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Fencer Vulns
low 2.9

CVE-2026-23553 json

x86: incomplete IBPB for vCPU isolation

published 2026-01-28 9 mo ago modified 2026-06-17 4 mo ago also EUVD-2026-4882

cvss
2.9
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.1%
EPSS 0.1% chance of exploitation in 30 days, percentile 3%
public exploits
0
none known

Summary

In the context switch logic Xen attempts to skip an IBPB in the case of a vCPU returning to a CPU on which it was the previous vCPU to run. While safe for Xen's isolation between vCPUs, this prevents the guest kernel correctly isolating between tasks. Consider:

1) vCPU runs on CPU A, running task 1. 2) vCPU moves to CPU B, idle gets scheduled on A. Xen skips IBPB. 3) On CPU B, guest kernel switches from task 1 to 2, issuing IBPB. 4) vCPU moves back to CPU A. Xen skips IBPB again.

Now, task 2 is running on CPU A with task 1's training still in the BTB.

Patch / fix

  • xen Debian 12 4.17.7-0+deb12u1
  • xen Debian 13 4.20.2+37-g61ff35323e-0+deb13u1
  • xen Debian 14 4.20.2+37-g61ff35323e-1

debian advisories DEBIAN-CVE-2026-23553

  • xen SLE Module for Server Applications 15 SP7, SLE Module for Basesystem 15 SP7 4.20.2_06-150700.3.25.1
  • xen SLE Micro 5.4, SLE Micro 5.3 4.16.7_06-150400.4.78.1
  • xen SLE Module for Server Applications 15 SP7, SLE Module for Basesystem 15 SP7 4.20.2_04-150700.3.22.1
  • xen openSUSE Leap 15.6 4.18.5_10-150600.3.37.1
  • xen SLE Micro 5.5 4.17.6_04-150500.3.59.1
  • xen SLE Micro 5.2 4.14.6_28-150300.3.94.1
  • xen SLES LTSS Extended Security 12 SP5 4.12.4_64-3.137.1

suse advisories SUSE-SU-2026:0589-1SUSE-SU-2026:0394-1SUSE-SU-2026:0329-1SUSE-SU-2026:0328-1SUSE-SU-2026:0306-1SUSE-SU-2026:0304-1

  • xen openSUSE Tumbleweed 4.21.0_04-1.1

opensuse advisories openSUSE-SU-2026:10118-1

  • xen Alpine 3.20 4.18.5-r4
  • xen Alpine 3.21 4.19.4-r1
  • xen Alpine 3.23, Alpine 3.22 4.20.2-r1
  • xen Alpine 3.24 4.21.0-r2

alpine advisories ALPINE-CVE-2026-23553

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-23553 updated cvss 2.9

xen: x86: incomplete IBPB for vCPU isolation

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
9 mo ago

Exploit prediction

0.14% chance of exploitation in the next 30 days, higher than 3% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CISA ADP 3.1 2.9 CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
CNA 3.1 2.9 CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

Affected products

  • Xen Xen

Classification

Weakness
  • CWE-665 Improper Initialization
  • CWE-693 Protection Mechanism Failure

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.