CVE-2026-63939 KVM: SEV: Compute the correct max length of the in-GHCB scratch area

Critical 9.3 Published 19 Jul 2026 Updated 19 Oct 2026

A buffer overflow in the Linux kernel's KVM SEV implementation allows attackers to corrupt memory, potentially leading to code execution or privilege escalation.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Local or same network only
Victim action needed: Not needed
Fix: Available

What is CVE-2026-63939?

CVE-2026-63939 is a critical memory corruption vulnerability in Linux kernel, published on 19 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

In the Linux kernel, the following vulnerability has been resolved:

KVM: SEV: Compute the correct max length of the in-GHCB scratch area

When setting the length of the GHCB scratch area, and the area is in the GHCB shared buffer, set the effective length of the scratch area to the max possible size given the start of the guest-provided pointer, and the end of the shared buffer.

The code was "fine" when first introduced, as KVM doesn't consult the length of the buffer when emulating MMIO, because the passed in @len always specifies the *max* size required. But for PSC requests, the incoming @len is just the minimum length (to process the header), and KVM needs to know the full size of the scratch area to avoid buffer overflows (spoiler alert).

Opportunistically rename @len => @min_len to better reflect its role.

Weakness: CWE-122

Which versions are affected by CVE-2026-63939?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2026-63939?

Apply the vendor's patch.

Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.

Is CVE-2026-63939 being exploited?

There is no known exploitation of CVE-2026-63939 in the wild and no public exploit code.

All Linux kernel vulnerabilities

Sources

Also known as BELL-CVE-2026-63939, DEBIAN-CVE-2026-63939, GHSA-94xr-m4g8-q6cc, UBUNTU-CVE-2026-63939.