CVE-2026-21435Network attack vector

Session-closure hang via withheld flow-control credit

Published Feb 12, 2026 · Updated Feb 12, 2026

Denial of service in quic-go webtransport-go 0.9.0 and earlier allows remote attackers to hang WebTransport session closure. The Session.CloseWithError path waits indefinitely to write a WT_CLOSE_SESSION capsule when its peer withholds QUIC flow-control credit on the CONNECT stream. Any unauthenticated WebTransport peer controlling credit on that stream can prevent the session from closing until credit becomes available.

CVSS severity5.3
Medium
EPSS probability0.43%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in quic-go webtransport-go 0.9.0 and earlier allows remote attackers to hang WebTransport session closure. The Session.CloseWithError path waits indefinitely to write a WT_CLOSE_SESSION capsule when its peer withholds QUIC flow-control credit on the CONNECT stream. Any unauthenticated WebTransport peer controlling credit on that stream can prevent the session from closing until credit becomes available.

The record

CVE
CVE-2026-21435
Published
Feb 12, 2026
Updated
Feb 12, 2026
Vendor
quic-go
Product
webtransport-go
Classifications
CWE-400, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 12, 2026CVE publishedPublication date reported by the CVE source.
  2. Feb 12, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

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  1. Affected versionversion=< 0.10.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

quic-go · webtransport-goversion=< 0.10.0

Published CVSS scores

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

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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