CVE-2026-44434 in quicly
Summary
by MITRE • 07/17/2026
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.
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Analysis
by VulDB Data Team • 07/17/2026
The vulnerability in Quicly represents a critical flaw in the implementation of the IETF QUIC protocol that undermines fundamental security assumptions about packet validation and connection integrity. This weakness specifically affects the stateless reset mechanism, which is designed to protect QUIC connections from certain types of attacks by ensuring that only legitimate packets containing specific secret patterns can trigger connection resets. The vulnerability stems from insufficient validation of packet entries within the stateless reset handling logic, creating an exploitable condition where an attacker can manipulate connection state through crafted packet injection.
The technical flaw manifests in how Quicly manages the storage and validation of secret patterns used for stateless resets. During QUIC connections, peers exchange up to four secret patterns that serve as authenticators for legitimate reset packets. However, Quicly's implementation failed to properly validate which of these four storage slots actually contain valid entries versus uninitialized zero values. This design oversight means that when fewer than four patterns are advertised by the peer, the remaining slots remain initialized to zero, creating a potential attack vector. An on-path attacker can exploit this by injecting packets that match the zero-initialized pattern, causing the Quicly implementation to incorrectly treat these as legitimate stateless reset signals.
The operational impact of this vulnerability is significant for any system utilizing Quicly within the H2O HTTP server environment, particularly in scenarios where network traffic may be subject to injection attacks. The ability to perform connection resets through packet injection undermines the reliability and security guarantees that QUIC protocol implementations are designed to provide. This vulnerability effectively allows attackers to disrupt legitimate QUIC connections without requiring sophisticated attack capabilities beyond basic packet manipulation, making it particularly dangerous in production environments where connection stability is critical.
This vulnerability aligns with CWE-129, which addresses issues related to insufficient input validation and improper handling of uninitialized data structures. The flaw also relates to ATT&CK technique T1059, specifically covering the use of injection attacks to manipulate system behavior. The root cause demonstrates poor implementation practices regarding memory initialization and validation procedures that are fundamental to secure protocol implementations. The fix implemented in commit dccf5d4 addresses this by ensuring proper validation of packet entries before treating them as legitimate stateless reset indicators, thereby preventing the exploitation of uninitialized storage slots.
The remediation approach required careful attention to how secret pattern validation occurs within the QUIC protocol implementation. The solution must maintain backward compatibility while ensuring that only properly advertised patterns are considered valid for stateless reset operations. This fix represents a critical security enhancement that aligns with industry best practices for protocol implementation, particularly in environments where network-level attacks are possible. The vulnerability underscores the importance of rigorous input validation and proper initialization of cryptographic parameters in protocol implementations.
The broader implications of this flaw extend beyond the immediate Quicly implementation, highlighting potential issues in other QUIC protocol libraries that may share similar design patterns. Network administrators and security professionals should consider this vulnerability when evaluating their QUIC-based deployments, particularly in environments where packet injection attacks are possible or likely. The fix serves as a reminder of the critical importance of validating all inputs and properly managing uninitialized data structures in security-sensitive protocol implementations, especially those handling cryptographic secrets and connection state information.