CVE-2026-40170

A flaw was found in ngtcp2, a C implementation of the IETF QUIC (Quick UDP Internet Connections) protocol. A remote attacker can exploit a stack buffer overflow vulnerability by sending specially crafted, large transport parameters during the QUIC handshake. This occurs when the qlog callback is enabled and untrusted peer transport parameters are processed, leading to writes beyond a fixed-size buffer. This can result in a denial of service.

This Important vulnerability affects ngtcp2 when qlog is enabled and processing untrusted QUIC peer transport parameters, leading to a denial of service. In the context of Red Hat support products ngtcp2 is not shipped directly through any RPM package or supported product but is shipped as a bundled dependency by the nodejs packages to provide QUIC support. This feature is considered experimental by the NodeJS upstream and needs to be enabled during NodeJS's compilation time through (--with-quic configuration option), with that in mind the versions of NodeJS as shipped with Red Hat Enterprise Linux are not affected by this vulnerability as QUIC support is not enabled in the distributed supported packages. This means although the affected code is present in the package code base itself, it's not compiled when the package is built and therefore is not present in the final binary.

For Samba package, this vulnerability strictly affects environments running Red Hat Enterprise Linux (RHEL) 9.8 and newer, as well as RHEL 10.2 and newer. Only these versions ship with Samba 4.23 or above, which introduces native support for the integrated ngtcp2 library to handle SMB over QUIC functionality. Earlier versions of RHEL utilize older iterations of Samba that do not bundle or support ngtcp2 and are therefore unaffected.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

To mitigate this issue, disable the qlog callback in applications utilizing the ngtcp2 library. This action prevents the vulnerable code path from being exercised when processing untrusted QUIC peer transport parameters. Disabling qlog may impact logging and debugging capabilities. Applications linked against ngtcp2 must be restarted for the change to take effect.