---
title: Xen XSA-523: grant-copy bugs let a malicious guest read and write any other VM's memory, including dom0, on every Xen from 4.6 on (CVE-2026-102703, CVE-2026-102704) | ctxsec
url: https://ctxsec.com/items/xen-xsa-523-grant-copy-guest-reads-writes-other-vms-dom0-cve-2026-102703-102704
description: "Xen caches already-validated data when a batch of grant copies hits the same page, and in some cases it did not re-validate: frame numbers and grant references could be mixed up, and the check that limits copy-by-frame-number to the caller's own memory was misplaced. The Xen Project says there is no mitigation, only patches."
status: "official (Xen Security Advisory XSA-523, Nov 3); no exploitation reported"
published: 2026-11-03
---

> Xen caches already-validated data when a batch of grant copies hits the same page, and in some cases it did not re-validate: frame numbers and grant references could be mixed up, and the check that limits copy-by-frame-number to the caller's own memory was misplaced. The Xen Project says there is no mitigation, only patches.

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official (Xen Security Advisory XSA-523, Nov 3); no exploitation reported

2026-11-03

### Xen XSA-523: grant-copy bugs let a malicious guest read and write any other VM's memory, including dom0, on every Xen from 4.6 on (CVE-2026-102703, CVE-2026-102704)

Patches are attached to the advisory for xen-unstable through 4.19 and for 4.17 to 4.18. Xen 4.5 and earlier are not affected. The advisory credits Anthropic, using Claude, with the discovery, validated by Stephan Lipp of Ada Logics, with an independent report from the CodeAnt AI Security Research Team.

**Who should act:** anyone running multi-tenant Xen hosts, plus products built on Xen (XCP-ng, Citrix/XenServer, Qubes OS and others) as their vendors ship XSA-523. The Hacker News reported that Debian's tracker still listed bookworm, trixie and forky as vulnerable as of Nov 4.

Sources: [Xen Project: XSA-523, arbitrary read/write via grant copy (Nov 3)](https://xenbits.xen.org/xsa/advisory-523.html) · [The Hacker News: Xen Hypervisor Flaws Let One Malicious VM Read and Write Any Other VM's Memory (Nov 4)](https://thehackernews.com/2026/11/xen-hypervisor-flaws-let-one-malicious.html)