Linux Kernel Out-of-Bounds Write Vulnerability (CVE-2026-53266) Analysis

An out-of-bounds write flaw in the Linux kernel ebtables frame-filtering subsystem allows local attackers with unprivileged system access to escalate privileges or breach isolation boundaries. Tracked as CVE-2026-53266, this Linux Kernel Out-of-Bounds Write Vulnerability resides in the Source Network Address Translation (SNAT) target handling for Ethernet bridge filtering.

Technical Breakdown

When processing ARP traffic through ebtables SNAT rules, the kernel attempts to rewrite the ARP sender hardware address. If the target frame resides in a non-linear socket buffer (sk_buff) whose memory fragments are backed by a file page imported via the splice() system call, the packet manipulation writes directly into that underlying page fragment without verifying memory writeability or boundaries.

Because ebtables operates at the link layer, a local attacker capable of crafting bridge traffic or manipulating network namespaces can exploit this flaw to write directly into page cache memory or adjacent kernel structures. Classified under CWE-787, the vulnerability carries a CVSS 3.1 score of 8.8 (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H). The Scope Changed (S:C) designation highlights the primary risk: an attacker operating inside an isolated container or restricted network namespace can alter host-level file pages or kernel memory, escaping the sandbox entirely.

Risk Evaluation

While the exploit mechanics require a local foothold, the technical requirements to trigger the condition are straightforward once access is achieved. Broad automated exploitation across the internet remains limited—reflected by an EPSS probability of 0.12% (2.2nd percentile)—because this flaw cannot be triggered unauthenticated over a raw WAN interface.

Instead, the primary threat model centers on multi-tenant cloud infrastructure, container hosting environments, and shared Linux systems. In these environments, local privilege escalation (LPE) vulnerabilities are routinely chained with web application flaws or compromised low-privilege service accounts to gain root permissions on the underlying host.

A wide swath of kernel maintenance branches are impacted, including legacy builds (5.4.73+) through long-term support (LTS) releases prior to 6.6.143 (specifically affecting ranges across 5.10, 5.15, 6.1, and 6.6).

Patching and Mitigation

System administrators should update Linux host kernels to builds containing upstream remediation commits 153ea96c806a, 67ba971ae025, 76280b78cc9f, or afd64b59c3de. Fixed baseline versions include kernel branches updated to at least 5.10.259, 5.15.210, 6.1.176, or 6.6.143.

If immediate kernel upgrades are not feasible:

• Restrict unprivileged access to user namespaces (sysctl -w kernel.unprivileged_userns_clone=0 on supporting distributions) to prevent untrusted local users from configuring network bridge rules.
• Unload or blacklist the ebtables kernel modules (modprobe -n -v ebtables or ebt_snat) if Ethernet bridging NAT is not actively required on the host.

Organizations adhering to CISA BOD 26-04 risk guidelines must ensure affected assets are updated or removed from service prior to the September 21, 2026 remediation deadline.