CVE-2026-42009 is a high-severity vulnerability in the gnutls library's DTLS packet reordering logic on Red Hat Enterprise Linux 10. The comparator function used to order DTLS packets by their sequence numbers fails to correctly handle duplicate sequence numbers, leading to unstable packet ordering or undefined behavior. This flaw can cause a denial of service by disrupting DTLS communications. Red Hat has issued security advisories and released updated gnutls packages (version 3.8.13-1.hum1 and related) that fix this issue. The vulnerability affects Red Hat Hardened Images and related products including Red Hat OpenShift Container Platform 4.21 on aarch64 architecture. The vendor advisory confirms the availability of patches and recommends applying updates to remediate the issue.

The vulnerability allows a remote attacker to cause a denial of service by exploiting improper handling of DTLS packets with duplicate sequence numbers. This leads to unstable packet ordering or undefined behavior in the gnutls library, potentially disrupting secure communications relying on DTLS. There is no indication of confidentiality or integrity impact, only availability is affected.

Red Hat has released updated gnutls packages that address this vulnerability. Users should apply the security updates for gnutls as provided in Red Hat Advisory RHSA-2026:13274 and RHSA-2026:20611. The advisories include gnutls version 3.8.13-1.hum1 for affected architectures. Applying these official patches is the recommended remediation. Patch status is confirmed by the vendor advisories. No additional mitigation steps are indicated beyond applying the updates.