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CVE-2026-23553

x86: incomplete IBPB for vCPU isolation

2.9 LowFix available

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.

Attack profile

How easy it is to exploit

Attack vector
Local
Complexity
High
Privileges needed
None
User interaction
None

What an attacker can affect

Confidentiality
Low
Integrity
None
Availability
None

Exploit likelihood

0.14%Up 0.1 points since Jan 29, 2026
90%50%10%1%

What each source reports

CVE List Score used 2.9

x86: incomplete IBPB for vCPU isolation

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.

Record
CVE-2026-23553
CVSS 3.1
2.9 LowCVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
Weakness
CWE-665, CWE-693
Updated
2026-11-02
EUVDEUVD-2026-4882 2.9

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.

Record
EUVD-2026-4882
CVSS 3.1
2.9 LowCVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
Updated
2026-11-02
GitHub AdvisoriesGHSA-pxqc-5jg3-xfqm 2.9

In the context switch logic Xen attempts to skip an IBPB in the case of a vCPU returning to a CPU...

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.

Record
GHSA-pxqc-5jg3-xfqm
CVSS 3.1
2.9 LowCVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
Weakness
CWE-665
Updated
2026-11-02
NVD 2.9

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.

Record
CVE-2026-23553
CVSS 3.1
2.9 LowCVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
Weakness
CWE-665, CWE-693
Updated
2026-11-02
OSVALPINE-CVE-2026-23553 2.9

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.

Record
ALPINE-CVE-2026-23553
CVSS 3.1
2.9 LowCVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
Updated
2026-11-02
OSV 2.9

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.

Record
CVE-2026-23553
CVSS 3.1
2.9 LowCVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
Updated
2026-11-02
Show 11 moreShow less
OSVDEBIAN-CVE-2026-23553 2.9

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.

Record
DEBIAN-CVE-2026-23553
CVSS 3.1
2.9 LowCVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
Updated
2026-11-02
OSVUBUNTU-CVE-2026-23553 2.9

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.

Record
UBUNTU-CVE-2026-23553
CVSS 3.1
2.9 LowCVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
Updated
2026-11-02
OSVMGASA-2026-0026 No score

Updated xen packages fix security vulnerabilities

x86: buffer overrun with shadow paging + tracing. (CVE-2025-58150) x86: incomplete IBPB for vCPU isolation. (CVE-2026-23553)

Record
MGASA-2026-0026
Updated
2026-11-02
OSVopenSUSE-SU-2026:10118-1 No score

xen-4.21.0_04-1.1 on GA media

These are all security issues fixed in the xen-4.21.0_04-1.1 package on the GA media of openSUSE Tumbleweed.

Record
openSUSE-SU-2026:10118-1
Updated
2026-11-02
OSVSUSE-SU-2026:0303-1 No score

Security update for xen

This update for xen fixes the following issues:

Security fixes:

  • CVE-2025-58150: Fixed buffer overrun with shadow paging and tracing (XSA-477) (bsc#1256745)
  • CVE-2026-23553: Fixed incomplete IBPB for vCPU isolation (XSA-479) (bsc#1256747)
  • CVE-2025-58149: Fixed incorrect removal od permissions on PCI device unplug allow PV guests to access memory of devices no longer assigned to it (XSA-476) (bsc#1252692)
  • CVE-2025-27466, CVE-2025-58142, CVE-2025-58143: Fixed multiple vulnerabilities in the Viridian interface (XSA-472) (bsc#1248807)

Other fixes:

  • Fixed virtxend service restart. Caused by a failure to start xenstored (bsc#1254180)
Record
SUSE-SU-2026:0303-1
Updated
2026-11-02
OSVSUSE-SU-2026:0304-1 No score

Security update for xen

This update for xen fixes the following issues:

  • CVE-2025-58150: Fixed buffer overrun with shadow paging and tracing (XSA-477) (bsc#1256745)
  • CVE-2026-23553: Fixed incomplete IBPB for vCPU isolation (XSA-479) (bsc#1256747)
Record
SUSE-SU-2026:0304-1
Updated
2026-11-02
OSVSUSE-SU-2026:0306-1 No score

Security update for xen

This update for xen fixes the following issues:

