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webtransport-go: CloseWithError can block indefinitely

Moderate severity GitHub Reviewed Published Feb 12, 2026 in quic-go/webtransport-go • Updated Feb 12, 2026

Package

gomod github.com/quic-go/webtransport-go (Go)

Affected versions

<= 0.9.0

Patched versions

0.10.0

Description

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.

References

@marten-seemann marten-seemann published to quic-go/webtransport-go Feb 12, 2026
Published to the GitHub Advisory Database Feb 12, 2026
Reviewed Feb 12, 2026
Published by the National Vulnerability Database Feb 12, 2026
Last updated Feb 12, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
Low

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(2nd percentile)

Weaknesses

Uncontrolled Resource Consumption

The product does not properly control the allocation and maintenance of a limited resource. Learn more on MITRE.

CVE ID

CVE-2026-21435

GHSA ID

GHSA-px4r-g4p3-hhqv

Source code

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