CVE-2021-28706 Explained : Impact and Mitigation

A guest in Xen may exceed its designated memory limit, potentially leading to Denial of Service (DoS) attacks. This vulnerability was discovered by Julien Grall of Amazon.

Understanding CVE-2021-28706

This CVE refers to a memory allocation issue in Xen that could allow a guest to exceed its memory limit.

What is CVE-2021-28706?

When a guest in Xen is allowed close to 16TiB of memory, it may issue hypercalls to increase its memory allocation beyond the established limit. This is due to a calculation error done with 32-bit precision, leading to potential memory allocation overflow.

The Impact of CVE-2021-28706

The vulnerability could enable a guest to allocate unbounded amounts of memory to itself, potentially resulting in a Denial of Service (DoS) affecting the entire host.

Technical Details of CVE-2021-28706

The issue affects various versions of Xen, including 4.12.x, xen-unstable, and 4.13.x.

Vulnerability Description

Guests may be able to allocate unbounded memory, leading to a DoS attack on the host.

Affected Systems and Versions

Versions of Xen from at least 3.2 onwards are impacted, especially Xen builds with the BIGMEM configuration option enabled.

Exploitation Mechanism

By exploiting the memory allocation vulnerability, a guest can exceed its designated memory limit and potentially disrupt the host system.

Mitigation and Prevention

It's important to take immediate steps for mitigation and implement long-term security practices to prevent such vulnerabilities.

Immediate Steps to Take

Set the maximum memory allocation for guests to less than 1023 GiB to avoid the vulnerability.

Long-Term Security Practices

Regularly update Xen software and apply patches promptly to mitigate security risks.