dbcve.org
Webtransport GoApplication · Quic Go

CVE-2026-21434

HIGH · 7.5 CVSS v3.1 Published 2026-02-12
Fix available
A fix is available. Upgrade to 0.10.0 or later.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

The webtransport-go library (versions 0.3.0-0.9.0) fails to enforce the WebTransport draft specification limit of 1024 bytes on the WT_CLOSE_SESSION capsule's Application Error Message field. This allows an attacker to send arbitrarily large payloads that are fully read into memory, causing excessive memory consumption.

MitigationUpgrade webtransport-go to version 0.10.0 or later, which implements the 1024-byte limit on the Application Error Message field as required by the specification.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Webtransport GoApplication
Affected:>= 0.3.0, < 0.10.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Locate webtransport-go dependency in your codebase
    Search your go.mod file or vendor directory for 'webtransport-go' or 'quic-go/webtransport' and note the exact version string listed.
    Affected if The version listed is >= 0.3.0 and < 0.10.0 (or if the version is unspecified and the library is present).
  2. Identify services using WebTransport
    Search your codebase for instances where webtransport-go is imported and a WebTransport server or handler is configured. Look for usage of server types or session handlers from the webtransport-go library.
    Affected if Your application creates and runs a WebTransport server or accepts WebTransport connections.
  3. Verify WT_CLOSE_SESSION capsule processing
    Review your WebTransport session handling code to determine if your server can receive and process WT_CLOSE_SESSION capsules from clients. This is typically part of the session lifecycle when clients initiate session closure.
    Affected if Your WebTransport server accepts incoming connections and processes session closure events from clients.
  4. Check for memory consumption patterns
    Monitor your application's memory usage during normal operation. Specifically observe memory behavior when WebTransport sessions are closed from the client side, as this triggers WT_CLOSE_SESSION capsule processing.
    Affected if Memory consumption grows unexpectedly large when WebTransport clients send large payloads in WT_CLOSE_SESSION capsules, indicating the oversized field is being fully buffered.

You are affected if your application uses webtransport-go version 0.3.0 through 0.9.0 and runs a WebTransport server that can receive WT_CLOSE_SESSION capsules from clients.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 0.10.0 or later
Fixed in 0.10.0
Interim mitigation

Upgrade webtransport-go to version 0.10.0 or later, which implements the 1024-byte limit on the Application Error Message field as required by the specification.

Recommended fix High confidence

0.10.0

  1. Identify the current version of webtransport-go in use by checking go.mod or dependencies
  2. Run 'go get github.com/quic-go/webtransport-go@latest' or 'go get github.com/quic-go/[email protected]' to upgrade to the fixed version
  3. Run 'go mod tidy' to update go.mod and go.sum
  4. Verify the upgrade was successful by checking the installed version
  5. Rebuild and test the application to ensure compatibility

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Webtransport Go Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing4.0 h
  • Review / QA2.0 h
9.0 hours of engineering $1,540
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,464.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2026-21434 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-21434 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data