CVE-2026-52940 Linux CVE debrief

Who should care

Linux kernel users and administrators should be aware of this vulnerability and take steps to mitigate it. The vulnerability affects the Linux kernel's tun_put_user() function and can be exploited by an unprivileged user. The vulnerability has been resolved by zeroing the whole vnet header in tun_put_user().

Technical summary

The tun_put_user() function in the Linux kernel did not properly initialize the virtio_net_hdr_v1_hash_tunnel struct, leading to a potential information leak. An unprivileged user can set the vnet header size to 24 with TUNSETVNETHDRSZ, allowing __tun_vnet_hdr_put() to copy all 24 bytes of the partially-initialized struct to userspace. The vulnerability has been resolved by zeroing the whole vnet header in tun_put_user(). This ensures that the vnet header is properly initialized, preventing the information leak.

Defensive priority

Medium

Recommended defensive actions

• Update the Linux kernel to the latest version
• Use a secure TUNSETVNETHDRSZ value
• Monitor for potential exploitation attempts
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record was published on 2026-06-24T08:16:24.287Z and last modified on 2026-07-07T18:35:34.340Z. The NVD entry is currently Awaiting Analysis. There is limited information available about the vulnerability, and defenders should verify the affected scope and severity with the official advisory. The Linux kernel's tun_put_user() function did not properly initialize the virtio_net_hdr_v1_hash_tunnel struct, potentially leaking 14 bytes of kernel stack on every read of a non-tunnel packet. The vulnerability has been resolved by zeroing the whole vnet header in tun_put_user().

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.