CVE-2026-23553

Name	CVE-2026-23553
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.
Source	CVE (at NVD; CERT, ENISA, LWN, oss-sec, fulldisc, Debian ELTS, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, GitHub advisories/code/issues, web search, more)

Vulnerable and fixed packages

The table below lists information on source packages.

Source Package	Release	Version	Status
xen (PTS)	bullseye	4.14.6-1	vulnerable
	bullseye (security)	4.14.5+94-ge49571868d-1	vulnerable
	bookworm, bookworm (security)	4.17.5+72-g01140da4e8-1	vulnerable
	trixie (security), trixie	4.20.2+7-g1badcf5035-0+deb13u1	vulnerable
	forky, sid	4.20.2+7-g1badcf5035-2	vulnerable

The information below is based on the following data on fixed versions.

Package	Type	Release	Fixed Version	Urgency	Origin	Debian Bugs
xen	source	bullseye	(unfixed)	end-of-life
xen	source	(unstable)	(unfixed)

Notes

[trixie] - xen <postponed> (Minor issue, fix along with next Xen update)
[bookworm] - xen <postponed> (Minor issue, fix along with next Xen update)
[bullseye] - xen <end-of-life> (EOLed in Bullseye)
https://xenbits.xen.org/xsa/advisory-479.html