VulnerabilityPublished 28 Oct 2026

CVE-2026-99120: Linux kernel · KVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes

Source GCVE Database

Authoritative record:CVE-2026-99120

Not known to be exploited · fixed in e90ab3669181c6469e3647ef2b30ae408275f496 : update Linux kernel when convenient.

Data as of 28 Oct 2026, 14:14 UTC

Severity

Low

Rated Low by Threat Radar's AI analysis · CVSS not yet scored by NVD

Patch: Available – fixed in e90ab3669181c6469e3647ef2b30ae408275f496, d6e08ec2fff45e0ec69f11010851650d7c6282b9, 0caeefed99b37f03251a00eb1fdb403a1cf2cb68

Exploitation evidence

Disclosed: evidence found.
CVE published 28 Oct 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

ghsa-r4wm-h5f5-6qhj

What to do

Upgrade Linux kernel to e90ab3669181c6469e3647ef2b30ae408275f496, d6e08ec2fff45e0ec69f11010851650d7c6282b9 or 0caeefed99b37f03251a00eb1fdb403a1cf2cb68.

Affected software

Product	Affected	Fixed in
LinuxLinux	Affected ≥ 8db428fd5229ba509d515af10d7f33d7d4a54bfe, < e90ab3669181c6469e3647ef2b30ae408275f496	Fixed in e90ab3669181c6469e3647ef2b30ae408275f496
LinuxLinux	Affected ≥ 8db428fd5229ba509d515af10d7f33d7d4a54bfe, < d6e08ec2fff45e0ec69f11010851650d7c6282b9	Fixed in d6e08ec2fff45e0ec69f11010851650d7c6282b9
LinuxLinux	Affected ≥ 8db428fd5229ba509d515af10d7f33d7d4a54bfe, < 0caeefed99b37f03251a00eb1fdb403a1cf2cb68	Fixed in 0caeefed99b37f03251a00eb1fdb403a1cf2cb68
LinuxLinux	Affected =6.18	Fixed in–

AI-suggested

A fix has been implemented in the Linux kernel to handle the SS bit correctly in nested page fault exit codes by dropping unexpected bits and preserving the SS bit when reflecting faults to L1. Users should apply the updated kernel version containing this fix. No additional mitigation steps are indicated.

Description

A vulnerability in the Linux kernel's KVM x86 memory management unit (MMU) handling causes out-of-bounds accesses due to unexpected SS bit settings in nested page fault (#NPF) exit codes when running Hyper-V on Windows 11. The issue arises because the AMD manual is unclear about when the SS bit is set, leading to warnings and undefined behavior sanitizer (UBSAN) reports. The fix involves dropping unexpected bits in error codes while preserving the SS bit when reflecting faults to the first-level hypervisor (L1).

Impact

The vulnerability causes out-of-bounds memory accesses in the kernel's KVM MMU code, leading to kernel warnings and potential instability or crashes. There is no indication of privilege escalation or remote code execution. The issue affects nested virtualization scenarios involving Hyper-V on Windows 11 guests running under KVM. No known exploits in the wild have been reported.

Summary

The Linux kernel's KVM module for x86 architecture mishandles nested page fault exit codes when the SS bit is set, which occurs when running Hyper-V from Windows 11. The AMD documentation does not clearly specify conditions for the SS bit, causing the kernel's permission_fault() function to access out-of-bounds array indices, resulting in kernel warnings and UBSAN errors. The patch corrects this by filtering out unexpected bits in the error code while preserving the SS bit when passing faults to L1, maintaining compatibility with Hyper-V's behavior. This issue was introduced following the enablement of GMET for guests in commit 687ee95c1b6d and is reproducible even with GMET disabled.

References

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