CVE-2015-8555 in Xen
Summary
by MITRE
Xen 4.6.x, 4.5.x, 4.4.x, 4.3.x, and earlier do not initialize x86 FPU stack and XMM registers when XSAVE/XRSTOR are not used to manage guest extended register state, which allows local guest domains to obtain sensitive information from other domains via unspecified vectors.
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Analysis
by VulDB Data Team • 07/01/2022
The vulnerability identified as CVE-2015-8555 represents a critical information disclosure flaw within the Xen hypervisor affecting versions 4.6.x through 4.3.x and earlier releases. This issue stems from inadequate initialization of x86 processor extended register states, specifically the FPU stack and XMM registers, when the hypervisor operates without utilizing XSAVE/XRSTOR instruction sets for guest register management. The flaw exists at the virtualization layer where hypervisor implementations fail to properly clear or initialize extended processor state components, creating potential pathways for cross-domain information leakage.
The technical root cause of this vulnerability lies in the hypervisor's failure to properly handle processor state initialization when extended register management is disabled. When XSAVE/XRSTOR instructions are not employed to manage guest extended register state, the hypervisor does not ensure that FPU stack and XMM registers are properly initialized or cleared between domain contexts. This creates a scenario where sensitive data from one guest domain could persist in processor registers and potentially be accessed by other domains running on the same physical host. The vulnerability operates at the hardware virtualization level where the hypervisor fails to provide proper isolation guarantees for extended processor state components, violating fundamental security principles of virtual machine isolation.
The operational impact of CVE-2015-8555 extends beyond simple information disclosure, as it represents a significant breach in virtualization security boundaries. Local guest domains can exploit this vulnerability to obtain sensitive information from other domains through unspecified vectors that likely involve processor state manipulation and cross-domain data leakage. This vulnerability affects the core isolation mechanisms that virtualization platforms rely upon to maintain security boundaries between different guest operating systems and applications. The implications are particularly severe in multi-tenant environments where multiple customers share the same physical infrastructure, as it could enable one tenant to access confidential data belonging to another tenant.
The vulnerability aligns with CWE-200 (Information Exposure) and CWE-310 (Cryptographic Issues) categories, as it enables unauthorized information disclosure through improper handling of processor state. From an ATT&CK framework perspective, this vulnerability maps to T1059 (Command and Scripting Interpreter) and T1046 (Network Service Scanning) as attackers could potentially use this information to craft more sophisticated attacks or identify system configurations. The exploitability of this vulnerability requires local access within a guest domain, making it particularly concerning for cloud environments where guest isolation is paramount. Organizations utilizing Xen hypervisors in production environments face significant risk exposure, as this vulnerability could enable data breaches and compromise the security of entire virtualized infrastructures.
Mitigation strategies for CVE-2015-8555 require immediate implementation of hypervisor updates to versions that properly initialize extended processor registers, with patch management protocols prioritizing this vulnerability due to its potential for cross-domain information disclosure. System administrators should also consider implementing additional monitoring and detection measures to identify potential exploitation attempts, including analysis of processor state behavior and cross-domain access patterns. Organizations should review their virtualization security configurations to ensure that XSAVE/XRSTOR instruction sets are properly enabled and utilized for managing extended register states. The vulnerability highlights the critical importance of proper processor state management in virtualization environments and underscores the need for comprehensive security testing of hypervisor implementations. Additionally, implementing proper access controls and domain isolation mechanisms can help reduce the potential impact of exploitation, though the fundamental fix requires updating to patched hypervisor versions that properly address the register initialization issue.