CVE-2025-39823
📋 TL;DR
This CVE addresses a speculative execution side-channel vulnerability in the Linux kernel's KVM (Kernel-based Virtual Machine) subsystem for x86 systems. Guest-controlled indices (min and dest_id) were not properly sanitized against speculative execution attacks, potentially allowing a malicious guest VM to infer host kernel memory contents. This affects systems running KVM virtualization with untrusted guest VMs.
💻 Affected Systems
- Linux Kernel
📦 What is this software?
Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →⚠️ Risk & Real-World Impact
Worst Case
A malicious guest VM could perform speculative execution attacks to leak sensitive host kernel memory, potentially exposing credentials, encryption keys, or other privileged data.
Likely Case
Information disclosure from host kernel memory to guest VMs, though exploitation requires sophisticated timing attacks and knowledge of the target system.
If Mitigated
With proper isolation and untrusted guest restrictions, impact is minimal as the attack requires guest VM compromise first.
🎯 Exploit Status
Exploitation requires guest VM compromise first, then sophisticated timing-based side-channel attacks. No public exploits known.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Patches available in stable kernel branches via provided git commits
Vendor Advisory: https://git.kernel.org/stable/c/31a0ad2f60cb4816e06218b63e695eb72ce74974
Restart Required: Yes
Instructions:
1. Update Linux kernel to patched version from distribution vendor. 2. Reboot system to load new kernel. 3. Verify kernel version matches patched release.
🔧 Temporary Workarounds
Disable KVM for untrusted workloads
linuxTemporarily disable KVM virtualization or restrict to trusted guest VMs only
🧯 If You Can't Patch
- Isolate KVM hosts from sensitive networks and systems
- Implement strict access controls and monitoring for guest VMs
🔍 How to Verify
Check if Vulnerable:
Check kernel version and verify if KVM is enabled: 'lsmod | grep kvm' and 'uname -r'Check Version:
uname -rVerify Fix Applied:
Verify kernel version matches patched release from distribution vendor📡 Detection & Monitoring
Log Indicators:
- Unusual guest VM behavior, timing anomalies in KVM operations
Network Indicators:
- Not network exploitable - local side-channel attack
SIEM Query:
Not applicable - local speculative execution attack🔗 References
- https://git.kernel.org/stable/c/31a0ad2f60cb4816e06218b63e695eb72ce74974
- https://git.kernel.org/stable/c/33e974c2d5a82b2f9d9ba0ad9cbaabc1c8e3985f
- https://git.kernel.org/stable/c/67a05679621b7f721bdba37a5d18665d3aceb695
- https://git.kernel.org/stable/c/72777fc31aa7ab2ce00f44bfa3929c6eabbeaf48
- https://git.kernel.org/stable/c/c87bd4dd43a624109c3cc42d843138378a7f4548
- https://git.kernel.org/stable/c/d51e381beed5e2f50f85f49f6c90e023754efa12
- https://git.kernel.org/stable/c/f49161646e03d107ce81a99c6ca5da682fe5fb69
- https://git.kernel.org/stable/c/f57a4bd8d6cb5af05b8ac1be9098e249034639fb
- https://lists.debian.org/debian-lts-announce/2025/10/msg00007.html
- https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html
