CVE-2026-44436: Quicly is vulnerable to connection state corruption

Vulnerability summary

CVE-2026-44436 is a high (CVSS 7.5) vulnerability in h2o quicly. EPSS estimates a 0.49% chance of exploitation in the next 30 days, higher than 42.4% of scored vulnerabilities.

Timeline

1. 16-07-2026
   PublishedVulnerability first disclosed.
2. 17-07-2026
   Last modifiedVulnerability information updated.

Description

Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server. Prior to commit 8b178e6, Quicly is vulnerable to a Denial of Service attack through connection state corruption. In QUIC Invariants, the maximum length of a Connection ID is 255 bytes, while QUIC version 1 further restricts the maximum to 20 bytes. Quicly implements QUIC version 1 and therefore its CID buffers are limited to 20 bytes. However, to be able to respond to unknown versions of QUIC, its packet decoder accepts Connection IDs of up to 255 bytes. As its CID buffers are merely 20 bytes long, Quicly must reject QUIC version 1 packets with Connection IDs longer than that. The command line tool bundled with Quicly has had that check, however the library itself lacked such enforcement. As a consequence, when used by applications that lack their own enforcement, the connection state becoming inconsistent to buffer overrun. Fortunately, the overflow stops within the allocated chunk of memory, but nevertheless, the bug leads to assertion failures. This issue has been fixed by commit 8b178e6.

• CWE-120Buffer Copy without Checking Size of InputThe product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer.
• CWE-787Out-of-bounds WriteThe product writes data past the end, or before the beginning, of the intended buffer.