CVE-2026-21435: webtransport-go CloseWithError Can Block Indefinitely Vulnerability

MODERATE
Advisory Identifier: GHSA-px4r-g4p3-hhqv
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CVSS 5.3
CWE-400
Published
Advisory Summary

CVE-2026-21435 is a MODERATE (CVSS 5.3) closewitherror can block indefinitely vulnerability (CWE-400: Uncontrolled Resource Consumption) affecting webtransport-go libraries in the Go ecosystem. The issue involves closewitherror can block indefinitely. Remediation is available by upgrading affected packages to version 0.10.0.

CVE-2026-21435 at a Glance

CVE Identifier CVE-2026-21435
GHSA Identifier GHSA-px4r-g4p3-hhqv
Severity MODERATE
CVSS v3.1 Base Score 5.3
CWE Classification CWE-400: Uncontrolled Resource Consumption
Affected Product webtransport-go
Package Ecosystem Go
Published Date
Last Updated

Security Impact

CVSS: 5.3

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability Low (A:L)

How to Fix CVE-2026-21435

To remediate this vulnerability, upgrade all affected webtransport-go dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

github.com/quic-go/webtransport-go fixed: 0.10.0 
Upgrade github.com/quic-go/webtransport-go to 0.10.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for github.com/quic-go/webtransport-go

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-21435, inspect your project dependencies for webtransport-go:

inspect dependencies for github.com/quic-go/webtransport-go

Analyze the terminal output to locate any version of webtransport-go in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 0.10.0), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-400: Uncontrolled Resource Consumption, where the product does not properly control the allocation and maintenance of a limited resource. In webtransport-go, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

## Summary 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. ## Details WebTransport over HTTP/3 signals session termination by sending a WT_CLOSE_SESSION capsule on the CONNECT stream. The capsule is only needed to transmit a reason phrase and an error code to the peer. After the capsule is sent, the CONNECT stream is closed. In affected versions, the closure procedure blocked indefinitely while waiting for sufficient QUIC flow control credit from the peer. A malicious peer can withhold this credit, preventing the capsule from being sent. ## The Fix webtransport-go now attempts to send the WT_CLOSE_SESSION capsule with a short deadline. If the capsule cannot be sent within this deadline, the CONNECT stream is reset instead. This closes the WebTransport session promptly without transmitting the optional error details. This prevents indefinite blocking on session closure.

CVE-2026-21435 Affected webtransport-go Versions

1 component(s)
Fixed Versions:
0.10.0
Product / Package Ecosystem Affected Versions Fixed Version
github.com/quic-go/webtransport-go
golang/github.com/quic-go/webtransport-go
Go
< 0.10.0
0.10.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/quic-go/webtransport-go/security/advisories/GHSA-px4r-g4p3-hhqv
NVD — CVE-2026-21435 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-21435
webtransport-go Source Repository REPOSITORY
https://github.com/quic-go/webtransport-go
webtransport-go Release RELEASE
https://github.com/quic-go/webtransport-go/releases/tag/v0.10.0

Frequently Asked Questions

What is CVE-2026-21435?
CVE-2026-21435 is a security vulnerability in webtransport-go involving CloseWithError can block indefinitely under CWE-400: Uncontrolled Resource Consumption.
Which webtransport-go versions are affected?
Affected versions include releases prior to 0.10.0 across affected packages: github.com/quic-go/webtransport-go.
What version fixes CVE-2026-21435?
Upgrading to 0.10.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is github.com/quic-go/webtransport-go affected?
Yes, github.com/quic-go/webtransport-go is affected in releases prior to 0.10.0. Upgrade your dependency to 0.10.0 or later.
What is the CVSS score of CVE-2026-21435?
CVE-2026-21435 has a CVSS v3.1 base score of 5.3 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L.
What CWE is associated with CVE-2026-21435?
CVE-2026-21435 is classified under CWE-400: Uncontrolled Resource Consumption (The product does not properly control the allocation and maintenance of a limited resource.).
How can I check Go dependencies for CVE-2026-21435?
Run the dependency tree command: 'inspect dependencies for github.com/quic-go/webtransport-go'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-px4r-g4p3-hhqv",
  "modified" : "2026-02-12T22:07:35Z",
  "published" : "2026-02-12T15:29:01Z",
  "aliases" : [ "CVE-2026-21435" ],
  "summary" : "webtransport-go: CloseWithError can block indefinitely",
  "details" : "## Summary\nAn 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.\n\n## Details\nWebTransport over HTTP/3 signals session termination by sending a WT_CLOSE_SESSION capsule on the CONNECT stream. The capsule is only needed to transmit a reason phrase and an error code to the peer. After the capsule is sent, the CONNECT stream is closed.\nIn affected versions, the closure procedure blocked indefinitely while waiting for sufficient QUIC flow control credit from the peer. A malicious peer can withhold this credit, preventing the capsule from being sent.\n\n## The Fix\nwebtransport-go now attempts to send the WT_CLOSE_SESSION capsule with a short deadline. If the capsule cannot be sent within this deadline, the CONNECT stream is reset instead. This closes the WebTransport session promptly without transmitting the optional error details.\nThis prevents indefinite blocking on session closure.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/quic-go/webtransport-go"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.10.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 0.9.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/quic-go/webtransport-go/security/advisories/GHSA-px4r-g4p3-hhqv"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-21435"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/quic-go/webtransport-go"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/quic-go/webtransport-go/releases/tag/v0.10.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-02-12T15:29:01Z",
    "nvd_published_at" : "2026-02-12T19:15:51Z"
  }
}