What’s Vulnerable › Vulnerabilities

CVE-2026-23553: x86: incomplete IBPB for vCPU isolation

• Severity: low (CVSS 2.9 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.1% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-01-28 · updated 2026-06-17
• Weakness: CWE-665

Description

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.

Affected products

• xen: from 4.6.0

References

• https://xenbits.xenproject.org/xsa/advisory-479.html
• http://www.openwall.com/lists/oss-security/2026/01/27/3
• http://xenbits.xen.org/xsa/advisory-479.html

Get alerted when CVE-2026-23553 or the next vulnerability affects your stack