Xen flaws let one malicious virtual machine access other guests’ memory
The Xen Project disclosed two flaws on November 3 that let a malicious guest virtual machine read and write the memory of other guests on the same host, including dom0, the management domain that normally has full permissions. The vulnerabilities affect Xen 4.6 and later and are tracked as CVE-2026-102703 and CVE-2026-102704 under advisory XSA-523, according to the Xen Project.
The flaws affect Xen’s grant-copy operation, which copies data between virtual machines. Xen can reuse an earlier permission check for several copies within the same memory page, and in some cases fails to check again when required. That error lets a malicious guest access memory it does not own. XCP-ng’s update notes said that “a single compromised guest can break out of its isolation and take over the whole host.”
The Xen advisory does not report attacks exploiting the flaws and does not specify what access an attacker needs inside the guest. It also lists no known mitigation, so administrators need to use a vendor fix where one is available, according to the Xen Project.
The Xen advisory includes patches for Xen unstable and versions 4.17.x through 4.19.x. It lists no patch for Xen 4.6 through 4.16. For XCP-ng 8.3, Vates released xen-4.17.7-4.2.xcpng8.3 on November 3 and requires hosts to be rebooted. XCP-ng rates the impact as critical, according to its update notes.
Qubes OS said every supported release and configuration is affected. Its current fix is for Qubes 4.3 and uses Xen packages 4.19.6-3 in dom0; dom0 must be restarted. Qubes had placed the fixed packages in its security-testing repository, with movement to the stable repository expected over the following two weeks after community testing. Debian’s security tracker listed Xen as vulnerable in bookworm, trixie and forky on November 4, while sid was listed as fixed.