  • CVE-2025-58150: Fixed buffer overrun with shadow paging and tracing (XSA-477) (bsc#1256745)
  • CVE-2026-23553: Fixed incomplete IBPB for vCPU isolation (XSA-479) (bsc#1256747)
Record
SUSE-SU-2026:0306-1
Updated
2026-11-02
OSVSUSE-SU-2026:0328-1 No score

Security update for xen

This update for xen fixes the following issues:

Security fixes:

  • CVE-2025-58150: Fixed buffer overrun with shadow paging and tracing (XSA-477) (bsc#1256745)
  • CVE-2026-23553: Fixed incomplete IBPB for vCPU isolation (XSA-479) (bsc#1256747)
  • CVE-2025-58149: Fixed incorrect removal od permissions on PCI device unplug allow PV guests to access memory of devices no longer assigned to it (XSA-476) (bsc#1252692)
Record
SUSE-SU-2026:0328-1
Updated
2026-11-02
OSVSUSE-SU-2026:0329-1 No score

Security update for xen

This update for xen fixes the following issues:

  • CVE-2025-58150: Fixed buffer overrun with shadow paging and tracing (XSA-477) (bsc#1256745)
  • CVE-2026-23553: Fixed incomplete IBPB for vCPU isolation (XSA-479) (bsc#1256747)
Record
SUSE-SU-2026:0329-1
Updated
2026-11-02
OSVSUSE-SU-2026:0394-1 No score

Security update for xen

This update for xen fixes the following issues:

Security fixes:

  • CVE-2025-58150: Fixed buffer overrun with shadow paging and tracing (XSA-477) (bsc#1256745)
  • CVE-2026-23553: Fixed incomplete IBPB for vCPU isolation (XSA-479) (bsc#1256747)
  • CVE-2025-58149: Fixed incorrect removal od permissions on PCI device unplug allow PV guests to access memory of devices no longer assigned to it (XSA-476) (bsc#1252692)

Other fixes:

  • Fixed virtxend service restart. Caused by a failure to start xenstored (bsc#1254180)
Record
SUSE-SU-2026:0394-1
Updated
2026-11-02
OSVSUSE-SU-2026:0589-1 No score

Security update for xen

This update for xen fixes the following issues:

