XSA-299 addressed several critical issues in restartable PV type change operations. Despite extensive testing and auditing, some corner cases were missed.
A malicious PV guest administrator may be able to escalate their privilege to that of the host.
All security-supported versions of Xen are vulnerable. Only x86 systems are affected. Arm systems are not affected. Only x86 PV guests can leverage the vulnerability. x86 HVM and PVH guests cannot leverage the vulnerability. Note that these attacks require very precise timing, which may be difficult to exploit in practice.