Known exploited vulnerability · CISA KEV

CVE-2026-46300: Linux Kernel Missing Initialization of a Variable Vulnerability

Linux Kernel contains a missing initialization of a variable vulnerability that could allow for unintended modification of page-cache-backed data via failure to preserve the shared-fragment marker during socket buffer coalescing, causing ESP input processing to decrypt shared fragments in place. This vulnerability can be chained with CVE-2026-109618 and CVE-2026-109619 to implant PHP webshells and run commands as root.

CVE-2026-46300

Exploited in the wild. Fix this first.

CISA added it to the Known Exploited Vulnerabilities catalog on 2026-10-28: attackers are using it right now.

CISA KEV: Listed; Added 2026-10-28 · federal fix-by 2026-10-31
EPSS (30 days): 16%; Higher than 97% of CVEs · 2026-11-01
CVSS v3.1: 7.8 High; CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

What CISA says

Linux Kernel Missing Initialization of a Variable Vulnerability

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.

NVD description · published 2026-05-23

In the Linux kernel, the following vulnerability has been resolved: net: skbuff: preserve shared-frag marker during coalescing skb_try_coalesce() can attach paged frags from @from to @to. If @from has SKBFL_SHARED_FRAG set, the resulting @to skb can contain the same externally-owned or page-cache-backed frags, but the shared-frag marker is currently lost. That breaks the invariant relied on by later in-place writers. In particular, ESP input checks skb_has_shared_frag() before deciding whether an uncloned nonlinear skb can skip skb_cow_data(). If TCP receive coalescing has moved shared frags into an unmarked skb, ESP can see skb_has_shared_frag() as false and decrypt in place over page-cache backed frags. Propagate SKBFL_SHARED_FRAG when skb_try_coalesce() transfers paged frags. The tailroom copy path does not need the marker because it copies bytes into @to's linear data rather than transferring frag descriptors.

Full record on NVD

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How to check if you are exposed to CVE-2026-46300

Find every Linux Kernel system you or your clients run, including ones exposed to the internet. Those come first: CISA only lists flaws attackers are already using.
Compare each version with the fixed versions in the vendor advisory below. Where no fix exists yet, apply the vendor’s mitigation or take the system off the internet.
Run a credentialed vulnerability scan to catch the systems you missed, then check that the next scan no longer reports CVE-2026-46300.

US federal agencies had to fix this by 2026-10-31. Everyone else should treat that deadline as a guide: if it has passed, this is overdue.

Affected versions

Versions the National Vulnerability Database lists as vulnerable to CVE-2026-46300. Check the vendor advisory for the exact fixed builds before you sign anything off.

Product	Vulnerable versions
Linux Kernel	from 3.9 up to and including 5.10.257; from 5.11 before 5.15.208; from 5.16 before 6.1.174; from 6.2 before 6.6.141; from 6.7 before 6.12.91; from 6.13 before 6.18.33 and 5 more
Siemens Simatic S7-1500 CPU 1518-4 Pn/dp MFP Firmware	from 3.1.5 up to and including 3.1.6
Redhat Openshift Container Platform	from 4.12 before 4.12.91; from 4.13 before 4.13.67; from 4.14 before 4.14.68; from 4.15 before 4.15.65; from 4.16 before 4.16.63; from 4.17 before 4.17.55 and 4 more
Redhat Enterprise Linux	6.0; 7.0; 8.0; 9.0; 10.0
Redhat Enterprise Linux AUS	8.4; 8.6
Redhat Enterprise Linux EUS	8.4; 9.4; 9.6; 10.0

NVD lists 2 more affected products; see the full record on NVD.

Timeline

2026-05-23 · Published in the National Vulnerability Database
2026-10-28 · Added to CISA's Known Exploited Vulnerabilities catalog, 158 days after it was published
2026-10-31 · Fix-by date for US federal agencies, passed 1 day ago

Vendor advisories and fixes

Weakness type

CWE-456

More exploited Linux vulnerabilities

CISA lists 32 exploited Linux vulnerabilities. The closest matches:

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Sources: CISA Known Exploited Vulnerabilities catalog (version 2026.10.30); EPSS data from FIRST.org; CVSS and descriptions from the NIST National Vulnerability Database. Checked 2026-11-01. This product uses the NVD API but is not endorsed or certified by the NVD.