CVE-2019-17347 in Xen
Summary
by MITRE
An issue was discovered in Xen through 4.11.x allowing x86 PV guest OS users to cause a denial of service or gain privileges because a guest can manipulate its virtualised %cr4 in a way that is incompatible with Linux (and possibly other guest kernels).
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Analysis
by VulDB Data Team • 09/27/2020
The vulnerability identified as CVE-2019-17347 represents a critical flaw in the Xen hypervisor affecting versions 4.11.x and earlier, specifically impacting x86 paravirtualized guest operating systems. This issue stems from improper handling of control register manipulation within the virtualized environment, creating a potential pathway for privilege escalation and denial of service conditions. The flaw exists in the hypervisor's virtualization layer where it fails to properly validate or restrict guest modifications to the %cr4 control register, which is fundamental to x86 architecture memory management and protection mechanisms.
The technical implementation of this vulnerability exploits the inherent differences between how guest operating systems and the hypervisor manage control registers, particularly %cr4 which contains crucial flags for memory management and privilege levels. When a paravirtualized guest OS manipulates its virtualized %cr4 register in ways that conflict with the expectations of the underlying Linux kernel or other guest operating systems, the hypervisor fails to properly enforce boundary conditions. This misconfiguration allows malicious guest users to craft specific control register modifications that can either crash the virtual machine or potentially escalate privileges within the guest environment. The vulnerability operates at the intersection of hardware virtualization and operating system kernel management, creating a dangerous gap in the hypervisor's privilege enforcement mechanisms.
From an operational impact perspective, this vulnerability presents significant security implications for virtualized environments that rely on Xen hypervisors. Attackers can leverage this flaw to either deny service to legitimate users by crashing virtual machines or potentially gain elevated privileges within guest operating systems, which could lead to complete compromise of the virtualized environment. The vulnerability affects systems where multiple guests share the same hypervisor instance, making it particularly dangerous in cloud computing environments or data centers where virtualization is extensively used. The privilege escalation aspect means that even a low-privilege guest user could potentially gain root access to the guest OS, while the denial of service component could be exploited to disrupt critical services running on virtual machines.
The vulnerability maps directly to CWE-119 Improper Access Control and CWE-248 Uncontrolled Resource Consumption, as it represents both unauthorized access to system resources through privilege escalation and potential resource exhaustion through denial of service attacks. From the MITRE ATT&CK framework perspective, this vulnerability aligns with T1059 Command and Scripting Interpreter and T1499 Endpoint Termination, as it enables both privilege escalation and system disruption. The attack surface is particularly concerning in multi-tenant environments where guest isolation is critical, as this vulnerability could allow one guest to affect the security posture of other guests on the same hypervisor. Organizations using Xen versions prior to 4.12.0 should consider this vulnerability as a high-priority remediation target, as it fundamentally undermines the security guarantees provided by virtualization technology.
Mitigation strategies should focus on immediate patching to Xen version 4.12.0 or later, which includes proper validation of %cr4 register modifications. Administrators should also implement additional monitoring for unusual control register modifications within virtual machines and consider implementing hypervisor-level restrictions on guest OS privilege escalation attempts. The fix addresses the core issue by ensuring that guest modifications to %cr4 are properly validated against the hypervisor's expectations and the guest OS's actual requirements, preventing the inconsistent state that led to the vulnerability. Organizations should also review their virtualization security policies and consider implementing additional isolation measures between guest operating systems to minimize the impact of potential exploitation.