CVE-2026-40170: ngtcp2 has a qlog transport parameter serialization stack buffer overflow

Vulnerability summary

CVE-2026-40170 is a high (CVSS 7.5) Stack-based Buffer Overflow vulnerability in ngtcp2. EPSS estimates a 0.78% chance of exploitation in the next 30 days, higher than 55.8% of scored vulnerabilities. 2 public exploits are recorded.

Timeline

1. 16-04-2026
   PublishedVulnerability first disclosed.
2. 15-07-2026
   Last modifiedVulnerability information updated.

Description

ngtcp2 is a C implementation of the IETF QUIC protocol. In versions prior to 1.22.1, ngtcp2_qlog_parameters_set_transport_params() serializes peer transport parameters into a fixed 1024-byte stack buffer without bounds checking. When qlog is enabled, a remote peer can send sufficiently large transport parameters during the QUIC handshake to cause writes beyond the buffer boundary, resulting in a stack buffer overflow. This affects deployments that enable the qlog callback and process untrusted peer transport parameters. This issue has been fixed in version 1.22.1. If developers are unable to immediately upgrade, they can disable the qlog on client.

• CWE-121Stack-based Buffer OverflowThe product writes past the end of a buffer that was allocated on the stack.
• 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.