Fragnesia — Linux kernel xfrm ESP-in-TCP local privilege escalation (PoC public)

Disclosed 2026-05-15 with public PoC; mainline kernel patch landed 2026-05-14, distro propagation underway. LPE primitive against the xfrm ESP-in-TCP code path; trips IPsec VPN endpoints in particular. Mitigation modprobe -r esp4 esp6 (breaks IPsec). Distinct from Dirty Frag (different code paths) but conceptually adjacent — both abuse kernel xfrm assumptions (daily 2026-05-15).

CVE-2026-46300 (codename "Fragnesia") is a local privilege escalation vulnerability in the Linux kernel's xfrm IPsec subsystem, specifically in the ESP-over-TCP code path that provides NAT traversal fallback for IPsec connections (Wiz Research, 2026-05-13 · Help Net Security, 2026-05-14). The vulnerability was discovered by William Bowling of Zellic.io using Zellic's AI-agentic source code auditing tool; Wiz Research (whose researcher Hyunwoo Kim had previously discovered the related Dirty Frag vulnerability family) published the technical writeup. A working proof-of-concept demonstrating escalation from an unprivileged local user to root on unpatched kernels has been released (hosted at github.com/v12-security/pocs). Exploitation requires local code execution on the target — there is no known remote exploitation path absent a prior foothold or a co-chained remote vulnerability (e.g., an RCE that drops a low-privilege shell). Fragnesia is therefore primarily relevant as a post-compromise privilege-escalation primitive and as a jailbreak-class risk in shared compute environments: VPS and bare-metal hosting providers, university Linux clusters, multi-tenant cloud workloads running on shared kernels, and container environments where the kernel namespace boundary can be crossed. MITRE ATT&CK: T1068 (Exploitation for Privilege Escalation). No in-the-wild exploitation reported as of 2026-05-15. Affected: Linux kernels shipping the xfrm ESP-in-TCP implementation across the 5.x and 6.x LTS series — consult your distribution's security bulletin for the exact affected package version range. Distributions shipping patches as of 2026-05-15 include upstream Linux and major vendors (Ubuntu, Debian, RHEL, SUSE); apply the available kernel update and reboot. Interim workaround: disable the xfrm_espintcp kernel module where IPsec ESP-over-TCP is not operationally required (modprobe -r esp6_offload esp4_offload where applicable); also consider restricting CAP_NET_ADMIN capability to reduce the xfrm attack surface in multi-tenant environments.

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