CVE-2026-21435
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Denial of Service in webtransport-go via QUIC Flow Control Abuse

Publication date: 2026-02-12

Last updated on: 2026-02-19

Assigner: GitHub, Inc.

Description
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.
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Meta Information
Published
2026-02-12
Last Modified
2026-02-19
Generated
2026-06-16
AI Q&A
2026-02-12
EPSS Evaluated
2026-06-15
NVD
CVE-2026-21435
EUVD
EUVD-2026-6717
Affected Vendors & Products
Showing 1 associated CPE
Vendor Product Version / Range
quic-go webtransport-go to 0.10.0 (exc)
Helpful Resources
Exploitability
CWE
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KEV
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CWE ID Description
CWE-400 The product does not properly control the allocation and maintenance of a limited resource.
Attack-Flow Graph
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Mitigation Strategies

To mitigate this vulnerability, you should upgrade the webtransport-go library to version 0.10.0 or later.

The fix in version 0.10.0 introduces a short deadline for sending the WT_CLOSE_SESSION capsule. If the capsule cannot be sent within this deadline due to withheld flow control credit, the CONNECT stream is reset instead, preventing indefinite blocking and denial of service.

Executive Summary

CVE-2026-21435 is a denial of service vulnerability in the webtransport-go library, which implements the WebTransport protocol. Before version 0.10.0, an attacker could cause the WebTransport session closure to hang indefinitely by withholding QUIC flow control credit on the CONNECT stream. This prevents the transmission of the WT_CLOSE_SESSION capsule, which is necessary to properly close a session, causing the close operation to block and never complete.

Impact Analysis

This vulnerability can cause a denial of service by blocking the closure of WebTransport sessions indefinitely. An attacker can exploit this by withholding flow control credit, causing resources to be consumed unnecessarily and preventing sessions from closing properly. This can lead to resource exhaustion and degraded service availability.

Compliance Impact

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Detection Guidance

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