CVE-2026-53266

In the Linux kernel, the following vulnerability has been resolved: netfilter: bridge: make ebt_snat ARP rewrite writable The ebtables SNAT target keeps the Ethernet source address rewrite behind skb_ensure_writable(skb, 0). This is intentional: at the bridge ebtables hooks the Ethernet header is addressed through skb_mac_header()/eth_hdr(), while skb->data points at the Ethernet payload. Asking skb_ensure_writable() for ETH_HLEN bytes would check the payload, not the Ethernet header, and would reintroduce the small packet regression fixed by commit 63137bc5882a. However, the optional ARP sender hardware address rewrite is different. It writes through skb_store_bits() at an offset relative to skb->data: skb_store_bits(skb, sizeof(struct arphdr), info->mac, ETH_ALEN) skb_header_pointer() only safely reads the ARP header; it does not make the later sender hardware address range writable. If that range is still held in a nonlinear skb fragment backed by a splice-imported file page, skb_store_bits() maps the frag page and copies the new MAC address directly into it. Ensure the ARP SHA range is writable before reading the ARP header and before calling skb_store_bits().

What this means for your business

It affects Linux Kernel. It matters if your company, or a supplier that handles your data, runs it.
Attackers are already using it. CISA added it to its list of flaws used in real attacks on 18 September 2026.
An attacker can use it only with access to the machine itself, with an ordinary user login, and without anyone at your company clicking anything.
FIRST's prediction model gives it a 0.1% chance of attack attempts being seen in the next 30 days, ranking above 2% of all known flaws.

What to do

1Check whether your company or your suppliers run Linux Kernel, and which version. The affected versions are listed further down this page.
2If you do, apply the vendor's fix. A patch or vendor advisory has been published.
3Treat it as urgent. US federal agencies must fix it by 21 September 2026. That deadline does not apply to private companies, but it shows how seriously CISA takes it.
4Because attackers have used it, check the affected systems for signs of a break-in, not only for the update.

Not sure if your company is exposed?

Fastnexa’s certified penetration testers can check whether attackers could use this flaw, or others like it, against your websites, apps and network. The full test is free for our first 10 founding clients until 31 December 2026.

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Required action

Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.

CISA’s wording, unedited. The notes it mentions are in CISA’s catalogue entry.

Scoring

CVSS: 8.8 (v3.1)
Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
Weakness: CWE-787
Assigned by: 416baaa9-dc9f-4396-8d5f-8c081fb06d67

Dates

Published: 2026-06-25
Last modified: 2026-09-19
Added to KEV: 2026-09-18
Remediation due: 2026-09-21
Sources: CISA KEV, NVD

Affected products

Linux Linux Kernel5.4.73 - 5.5, 5.8.17 - 5.9, 5.9.2 - 5.10.259, 5.11 - 5.15.210, 5.16 - 6.1.176, 6.2 - 6.6.143, 6.7 - 6.12.94, 6.13 - 6.18.36, 6.19 - 7.0.13, 7.1

As listed in the NVD configuration data. Not a statement about your estate.