VulnerabilityPublished 28 Oct 2026Updated 29 Oct 2026

CVE-2026-99186: Linux kernel · x86/insn-eval: Fix shadowed segment index in get_seg_reg_override_idx()

Affects Linux kernelSource GCVE Database

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Authoritative record:CVE-2026-99186· High

Not known to be exploited.

Data as of 29 Oct 2026, 22:46 UTC

Severity

High

Rated by Threat Radar · CVSS not yet scored by NVD

Thin evidence

One scorer or none

Exploitation evidence

Disclosed: evidence found.
CVE published 28 Oct 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: 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 29 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.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), 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.

What to do

Affected software

Product	Affected	Fixed in
Ubuntu 26.10linux`pkg:deb/ubuntu/linux?arch=source&distro=stonking`Other threats	Affected 7.0.0-14.14 – 7.3.0-10.10 7 affected builds	Fixed in–
Ubuntu 26.10linux-aws`pkg:deb/ubuntu/linux-aws?arch=source&distro=stonking`Other threats	Affected 7.0.0-1004.4 – 7.3.0-1003.3 5 affected builds	Fixed in–
Ubuntu 26.10linux-azure`pkg:deb/ubuntu/linux-azure?arch=source&distro=stonking`Other threats	Affected 7.0.0-1004.4 – 7.3.0-1003.3 5 affected builds	Fixed in–
Ubuntu 26.10linux-azure-fde`pkg:deb/ubuntu/linux-azure-fde?arch=source&distro=stonking`Other threats	Affected =7.0.0-1004.4 =7.3.0-1002.3 =7.3.0-1003.4	Fixed in–
Ubuntu 26.10linux-gcp`pkg:deb/ubuntu/linux-gcp?arch=source&distro=stonking`Other threats	Affected 7.0.0-1003.3 – 7.3.0-1003.3 5 affected builds	Fixed in–
Ubuntu 26.10linux-oracle`pkg:deb/ubuntu/linux-oracle?arch=source&distro=stonking`Other threats	Affected 7.0.0-1003.3 – 7.3.0-1003.3 5 affected builds	Fixed in–

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

A fix is available that renames the internal macro iterator variable to prevent variable shadowing and correct the segment override handling. Users should apply the updated Linux kernel versions that include this patch. Since this is a kernel-level issue, upgrading to patched kernel releases is the recommended mitigation. No other specific workarounds are indicated.

Description

A vulnerability in the Linux kernel's x86 instruction evaluation code caused by a variable shadowing issue in get_seg_reg_override_idx() leads to incorrect handling of segment register overrides. This results in the function always returning a default segment register value, ignoring FS and GS overrides, and causing an infinite loop when a CS override is present. The flaw can cause the kernel to spin indefinitely, preventing user space from regaining control and leaving CPUs tied up until reboot. Additionally, FS or GS prefix usage can cause emulated results to be written to incorrect memory addresses.

Impact

This vulnerability allows any user to cause a denial of service by tying up CPUs indefinitely until the system is rebooted. The kernel spins in an infinite loop when handling certain segment override prefixes, preventing normal task scheduling and response. Additionally, incorrect memory writes can occur with FS or GS prefixes, potentially leading to memory corruption. There is no indication of privilege escalation or code execution. No known exploits in the wild have been reported.

Summary

The Linux kernel's x86/insn-eval component contained a bug introduced by commit 45e1dccc0653, where the loop counter variable 'idx' in for_each_insn_prefix() was shadowed by a local variable of the same name in get_seg_reg_override_idx(). This caused the function to always return INAT_SEG_REG_DEFAULT, ignoring FS and GS segment overrides. When a CS override is encountered, the loop counter increments incorrectly, causing an infinite loop that prevents the kernel from returning to user space, resulting in a hung CPU and a SIGKILL that is never delivered. The issue affects UMIP emulation paths triggered from the #GP handler for user space instructions like SGDT, SIDT, or SMSW with memory operands and CS prefixes. The fix involved renaming the internal macro iterator variable to avoid shadowing.

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 29 Oct 2026, 22:46 UTC

Technical details

References

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