The Linux kernel’s Free‑Out‑of‑Band (FOU) subsystem stores packet attributes in the FOU_ATTR_IPPROTO field. When set to zero, the packet handler fails to free the associated socket buffer and does not resubmit it, leading to a kernel‑level memory leak that can grow until system memory is exhausted. This is a classic resource exhaustion flaw that can destabilize or halt the kernel. The flaw is identified in the fou_udp_recv implementation and was addressed by forbidding a value of zero for FOU_ATTR_IPPROTO.

Affected systems are Linux kernels that include the fou module without the applied patch, specifically kernel builds 6.19 RC1 through RC6 and any subsequent builds that have not incorporated the change. The CPE list confirms vulnerability in kernel versions 6.19 RC1‑RC6. Any system running these kernels remains vulnerable until the fix is applied.

The CVSS score of 7.8 deems this high severity, while the EPSS score of less than 1% indicates a very low exploitation probability at this time. It is not listed in CISA’s KEV catalog. Exploitation requires the attacker to be able to cause the kernel to process a packet with an invalid FOU_ATTR_IPPROTO value, which could come from crafted network traffic or local privileged code. If triggered, the kernel can suffer memory exhaustion, leading to denial of service. The risk is significant but the likelihood of successful exploitation remains low without additional conditions.