CVE-2026-21435: webtransport-go CloseWithError can block indefinitely
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 WTCLOSESESSION capsule and causing the close operation to hang.
Details WebTransport over HTTP/3 signals session termination by sending a WTCLOSESESSION 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 WTCLOSESESSION 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.
Other sources
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 WTCLOSESESSION capsule and causing the close operation to hang. This vulnerability is fixed in v0.10.0.
— MITRE
Affected Software
Event History
Frequently Asked Questions
What is the severity of CVE-2026-21435?
CVE-2026-21435 has been classified as a denial of service vulnerability.
How do I fix CVE-2026-21435?
To fix CVE-2026-21435, upgrade to version 0.10.0 of the webtransport-go package.
Which versions are affected by CVE-2026-21435?
CVE-2026-21435 affects versions up to and including 0.9.0 of the webtransport-go package.
What kind of attack does CVE-2026-21435 allow?
CVE-2026-21435 allows attackers to prevent or delay WebTransport session closure, causing service disruption.
Is CVE-2026-21435 specific to any particular software?
Yes, CVE-2026-21435 is specific to the webtransport-go package in the Go programming language.