  • CVE-2025-58150: buffer overrun with shadow paging + tracing (XSA-477) (bsc#1256745).
  • CVE-2026-23553: incomplete IBPB for vCPU isolation (XSA-479) (bsc#1256747).
Record
SUSE-SU-2026:0589-1
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
xen
Alpine:v3.20
 4.6.0 up to 4.18.5-r4
4.18.5-r4
xen
Alpine:v3.21
 4.6.0 up to 4.19.4-r1
4.19.4-r1
xen
Alpine:v3.22
 4.6.0 up to 4.20.2-r1
4.20.2-r1
xen
Alpine:v3.23
 4.6.0 up to 4.20.2-r1
4.20.2-r1
xen
Alpine:v3.24
 4.6.0 up to 4.21.0-r2
4.21.0-r2
xen
Debian:12
 before 4.17.7-0+deb12u1, plus listed versions
4.17.1+2-gb773c48e36-14.17.2+55-g0b56bed864-14.17.2+76-ge1f9cb16e2-14.17.2+76-ge1f9cb16e2-1~deb12u14.17.2-1
Show 9 moreShow less
4.17.3+10-g091466ba55-14.17.3+10-g091466ba55-1.14.17.3+10-g091466ba55-1.1~exp14.17.3+10-g091466ba55-1.1~exp24.17.3+10-g091466ba55-1~deb12u14.17.3+36-g54dacb5c02-14.17.5+23-ga4e5191dc0-14.17.5+23-ga4e5191dc0-1+deb12u14.17.5+72-g01140da4e8-1
4.17.7-0+deb12u1
xen
Debian:13
 before 4.20.2+37-g61ff35323e-0+deb13u1, plus listed versions
4.20.0+68-g35cb38b222-14.20.2+7-g1badcf5035-0+deb13u14.20.2+7-g1badcf5035-14.20.2+7-g1badcf5035-2
4.20.2+37-g61ff35323e-0+deb13u1
xen
Debian:14
 before 4.20.2+37-g61ff35323e-1, plus listed versions
4.20.0+68-g35cb38b222-14.20.2+7-g1badcf5035-14.20.2+7-g1badcf5035-2
4.20.2+37-g61ff35323e-1
xen
Mageia:9
 before 4.17.5-1.git20251028.2.mga9
4.17.5-1.git20251028.2.mga9
xen
SUSE:Linux Enterprise Micro 5.2
 before 4.14.6_28-150300.3.94.1
4.14.6_28-150300.3.94.1
Show 18 moreShow less
Affected packages
PackageAffectedFixed
xen
SUSE:Linux Enterprise Micro 5.5
 before 4.17.6_04-150500.3.59.1
4.17.6_04-150500.3.59.1
xen
openSUSE:Leap 15.6
 before 4.18.5_10-150600.3.37.1
4.18.5_10-150600.3.37.1
xen
SUSE:Linux Enterprise Server LTSS Extended Security 12 SP5
 before 4.12.4_64-3.137.1
4.12.4_64-3.137.1
xen
SUSE:Linux Enterprise Module for Basesystem 15 SP7
 before 4.20.2_04-150700.3.22.1
4.20.2_04-150700.3.22.1
xen
SUSE:Linux Enterprise Module for Server Applications 15 SP7
 before 4.20.2_04-150700.3.22.1
4.20.2_04-150700.3.22.1
xen
SUSE:Linux Enterprise Micro 5.3
 before 4.16.7_06-150400.4.78.1
4.16.7_06-150400.4.78.1
xen
SUSE:Linux Enterprise Micro 5.4
 before 4.16.7_06-150400.4.78.1
4.16.7_06-150400.4.78.1
xen
SUSE:Linux Enterprise Module for Basesystem 15 SP7
 before 4.20.2_06-150700.3.25.1
4.20.2_06-150700.3.25.1
xen
SUSE:Linux Enterprise Module for Server Applications 15 SP7
 before 4.20.2_06-150700.3.25.1
4.20.2_06-150700.3.25.1
xen
Ubuntu:16.04:LTS
 All versions, plus listed versions
4.5.1-0ubuntu14.5.1-0ubuntu24.6.0-1ubuntu14.6.0-1ubuntu24.6.0-1ubuntu4
Show 7 moreShow less
4.6.0-1ubuntu4.14.6.0-1ubuntu4.24.6.0-1ubuntu4.34.6.5-0ubuntu14.6.5-0ubuntu1.14.6.5-0ubuntu1.24.6.5-0ubuntu1.4
 None known
xen
Ubuntu:18.04:LTS
 All versions, plus listed versions
4.9.0-0ubuntu34.9.0-0ubuntu44.9.2-0ubuntu1
 None known
xen
Ubuntu:20.04:LTS
 All versions, plus listed versions
4.9.2-0ubuntu24.9.2-0ubuntu64.9.2-0ubuntu74.11.3+24-g14b62ab3e5-1ubuntu14.11.3+24-g14b62ab3e5-1ubuntu24.11.3+24-g14b62ab3e5-1ubuntu2.24.11.3+24-g14b62ab3e5-1ubuntu2.3
 None known
xen
Ubuntu:22.04:LTS
 All versions, plus listed versions
4.11.4+24-gddaaccbbab-1ubuntu24.16.0-1~ubuntu24.16.0-1~ubuntu2.1
 None known
xen
Ubuntu:24.04:LTS
 All versions, plus listed versions
4.17.2-14.17.2+55-g0b56bed864-14.17.2+76-ge1f9cb16e2-14.17.2+76-ge1f9cb16e2-1ubuntu14.17.3+10-g091466ba55-14.17.3+10-g091466ba55-1.1ubuntu24.17.3+10-g091466ba55-1.1ubuntu3
 None known
xen
Ubuntu:25.10
 All versions, plus listed versions
4.20.0-1ubuntu14.20.0+68-g35cb38b222-1
 None known
xen
Ubuntu:26.10
 All versions, plus listed versions
4.20.2+7-g1badcf5035-2build24.20.2+37-g61ff35323e-1ubuntu14.20.3+127-gc42374a105-1
 None known
xen
Ubuntu:26.04:LTS
 All versions, plus listed versions
4.20.0+68-g35cb38b222-14.20.2+7-g1badcf5035-24.20.2+7-g1badcf5035-2build2
 None known
xen
openSUSE:Tumbleweed
 before 4.21.0_04-1.1
4.21.0_04-1.1

History

History
RecordedChangedSources
first indexedCVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2026-23553 is a low vulnerability (CVSS 2.9) in Xen, published January 27, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.14%, the 3rd percentile. Fixed in xen 4.18.5-r4.

References

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