CVE-2026-18727 is a moderate severity vulnerability in the iscsiuio component of open-iscsi involving an integer underflow and out-of-bounds read during DHCPv6 packet parsing. Specifically, crafted DHCPv6 Advertise traffic with a short UDP length causes the DHCPv6 payload length to underflow. An attacker on the same Layer 2 network segment can exploit this by sending specially crafted IPv6 UDP traffic while the client is engaged in DHCPv6 exchange, resulting in denial of service due to process crash or service disruption. The vulnerability requires adjacency and no authentication, but has limited impact scope due to this requirement. Red Hat advisory confirms the issue and provides mitigation guidance but does not yet confirm a patch availability.

The vulnerability allows an unauthenticated attacker on the same adjacent network segment to cause a denial of service by crashing the iscsiuio process or disrupting the DHCPv6 client service. There is no impact on confidentiality or integrity. The attack vector is adjacent network with low complexity and no privileges or user interaction required. The impact is limited to availability degradation.

If DHCPv6 is not required for iscsiuio, disable DHCPv6 handling on the relevant network interface. If DHCPv6 is necessary, restrict network exposure to trusted adjacent networks and implement firewall rules to block forged DHCPv6 server traffic on shared Layer 2 segments. A system restart or service reload may be required for changes to take effect. Patch status is not yet confirmed — check the vendor advisory for current remediation guidance.