CVE-2026-21434 being exploited?

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

## Other vulnerabilities in quic-go webtransport-go

- [CVE-2026-21435](https://insights.sbomify.com/CVE-2026-21435/): webtransport-go CloseWithError can block indefinitely (medium 5.3)
- [CVE-2026-21438](https://insights.sbomify.com/CVE-2026-21438/): webtransport-go affected by a Memory Exhaustion Attack due to Missing Cleanup of Streams Map (medium 5.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-21434 and fix it if it is affected.

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

Affected packages and fixed versions:
- github.com/quic-go/webtransport-go (Go): affected all versions; fixed in 0.10.0
- github.com/quic-go/webtransport-go (Go): affected from 0.3.0; fixed in 0.10.0
- github.com/quic-go/webtransport-go (Go): affected from 0.3.0; fixed in 0.10.0
- github.com/marten-seemann/webtransport-go (Go): affected from 0.3.0; no fix listed

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-21434/ (Markdown: https://insights.sbomify.com/CVE-2026-21434/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21434
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21434
- OSV: https://osv.dev/vulnerability/CVE-2026-21434

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21434/
Summaries are generated and may contain mistakes; the official description above is the record.