CVE-2025-21756
📋 TL;DR
A use-after-free vulnerability in the Linux kernel's vsock subsystem allows local attackers to potentially escalate privileges or crash the system. The vulnerability occurs when socket bindings are improperly managed during transport reassignment, leading to memory corruption. This affects systems running vulnerable Linux kernel versions with vsock functionality enabled.
💻 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 →⚠️ Risk & Real-World Impact
Worst Case
Local privilege escalation to root, complete system compromise, or kernel panic causing denial of service.
Likely Case
Local privilege escalation allowing attackers to gain root access on affected systems.
If Mitigated
Limited impact if proper access controls restrict local user privileges and vsock functionality is disabled.
🎯 Exploit Status
Requires local access and knowledge of kernel exploitation techniques. No public exploit code is known at this time.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Kernel versions containing commits: 3f43540166128951cc1be7ab1ce6b7f05c670d8b, 42b33381e5e1f2b967dc4fb4221ddb9aaf10d197, 645ce25aa0e67895b11d89f27bb86c9d444c40f8, b1afd40321f1c243cffbcf40ea7ca41aca87fa5e, e48fcb403c2d0e574c19683f09399ab4cf67809c
Vendor Advisory: https://git.kernel.org/stable/c/3f43540166128951cc1be7ab1ce6b7f05c670d8b
Restart Required: Yes
Instructions:
1. Update Linux kernel to patched version from your distribution vendor. 2. Reboot system to load new kernel. 3. Verify kernel version after reboot.
🔧 Temporary Workarounds
Disable vsock module
LinuxPrevent loading of vsock kernel module to mitigate vulnerability
echo 'install vsock /bin/false' >> /etc/modprobe.d/disable-vsock.conf
rmmod vsock 2>/dev/null || true
🧯 If You Can't Patch
- Restrict local user access and implement strict privilege separation
- Monitor for suspicious local privilege escalation attempts and kernel crashes
🔍 How to Verify
Check if Vulnerable:
Check kernel version and compare with distribution's security advisory. Vulnerable if running kernel before patched versions.Check Version:
uname -rVerify Fix Applied:
Verify kernel version is updated to patched version and system has been rebooted.📡 Detection & Monitoring
Log Indicators:
- Kernel panic logs
- KASAN use-after-free reports in dmesg
- Refcount underflow warnings
Network Indicators:
- None - this is a local vulnerability
SIEM Query:
Search for: 'KASAN: slab-use-after-free', 'refcount_t: underflow', 'vsock_bind' in kernel logs🔗 References
- https://git.kernel.org/stable/c/3f43540166128951cc1be7ab1ce6b7f05c670d8b
- https://git.kernel.org/stable/c/42b33381e5e1f2b967dc4fb4221ddb9aaf10d197
- https://git.kernel.org/stable/c/645ce25aa0e67895b11d89f27bb86c9d444c40f8
- https://git.kernel.org/stable/c/b1afd40321f1c243cffbcf40ea7ca41aca87fa5e
- https://git.kernel.org/stable/c/e48fcb403c2d0e574c19683f09399ab4cf67809c
- https://git.kernel.org/stable/c/e7754d564579a5db9c5c9f74228df5d6dd6f1173
- https://git.kernel.org/stable/c/fcdd2242c0231032fc84e1404315c245ae56322a
- https://lists.debian.org/debian-lts-announce/2025/05/msg00030.html
- https://lists.debian.org/debian-lts-announce/2025/05/msg00045.html
