CVE-2026-44434
MediumCVSS 5.3Exploitation Probability (EPSS)
Low risk4th percentile - higher than 4% of all known CVEs
Summary
Quicly, a QUIC protocol implementation primarily used in the H2O HTTP server, was vulnerable to stateless reset injection before commit dccf5d4 due to missing packet entry validation. This allowed an on-path attacker to reset QUIC connections.
Risk Assessment
Attackers can disrupt existing QUIC connections, leading to denial of service (DoS) and destabilizing communication between clients and the H2O server.
Recommendation
Immediately update Quicly to a version containing commit dccf5d4, which fixes the entry validation for reset patterns.
Other vulnerabilities in Quicly
See all- CVE-2026-44436High
Quicly, the QUIC implementation in H2O, before commit 8b178e6 is vulnerable to a DoS attack via connection state corruption. The library accepts Connection IDs up to 255 bytes, but CID buffers are limited to 20 bytes, leading to buffer overrun and assertion failures.
- CVE-2026-44435High
In the Quicly IETF QUIC protocol implementation, primarily used within the H2O HTTP server, an assertion failure occurs prior to commit 937d0e9. The failure is triggered when the total number of valid handshake messages received over a CRYPTO stream in a single packet number space exceeds 32KB, causing a Denial of Service (DoS).
- CVE-2026-44433Medium
In the Quicly implementation of the QUIC protocol, primarily used in the H2O HTTP server, prior to commit 8b178e6, an attacker could send a STREAM frame with a single byte at the largest allowed offset to obtain additional flow control credit. Under certain conditions, this led to memory exhaustion because the application allocated a receive buffer for all out-of-order data up to the largest received offset.
Original NVD description (English source)
Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server. Prior to commit dccf5d4, Quicly was vulnerable to stateless reset injection through lack of packet entry validation. The QUIC protocol is designed to withstand packet injection attacks, once the handshake is complete. Only packets that carry some secret patterns are considered as stateless resets. Quicly allows the peer to share up to 4 such patterns per connection. However, until now, it failed to determine which of the 4 slots that it uses to retain the secret patterns contains a valid entry. As the slots are zero-initialized, the failure meant that, unless the peer advertised 4 of such patterns, an all-zero pattern was treated as a stateless reset.In effect, this allowed an on-path attacker to reset QUIC connections governed by Quicly. This issue has been fixed by commit dccf5d4.