CVE-2026-21434: webtransport-go Memory Exhaustion Attack Due To Vulnerability

MODERATE
Advisory Identifier: GHSA-g6x7-jq8p-6q9q
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CVSS 5.3
CWE-770
Published
Advisory Summary

CVE-2026-21434 is a MODERATE (CVSS 5.3) memory exhaustion attack due to vulnerability (CWE-770: Allocation of Resources Without Limits or Throttling) affecting webtransport-go libraries in the Go ecosystem. The issue involves memory exhaustion attack due to missing length check in wt_close_session capsule. Remediation is available by upgrading affected packages to version 0.10.0.

CVE-2026-21434 at a Glance

CVE Identifier CVE-2026-21434
GHSA Identifier GHSA-g6x7-jq8p-6q9q
Severity MODERATE
CVSS v3.1 Base Score 5.3
CWE Classification CWE-770: Allocation of Resources Without Limits or Throttling
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-2026-21434

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-2026-21434, 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-770: Allocation of Resources Without Limits or Throttling, where the product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated. 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 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. ## Details WebTransport over HTTP/3, as defined in draft-ietf-webtrans-http3, uses the WT_CLOSE_SESSION 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 maximum when processing incoming WT_CLOSE_SESSION capsules. A peer can send a capsule with an excessively large payload, forcing the recipient to allocate and buffer the full amount of transmitted data without bound. ## The Fix webtransport-go now limits the length of the parsed Application Error Message to 1024 bytes in WT_CLOSE_SESSION capsules by reading no more than this amount. This prevents excessive memory consumption.

CVE-2026-21434 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
≥ 0.3.0
< 0.10.0
0.10.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/quic-go/webtransport-go/security/advisories/GHSA-g6x7-jq8p-6q9q
NVD — CVE-2026-21434 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-21434
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-2026-21434?
CVE-2026-21434 is a security vulnerability in webtransport-go involving Memory Exhaustion Attack due to Missing Length Check in WT_CLOSE_SESSION Capsule under CWE-770: Allocation of Resources Without Limits or Throttling.
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-2026-21434?
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-2026-21434?
CVE-2026-21434 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-2026-21434?
CVE-2026-21434 is classified under CWE-770: Allocation of Resources Without Limits or Throttling (The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.).
How can I check Go dependencies for CVE-2026-21434?
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-g6x7-jq8p-6q9q",
  "modified" : "2026-02-12T22:07:29Z",
  "published" : "2026-02-12T15:28:52Z",
  "aliases" : [ "CVE-2026-21434" ],
  "summary" : "webtransport-go: Memory Exhaustion Attack due to Missing Length Check in WT_CLOSE_SESSION Capsule",
  "details" : "## Summary\nAn 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.\n\nThis 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.\n\n## Details\nWebTransport over HTTP/3, as defined in draft-ietf-webtrans-http3, uses the WT_CLOSE_SESSION 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.\nIn affected versions of webtransport-go, the parser does not enforce this 1024-byte maximum when processing incoming WT_CLOSE_SESSION capsules. A peer can send a capsule with an excessively large payload, forcing the recipient to allocate and buffer the full amount of transmitted data without bound.\n\n## The Fix\nwebtransport-go now limits the length of the parsed Application Error Message to 1024 bytes in WT_CLOSE_SESSION capsules by reading no more than this amount. This prevents excessive memory consumption.",
  "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" : "0.3.0"
      }, {
        "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-g6x7-jq8p-6q9q"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-21434"
  }, {
    "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-770" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-02-12T15:28:52Z",
    "nvd_published_at" : "2026-02-12T19:15:51Z"
  }
}