CVE-2026-62436 in Xen
Summary
by MITRE • 07/28/2026
[This CNA information record relates to multiple CVEs; the
text explains which aspects/vulnerabilities correspond to which CVE.]
With the introduction of Grant Table v2 came the requirement to be able to switch between versions. Switching from v1 to v2 reduces the number of valid grant references, as a bigger shared entry structure is then needed while the shared table doesn't change size. Switching from v2 back to v1 the status frames, which are separate in v2, go away.
Code holding, but intermediately dropping and then re-acquiring the grant table lock, sometimes wrongly assumes that said properties wouldn't change across the window in time where the lock is not being held.
The v1 -> v2 issue is CVE-2026-62435.
The v2 -> v1 issue is CVE-2026-62436.
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Analysis
by VulDB Data Team • 07/29/2026
This vulnerability exists within virtualization environments that utilize grant tables for memory sharing between guest operating systems and hypervisor components. The introduction of Grant Table version 2 introduced significant structural changes to how shared memory entries are managed, creating compatibility challenges when switching between versions. When transitioning from v1 to v2, the system must accommodate larger shared entry structures while maintaining the same table size, resulting in a reduction of valid grant references that can be managed simultaneously. This architectural change creates a window of vulnerability where code that holds grant table locks may incorrectly assume that the properties of grant references remain consistent during temporary lock release periods.
The technical flaw stems from improper assumptions made by code that manages grant table access patterns. Specifically, when code temporarily releases the grant table lock for intermediate operations and then re-acquires it, it fails to account for potential version transitions that could alter the grant reference landscape. This race condition occurs because the system does not properly validate or synchronize grant table properties during the brief period when the lock is unavailable. The vulnerability manifests as a potential security issue where code may operate on outdated assumptions about grant table structure and validity, leading to incorrect memory management decisions.
The operational impact of these vulnerabilities extends beyond simple performance degradation to potentially critical security implications within virtualized environments. When switching from v1 to v2, the reduction in valid grant references could lead to denial of service conditions where legitimate memory sharing operations fail due to exhausted reference slots. The reverse transition from v2 to v1 creates additional complications as status frames that were previously separate become consolidated, potentially causing data corruption or privilege escalation if code incorrectly handles these structural changes. These issues affect virtualization platforms where dynamic grant table version switching is supported, creating potential attack vectors for malicious actors seeking to exploit memory management inconsistencies.
The first vulnerability CVE-2026-62435 specifically addresses the v1 to v2 transition problem where code assumes grant reference properties remain constant during lock release windows. This aligns with common software security patterns identified in CWE-362, which describes race conditions occurring when a program has a window of time between checking a condition and acting on it. The second vulnerability CVE-2026-62436 addresses the reverse transition issue where code fails to properly handle the consolidation of status frames from separate entities into unified structures. Both vulnerabilities can be mapped to ATT&CK techniques involving privilege escalation through memory corruption or resource exhaustion attacks, as described in the adversary tactics and techniques framework for virtualization environments.
Mitigation strategies should focus on implementing proper synchronization mechanisms that validate grant table properties before and after lock operations, ensuring that code does not make assumptions about structural consistency during temporary lock release periods. The recommended approach includes adding explicit version checking routines that verify grant table state transitions occur atomically without intermediate changes that could invalidate existing reference assumptions. Additionally, defensive programming practices should be enforced to prevent code from operating on grant references that may have become invalid due to version switching events. System administrators should consider implementing monitoring for grant table version transitions and establish alerting mechanisms for unusual patterns in memory sharing operations that could indicate exploitation attempts.
This vulnerability class demonstrates the complexity of maintaining backward compatibility while introducing architectural improvements in virtualization systems, highlighting the importance of thorough testing across all transition scenarios. The security implications extend to potential compromise of guest-to-hypervisor communication channels and memory isolation properties that are fundamental to virtualization security models. Organizations using affected virtualization platforms should prioritize patching implementations that contain these grant table version switching mechanisms, particularly in environments where multiple hypervisor versions or dynamic configuration changes occur regularly.