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


- **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
- **Fix available:** Yes
- **Published:** 12 Feb 2026
- **Updated:** 17 Feb 2026
- **Also known as:** GHSA-px4r-g4p3-hhqv, GO-2026-4488, MINI-7v4c-m83q-mv84, MINI-j8jp-fhv4-h367

## What is CVE-2026-21435?

CVE-2026-21435 is a medium-severity vulnerability in quic-go webtransport-go, published on 12 Feb 2026. 

### Official 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.

Weakness: CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2026-21435?

3 open source packages ship an affected version, including github.com/quic-go/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 | All versions | 0.10.0 |
| `github.com/quic-go/webtransport-go` | Go | All versions | 0.10.0 |

Products: Quic-Go Webtransport-Go.

## How do I fix CVE-2026-21435?

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

## Is CVE-2026-21435 being exploited?

There is no known exploitation of CVE-2026-21435 in the wild and no public exploit code.

## Other vulnerabilities in quic-go webtransport-go

- [CVE-2026-21434](https://insights.sbomify.com/CVE-2026-21434/): webtransport-go affected by Memory Exhaustion Attack due to Missing Length Check in WT_CLOSE_SESSION Capsule (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-2026-21435 and fix it if it is affected.

What it is: CVE-2026-21435 is a medium-severity vulnerability in quic-go webtransport-go, published on 12 Feb 2026. 
Exploitation: There is no known exploitation of CVE-2026-21435 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 all versions; fixed in 0.10.0
- github.com/quic-go/webtransport-go (Go): affected all versions; fixed in 0.10.0

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-2026-21435/ (Markdown: https://insights.sbomify.com/CVE-2026-21435/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21435
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21435
- OSV: https://osv.dev/vulnerability/CVE-2026-21435

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21435/
Summaries are generated and may contain mistakes; the official description above is the record.