XSA-523: two grant copy flaws let a Xen guest read and write Dom0 memory

XSA-523: two grant copy flaws let a Xen guest read and write Dom0 memory

3 November 2026: The Xen Project has published XSA-523, covering CVE-2026-102703 and CVE-2026-102704, two flaws in how the hypervisor handles batches of grant copy operations. A malicious guest can use them to read and write arbitrary memory belonging to other domains, Dom0 included. Every Xen release from 4.6 onwards is vulnerable, and the advisory says there is no known mitigation.

Not yet independently verified. The advisory gives no CVSS score and says nothing about exploitation. We have not checked which distributions and vendors have shipped fixed packages. We will update this when it can be confirmed, and remove this note.

Where the cache went wrong

Grant tables are the way one Xen domain shares memory pages with another, and a grant copy moves data between such pages on a caller's behalf. A common pattern is a batch of copies over adjacent ranges in the same source or destination page. To save time, Xen keeps details it has already validated and reuses them across the batch. The rules for when to validate again were wrong in some cases.

The advisory splits the result in two. In CVE-2026-102703, frame numbers and grant references could be mixed up. In CVE-2026-102704, the rule that copying by frame number is allowed only on the caller's own memory was enforced in the wrong place. Dom0 is the privileged control domain, so a guest reaching it is the outcome a hypervisor exists to prevent.

Which releases have a patch

Xen 4.5 and earlier are not vulnerable; 4.6 and everything later is. The advisory attaches four patch files:

• xsa523-1 and xsa523-2, for xen-unstable down to Xen 4.19.x;
• xsa523-4.18-1 and xsa523-4.18-2, for Xen 4.18.x and 4.17.x.

It lists no patch for the branches between 4.6 and 4.16, so operators on those must look to their vendor or move to a supported branch. The Xen security team warns that patches are written for the stable branches and may not apply cleanly to a release tarball, and asks downstream distributors to move to the tip of the stable branch first. Because there is no mitigation, the exposure is any unpatched host that runs guests its operator does not control.

Two more advisories on 10 November

The advisory index also shows XSA-529 and XSA-530 as assigned and embargoed, with public release set for 18:00 UTC on 10 November 2026. Nothing about their subject is published. The embargo notice in XSA-523 repeats the project's standing rule: operators may deploy fixes during an embargo, even on public-facing systems, but distributing updated software is barred except to members of the predisclosure list.

XSA-523 follows two October advisories, XSA-519 and XSA-520, that also dealt with grant-table cache flush handling, both published on 13 October. Teams that patch Xen in batches may want to hold a slot for the second week of November.

Related on TrustList:

• Linux kernel flaw lets a guest VM read and crash its host
• LXD and MicroCloud fix a flaw that lets a public certificate log in
• AWS fixes EKS network policy bypass and a Firecracker memory flaw

Sources

• Xen Project: XSA-523, arbitrary read/write via grant copy (version 2), 3 November 2026
• Xen Project: Xen Security Advisories index, with XSA-529 and XSA-530 pre-released, 3 November 2026