dbcve.org
Webtransport GoApplication · Quic Go

CVE-2026-21435

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. 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.

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

webtransport-go before v0.10.0 contains a denial-of-service vulnerability where a malicious peer can withhold QUIC flow control credit on the CONNECT stream, preventing transmission of the WT_CLOSE_SESSION capsule and causing session close operations to hang indefinitely.

MitigationUpgrade webtransport-go to v0.10.0 or later. If upgrade is not immediately possible, monitor for hung sessions and implement application-level timeouts for WebTransport session closure.

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:< 0.10.0

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. Check if webtransport-go is in dependencies
    Inspect go.mod for 'quic-go/webtransport-go' or run 'go list -m all | grep webtransport' to list all modules
    Affected if The package appears in dependency list with version < v0.10.0 or no version pinned (implicitly using latest)
  2. Verify the installed version number
    Run 'go list -m quic-go/webtransport-go' or check the version entry in go.mod under the webtransport-go module
    Affected if Version is missing (indicating latest which is vulnerable) or shows a version string lower than v0.10.0 (e.g., v0.9.0, v0.8.0)
  3. Confirm WebTransport server is configured
    Search codebase for 'webtransport.Server' initialization or import of 'quic-go/webtransport-go' in server-side code
    Affected if WebTransport server code exists and is actively used to accept connections (vulnerable code path present)
  4. Identify session closure implementation
    Search for WT_CLOSE_SESSION handling or session.Close() calls in the codebase
    Affected if Session close operations are implemented without application-level timeouts (vulnerability can cause hangs)
  5. Check for dependency version pinning
    Review go.mod to see if webtransport-go version is pinned (e.g., 'quic-go/webtransport-go v0.9.0') or unpinned
    Affected if Version is not pinned to v0.10.0 or later, or go.mod uses a replace directive pointing to a vulnerable version

Your environment is affected if webtransport-go version is present in dependencies and is lower than v0.10.0, and your application runs a WebTransport server that accepts connections where a malicious peer could exploit the flow control withholding.

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 v0.10.0 or later. If upgrade is not immediately possible, monitor for hung sessions and implement application-level timeouts for WebTransport session closure.

Recommended fix High confidence

v0.10.0

  1. Check the current version of webtransport-go in your project by examining your go.mod file or running 'go list -m github.com/quic-go/webtransport-go'
  2. Upgrade to v0.10.0 or later by running 'go get github.com/quic-go/[email protected]'
  3. Run 'go mod tidy' to update dependencies
  4. Run your test suite to verify the upgrade does not break functionality
  5. Redeploy your application with the updated dependency
Caveat Review the v0.10.0 release notes for any breaking changes before upgrading

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

Fix this in Webtransport Go Scoped from the published advisory
  • Consultation1.0 h
  • Implementation2.0 h
  • Testing4.0 h
  • Review / QA1.0 h
8.0 hours of engineering $1,340
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,144.

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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-2026-21435 in production — separate from our analysis above.

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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