CVE-2026-21435 is a denial of service vulnerability classified under CWE-400 (Uncontrolled Resource Consumption) affecting webtransport-go, an implementation of the WebTransport protocol in Go. The vulnerability exists in versions prior to 0.10.0, where an attacker acting as a malicious peer can exploit the QUIC flow control mechanism on the CONNECT stream. Specifically, the attacker withholds QUIC flow control credit, which prevents the transmission of the WT_CLOSE_SESSION capsule that signals the closure of a WebTransport session. Because the close operation depends on this capsule, it hangs indefinitely, causing the server or client to consume resources unnecessarily and potentially leading to denial of service conditions. This attack requires no authentication or user interaction and can be executed remotely, making it accessible to any attacker capable of establishing a QUIC connection. The vulnerability impacts availability but does not compromise confidentiality or integrity. The issue was addressed in webtransport-go version 0.10.0 by ensuring proper handling of flow control credit and session closure. No known exploits are currently reported in the wild, but the medium CVSS score of 5.3 reflects the moderate risk posed by this vulnerability due to its ease of exploitation and potential to disrupt services relying on WebTransport. WebTransport is increasingly used for low-latency, multiplexed communication over QUIC, so this vulnerability could affect modern web applications and services that adopt this protocol stack.

For European organizations, the primary impact of CVE-2026-21435 is the risk of denial of service on services utilizing webtransport-go for WebTransport protocol communication. This could lead to service outages, degraded performance, or resource exhaustion on servers handling WebTransport sessions, affecting availability of critical web applications or real-time communication services. Industries relying on low-latency web transport, such as financial services, media streaming, and telecommunications, may experience operational disruptions. The vulnerability does not expose sensitive data or allow unauthorized data modification, so confidentiality and integrity impacts are minimal. However, availability issues can cause reputational damage, financial loss, and regulatory scrutiny under European data protection and service availability standards. Organizations using older versions of webtransport-go or custom implementations based on it are at higher risk. Given the increasing adoption of QUIC and WebTransport in modern web infrastructure, the threat surface is expected to grow, making timely patching essential to maintain service reliability in Europe.

The most effective mitigation is to upgrade all instances of webtransport-go to version 0.10.0 or later, where the vulnerability is fixed. Organizations should audit their software dependencies and deployment environments to identify any use of vulnerable versions. Implement monitoring and alerting for abnormal WebTransport session behavior, such as sessions that fail to close or exhibit prolonged resource consumption, to detect potential exploitation attempts early. Network-level controls can be employed to limit the rate of new QUIC connections or enforce stricter flow control policies to reduce the impact of malicious peers. Consider deploying WebTransport session timeouts or watchdog mechanisms to forcibly close hung sessions. For critical services, isolate WebTransport endpoints and apply resource quotas to prevent a single session from exhausting system resources. Finally, maintain an up-to-date inventory of applications using webtransport-go and integrate vulnerability scanning into the CI/CD pipeline to prevent deployment of vulnerable versions.