CVE-2026-23553

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.

Affected Software

Weaknesses

Workarounds

Using "spec-ctrl=ibpb-entry=hvm,ibpb-entry=pv" on the Xen command line will activate the SRSO mitigation on non-SRSO-vulnerable hardware, but it is a large overhead.

ADP Enrichment

CVE Program Container

Additional References

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

CISA ADP Vulnrichment

• SSVC:
• Exploitation: none
  • Automatable: no
  • Technical Impact: partial

References

• https://xenbits.xenproject.org/xsa/advisory-479.html

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