CVE-2020-29040 in Xen
Summary
by MITRE • 11/25/2020
An issue was discovered in Xen through 4.14.x allowing x86 HVM guest OS users to cause a denial of service (stack corruption), cause a data leak, or possibly gain privileges because of an off-by-one error. NOTE: this issue is caused by an incorrect fix for CVE-2020-27671.
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Analysis
by VulDB Data Team • 12/10/2020
The vulnerability identified as CVE-2020-29040 represents a critical stack corruption issue affecting Xen hypervisor versions through 4.14.x, specifically impacting x86 HVM guest operating systems. This flaw stems from an off-by-one error that creates a condition where guest users can manipulate memory access patterns to trigger unintended behavior within the hypervisor's execution environment. The vulnerability was introduced as an incorrect fix for CVE-2020-27671, demonstrating how remediation efforts can sometimes introduce new security weaknesses rather than resolving existing ones. The root cause lies in improper bounds checking within the hypervisor's handling of guest memory operations, particularly in scenarios involving stack manipulation and memory access validation.
The technical implementation of this vulnerability exploits an off-by-one error that occurs during memory management operations within the x86 HVM guest context. When a malicious guest user executes specific memory access patterns, the hypervisor's memory management subsystem fails to properly validate the bounds of stack operations, leading to stack corruption that can be leveraged to execute arbitrary code or cause system instability. This type of vulnerability falls under CWE-129, which specifically addresses improper validation of array indices, and demonstrates how memory safety issues in hypervisor code can have cascading effects on system security. The flaw allows for privilege escalation by enabling guest users to manipulate the hypervisor's execution context through carefully crafted memory operations that bypass normal access controls.
The operational impact of CVE-2020-29040 extends beyond simple denial of service scenarios, as it provides potential for data leakage and privilege escalation within virtualized environments. Attackers can exploit this vulnerability to gain unauthorized access to sensitive data stored within the hypervisor or other virtual machines, potentially compromising the entire virtualized infrastructure. The vulnerability affects systems running Xen hypervisors in x86 HVM mode, making it particularly dangerous in cloud computing environments where multiple tenants share the same physical hardware. From an attack perspective, this vulnerability maps to ATT&CK technique T1055 for privilege escalation and T1499 for denial of service, representing a significant threat to virtualization security. The impact is amplified because hypervisors serve as the foundational layer for all virtualized workloads, making successful exploitation potentially catastrophic for entire cloud deployments.
Mitigation strategies for CVE-2020-29040 require immediate patching of affected Xen hypervisor versions to address the incorrect fix for CVE-2020-27671 and implement proper bounds checking mechanisms. Organizations should prioritize updating their Xen installations to versions that contain the corrected implementation of memory validation routines and ensure that all virtualized environments undergo thorough security assessments. Additionally, implementing runtime monitoring for suspicious memory access patterns and stack manipulation behaviors can help detect exploitation attempts before they succeed. System administrators should also consider implementing network segmentation and access controls to limit the potential impact of successful exploitation attempts, while maintaining detailed logging of hypervisor activities for forensic analysis. The vulnerability underscores the importance of comprehensive testing and validation of security patches, particularly in critical infrastructure components like hypervisors where incorrect fixes can introduce new attack vectors.