CVE-2025-61684 identifies a vulnerability in quicly, a QUIC protocol implementation maintained by the h2o project. The root cause is improper input validation (CWE-20), which leads to an assertion failure when processing crafted network packets. This failure causes the quicly process to crash, resulting in a denial-of-service (DoS) condition. The vulnerability affects all versions prior to commit d9d3df6a8530a102b57d840e39b0311ce5c9e14e, which contains the fix. The attack vector is network-based (AV:N), requiring no privileges (PR:N) or user interaction (UI:N), making it straightforward for remote attackers to exploit. The impact is limited to availability, with no confidentiality or integrity compromise. The vulnerability was reserved in late 2025 and published in early 2026, with no known active exploitation reported. Quicly is used in environments requiring QUIC protocol support, including web servers and network services that benefit from QUIC’s performance and security features. The assertion failure likely stems from unchecked or malformed input data that violates expected protocol state or parameters, triggering internal consistency checks. The fix involves improved input validation and assertion handling to prevent crashes. Organizations using quicly should verify their versions and apply the patch or upgrade to the fixed commit to mitigate the risk of DoS attacks.

For European organizations, this vulnerability poses a significant risk of service disruption due to denial-of-service attacks against systems running vulnerable versions of quicly. Since quicly is an implementation of the QUIC protocol, which is increasingly adopted for HTTP/3 and other low-latency network communications, affected services could include web servers, API endpoints, and other networked applications relying on QUIC. Disruptions could impact customer-facing services, internal communications, and critical infrastructure components that depend on high availability. The lack of confidentiality or integrity impact limits data breach risks, but availability degradation can cause financial losses, reputational damage, and operational interruptions. Organizations in sectors such as finance, telecommunications, government, and cloud service providers are particularly vulnerable due to their reliance on robust network protocols and high uptime requirements. The ease of remote exploitation without authentication or user interaction increases the threat level, especially for publicly accessible services. Given no known exploits in the wild, proactive patching is essential to prevent potential attacks.

1. Immediately identify all systems and services using quicly, particularly those exposed to untrusted networks. 2. Upgrade quicly to the version including commit d9d3df6a8530a102b57d840e39b0311ce5c9e14e or later, which contains the fix for this vulnerability. 3. If upgrading is not immediately possible, implement network-level protections such as rate limiting, filtering, or blocking suspicious QUIC traffic to reduce exposure. 4. Monitor network traffic for anomalous QUIC packets that could indicate exploitation attempts targeting assertion failures. 5. Conduct thorough testing of updated quicly versions in staging environments to ensure stability and compatibility. 6. Review and enhance input validation and error handling in any custom integrations or wrappers around quicly to prevent similar issues. 7. Maintain an incident response plan that includes detection and mitigation of denial-of-service attacks targeting QUIC implementations. 8. Stay informed on vendor advisories and community updates regarding quicly and related QUIC protocol vulnerabilities.