CVE-2013-2076 in Xen
Summary
by MITRE
Xen 4.0.x, 4.1.x, and 4.2.x, when running on AMD64 processors, only save/restore the FOP, FIP, and FDP x87 registers in FXSAVE/FXRSTOR when an exception is pending, which allows one domain to determine portions of the state of floating point instructions of other domains, which can be leveraged to obtain sensitive information such as cryptographic keys, a similar vulnerability to CVE-2006-1056. NOTE: this is the documented behavior of AMD64 processors, but it is inconsistent with Intel processors in a security-relevant fashion that was not addressed by the kernels.
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Analysis
by VulDB Data Team • 05/14/2021
The vulnerability described in CVE-2013-2076 represents a critical security flaw in the Xen hypervisor version 4.0.x, 4.1.x, and 4.2.x when operating on AMD64 processors. This issue stems from the improper handling of x87 floating-point register state during context switching between virtual machines. The flaw specifically affects how the hypervisor manages the FXSAVE and FXRSTOR instructions, which are responsible for saving and restoring the x87 floating-point unit state. Under normal circumstances, these instructions should preserve all relevant floating-point registers including FOP, FIP, and FDP, but the vulnerability allows for selective restoration of only those registers when an exception is pending.
The technical implementation of this vulnerability creates a cross-domain information leakage scenario that fundamentally undermines the isolation guarantees provided by virtualization. When a virtual machine executes floating-point operations that result in exceptions, only certain x87 registers get saved and restored properly. However, when no exception occurs, the hypervisor fails to properly save the complete floating-point state, leaving portions of the register contents accessible to other domains running on the same physical processor. This behavior creates a side-channel attack vector where malicious domains can infer sensitive information about the floating-point state of other domains through careful observation of register contents.
The operational impact of this vulnerability extends beyond simple information disclosure, as it can be exploited to extract cryptographic keys and other sensitive data from neighboring virtual machines. The flaw operates at a low level within the hypervisor's virtualization layer, making it particularly dangerous because it bypasses traditional security boundaries between domains. Attackers can leverage this vulnerability to perform cross-vm attacks that would normally be prevented by proper virtualization isolation mechanisms. The vulnerability is especially concerning in multi-tenant cloud environments where multiple customers share the same physical infrastructure, as it could allow one tenant to extract confidential information from another tenant's virtual machines.
This vulnerability maps to CWE-200 (Information Exposure) and CWE-310 (Cryptographic Issues) within the Common Weakness Enumeration framework, and it aligns with several ATT&CK techniques including T1059.001 (Command and Scripting Interpreter: PowerShell), T1003.002 (Credential Access: OS Credential Dumping), and T1552.001 (Unsecured Credentials: Credentials in Files). The inconsistency between AMD64 and Intel processor behaviors creates a particularly challenging security landscape where hypervisor implementations must account for processor-specific quirks that can introduce security vulnerabilities. The vulnerability represents a design flaw in the hypervisor's virtualization handling that was not properly addressed by kernel-level security measures, leaving systems exposed to attacks that exploit these processor-level inconsistencies. Organizations running affected Xen versions should immediately implement mitigations including kernel updates, hypervisor patches, and potential isolation measures to prevent cross-domain information leakage that could compromise cryptographic security and overall system integrity.