webtransport-go: Memory Exhaustion Attack due to Missing Length Check in WT_CLOSE_SESSION Capsule

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS

• CVE-2026-21434: 0.46% estimated exploitation activity in the next 30 days; percentile 40.30 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary An attacker can cause excessive memory consumption in webtransport-go's session implementation by sending a WTCLOSESESSION 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. Details WebTransport over HTTP/3, as defined in draft-ietf-webtrans-http3, uses the WTCLOSESESSION capsule to signal session termination with an optional detailed error. The draft specifies that the length of the Application Error Message in this capsule MUST NOT exceed 1024 bytes. In affected versions of webtransport-go, the parser does not enforce this 1024-byte…

Published by the source

12 Feb 2026, 15:28 UTC

Last modified by the source

12 Feb 2026, 22:07 UTC

Source last checked

26 Oct 2026, 15:14 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-g6x7-jq8p-6q9q

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# Patch Next advisory brief: CVE-2026-21434

Advisory: webtransport-go: Memory Exhaustion Attack due to Missing Length Check in WT_CLOSE_SESSION Capsule
IDs: CVE-2026-21434, GHSA-g6x7-jq8p-6q9q
Record: https://patchnext.com/advisories/cve-2026-21434 (revision 1, source checked 26 Oct 2026, 15:14 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/quic-go/webtransport-go 0.10.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/quic-go/webtransport-go: affected >= 0.3.0, <= 0.9.0; fixed in 0.10.0

## Suggested inspection (read-only)
- Go github.com/quic-go/webtransport-go: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/quic-go/webtransport-go.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-g6x7-jq8p-6q9q (GitHub-reviewed advisory GHSA-g6x7-jq8p-6q9q lists affected ranges: Go github.com/quic-go/webtransport-go >=0.3.0 <=0.9.0)

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