Low severity · published Jan 28, 2026
CVE-2026-23553
x86: incomplete IBPB for vCPU isolation
2.9
CVSS
0.1%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
Attack vector
Local
Complexity
High
Privileges
None
User interaction
None
Confidentiality
Low
Integrity
None
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.1% · higher than 4% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
XE
Xen · Xen
4.6.0 and later · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-23553
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-23553
Mitigation
https://xenbits.xenproject.org/xsa/advisory-479.html
Mailing List
http://www.openwall.com/lists/oss-security/2026/01/27/3
Mitigation
http://xenbits.xen.org/xsa/advisory-479.html
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