CVE-2026-23553 | 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.

Published: 2026-01-28 Last update: 2026-06-17 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-23553 is rated Low Risk (12.7/100): CVSS Low severity, with low exploitation likelihood (EPSS 0.13%). Low composite risk—no urgent action required; patch on your normal maintenance cycle and revisit priority if CVSS or EPSS increases.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2026-23553

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 0.01% 0.13% +0.11%
2 2026-01-29 — 0.01% —

Full EPSS history (2 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-23553

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
2.9 3.1 LOW
CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N Click to expand
Attack vector (AV:L)
They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by.
Attack complexity (AC:H)
Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:L)
Some sensitive info could get out, but not a total data dump.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:N)
Service keeps running; no real outage angle.
 1.4 1.4 134c704f-9b21-4f2e-91b3-4a467353bcc0

Weakness enumeration for CVE-2026-23553

OS Trackers for CVE-2026-23553

vendor priority summary link
alpine — CVE-2026-23553: 1 source package rows (xen); 427 state rows across 6 repos (3.19-main, 3.20-main, 3.21-main, 3.22-main, 3.23-main, edge-main); fixed 20, open 407. https://security.alpinelinux.org/vuln/CVE-2026-23553
debian end-of-life CVE-2026-23553 end-of-life priority: Debian including 1 source packages (xen), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 3, open 2. https://security-tracker.debian.org/tracker/CVE-2026-23553
suse medium — https://www.suse.com/security/cve/CVE-2026-23553/
ubuntu medium CVE-2026-23553 medium priority: Ubuntu including 1 source packages (xen), 7 status rows across 7 suites (bionic, focal, jammy, noble, questing, upstream, xenial): needs-triage 7. https://ubuntu.com/security/CVE-2026-23553

Affected software / configurations for CVE-2026-23553

Vendor Product Version Raw CPE
xen xen >= 4.6.0 cpe:2.3:o:xen:xen:*:*:*:*:*:*:x86:*

References for CVE-2026-23553

URL Tags
https://xenbits.xenproject.org/xsa/advisory-479.html Mitigation Patch Vendor Advisory
http://www.openwall.com/lists/oss-security/2026/01/27/3 Mailing List Mitigation Patch Third Party Advisory
http://xenbits.xen.org/xsa/advisory-479.html Mitigation Patch Vendor Advisory
cvelogic Threat Intelligence