CVE-2026-99186

CPU hangs via shadowed instruction-prefix iterator

Published Oct 28, 2026 · Updated Oct 28, 2026

An infinite loop in Linux 6.19 x86 UMIP emulation allows local users to hang CPUs through CS-prefixed instructions. In get_seg_reg_override_idx(), the for_each_insn_prefix() macro shadows the result variable, causing segment-index assignments to overwrite its loop counter and prevent termination. Local execution of SGDT, SIDT, or SMSW with a memory operand and CS prefix traps the task in the kernel until reboot despite SIGKILL; FS or GS prefixes instead cause emulated results to be written to the wrong address.

CVSS severityUnavailable
Unscored
EPSS probability0.16%
Next 30 days · Nov 1, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

An infinite loop in Linux 6.19 x86 UMIP emulation allows local users to hang CPUs through CS-prefixed instructions. In get_seg_reg_override_idx(), the for_each_insn_prefix() macro shadows the result variable, causing segment-index assignments to overwrite its loop counter and prevent termination. Local execution of SGDT, SIDT, or SMSW with a memory operand and CS prefix traps the task in the kernel until reboot despite SIGKILL; FS or GS prefixes instead cause emulated results to be written to the wrong address.

The record

CVE
CVE-2026-99186
Published
Oct 28, 2026
Updated
Oct 28, 2026
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
T1499.004
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Oct 28, 2026CVE publishedPublication date reported by the CVE source.
  2. Oct 28, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=45e1dccc0653c50e377dae57ef086a8d0f71061d <0efe927053f7eabd1e8658cbda96ee3d78bd1765
  2. Affected versionversion=45e1dccc0653c50e377dae57ef086a8d0f71061d <32c52ec87d6245ee2d12ed5dd509e40062939b50
  3. Affected versionversion=6.19

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=45e1dccc0653c50e377dae57ef086a8d0f71061d <0efe927053f7eabd1e8658cbda96ee3d78bd1765; version=45e1dccc0653c50e377dae57ef086a8d0f71061d <32c52ec87d6245ee2d12ed5dd509e40062939b50; version=6.19

Published CVSS scores

No CVSS assessment is available in this record.

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Activity data is delayed. The latest report is Sep 28, 2026.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 22, 2026Sep 28, 2026
Latest reporting daySep 28, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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