CVE-2026-46300
CVE-2026-46300 is a security flaw in Linux Kernel, the core of Linux, which runs most servers, Android phones and many home routers. Attackers are using a flaw in Linux Kernel. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 28 Oct 2026.[1]
Linux Kernel is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed by 8 commits to Linux, in the releases that include them.
Description
CISA calls it a missing initialization of a variable in Linux Kernel. In its words: “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.”[1]
The CVE record adds: “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…”[2]
The underlying weakness is CWE-456, in MITRE’s list of common software weaknesses.
It scores 7.8 out of 10 on the Common Vulnerability Scoring System, which rates it high, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]
| Product | Fixed in |
|---|---|
| Linux | commit 3599e6b3cc1a |
| Linux | commit 2f2b16022a2e |
| Linux | commit 9d3e5fd19fe1 |
| Linux | commit 78bf6b6bb195 |
| Linux | commit 760e1addc27b |
| Linux | commit 3bd9e113d500 |
| Linux | commit 3884358a9286 |
| Linux | commit f84eca581739 |
Most families won’t have Linux Kernel; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 28 Oct 2026, 158 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 31 Oct 2026 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 16%. That’s in the top 4% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
Linux has published a fix or advice.[3] CISA’s 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).”[1]
- Check whether you use it.Linux Kernel is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed (fixed by 8 commits to Linux, in the releases that include them), or not in use. CISA gave US government agencies until 31 Oct 2026 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 7.8.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 16%.[4] |
See also
Other Linux Kernel flaws
- CVE-2026-53266: out-of-bounds write ·
- CVE-2025-39964: race condition ·
- CVE-2025-39682: improper check for unusual or exceptional conditions ·
- CVE-2026-53362: unspecified ·
- CVE-2022-0995: out-of-bounds write ·
- CVE-2022-0492: improper authentication ·
Added the same day
- CVE-2026-41709, Broadcom VMware ESX: insufficient logging ·
Every Linux flaw on CISA’s list · Everything added in October 2026
References
- ^ Cybersecurity and Infrastructure Security Agency. “Linux Kernel Missing Initialization of a Variable Vulnerability”. Known Exploited Vulnerabilities Catalog, added 28 Oct 2026.
- ^ National Institute of Standards and Technology. “CVE-2026-46300 Detail”. National Vulnerability Database, published 23 May 2026.
- ^ Linux. Advisories and fixes: git.kernel.org, git.kernel.org, git.kernel.org, git.kernel.org, git.kernel.org, and 3 more on NVD’s record.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.161, percentile 96.9, 31 Oct 2026.
- CVE Program. “CVE-2026-46300”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.