Executive Summary : The Linux kernel’s Vsock subsystem has a privilege escalation vulnerability known as CVE-2025-21756. It is brought on by an incorrect reference count decrease in the vsock_remove_sock function, which results in a Use After Free (UAF) condition. An attacker may cause a memory corruption by taking advantage of this UAF, which would enable the recovery of a freed vsock object.
By overwriting function pointers, the attacker can obtain control of the execution flow, circumvent kASLR via a brute-force technique, and leak kernel memory. In order to exploit this issue, it was necessary to get past AppArmor security checks and use side channels, such as the vsock_diag_dump function, to change the kernel’s memory state. A well-planned ROP chain was then used to gain root access.
“This research and exploit development was originally discovered and documented by Michael Hoefler.“
Introduction: Understanding the Linux Kernel Vulnerability (CVE-2025-21756)
The central component of every Linux-based operating system, the Linux kernel, is essential to controlling the hardware and software resources of the system. Any defect or weakness in the kernel might result in significant security vulnerabilities due to its significance. CVE-2025-21756 is one such vulnerability that was recently found to affect how the Linux kernel managed virtual sockets (vsocks).
“CVE-2025-21756 is a vulnerability discovered in the Linux kernel related to the handling of vsock objects, which are used in communication between virtual machines. This issue allows for privilege escalation, meaning that an attacker with limited access to a system can exploit this vulnerability to gain root access.“
Specifically, an incorrect treatment of reference counting on vsock objects is the cause of this problem. An early release of a vsock object creates a Use After Free (UAF) situation, which could allow attackers to reuse previously released memory and cause system instability or remote code execution.Although this defect first seemed minor, it could have a big effect, especially for systems that depend on virtual machine connectivity in settings like cloud computing.
What was CVE-2025-21756?
The Linux kernel’s virtual socket subsystem’s handling of vsock objects was the source of vulnerability CVE-2025-21756. In cloud, containerized, and virtualized environments, Vsocks are essential since they are mostly used for communication between virtual machines (VMs) on the same host.
The Core Issue :The kernel’s handling of reference counting for vsock objects is the weak point. A Use After Free (UAF) error could occur in certain situations when a vsock object is released too soon. This implies that memory is reused after it has been released, which may give hackers the opportunity to manage the system by manipulating the released memory.
How It Works:
- Improper Reference Counting: The main problem comes from a patch that changed how vsock objects were handled, particularly while a bound socket was being removed. The fix attempts to remove the socket after verifying its state, but the reference counting was done incorrectly. Even though a vsock object should be released when its reference counter hits zero, it was not always properly guarded.
- Before using
vsock_remove_bound, the code looks for the flagSOCK_DEAD,however in certain situations, this was insufficient to stop an object from being released too soon.
- Use After Free (UAF) Condition: Once released, a vsock object can be utilized in other kernel components. An attacker may be able to overwrite this released memory with malicious data if they have control to it, which could result in privilege escalation or arbitrary code execution.
- The risk was not entirely reduced by the solution used here because it still permitted released items to remain in some tables and be used at a later time.
- Test Case: To illustrate the vulnerability, the kernel maintainers provided a test scenario, which clarifies the circumstances in which the UAF could be activated:
How It Was Exploited
The vulnerability occurs when an attacker forces the kernel into prematurely releasing a vsock object that other kernel components are still referencing. An attacker may take advantage of this by changing the underlying structures of the kernel, which would allow the memory to be reused. The attacker could then insert any code or data into the freed memory space.
Because vsock is utilized in containerized environments and virtual machine communication, an attacker who has access to a vulnerable VM or container might use a vulnerable vsock socket to interact with the kernel or transmit specially crafted network traffic to cause the UAF.
The attacker could successfully impact the kernel’s internal state by overwriting the released memory with their own data after the vsock object has been released. They could specifically introduce malicious code into the memory that has been released, which would then be run when the kernel uses the object again.
This is made possible by poor defense against this kind of attack by the kernel’s memory management system, which is in charge of monitoring released memory. The attacker could run arbitrary code in the kernel’s address space or increase their privileges once they have control of the kernel memory.
Privilege escalation is the most dangerous consequence of exploiting CVE-2025-21756. This UAF vulnerability could allow an attacker with limited privileges (like a user or a container) to raise their privileges and take complete root access to the computer. In some circumstances, an attacker might be able to run arbitrary code in the kernel space by taking advantage of this vulnerability.
int vsock_trigger_uaf_attack()
{
int s, client_socket;
struct sockaddr_vm addr;
// Step 1: Bind a vsock to trigger the kernel's management of vsock
s = socket(AF_VSOCK, SOCK_SEQPACKET, 0);
if (s < 0) {
perror("socket");
return -1;
}
memset(&addr, 0, sizeof(addr));
addr.svm_family = AF_VSOCK;
addr.svm_cid = VMADDR_CID_LOCAL; // Local VM
addr.svm_port = 1234;
if (bind(s, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
perror("bind failed");
close(s);
return -1;
}
// Step 2: Close the socket, freeing the vsock object
close(s);
// Step 3: Reuse the freed vsock object (this step can lead to UAF)
client_socket = socket(AF_VSOCK, SOCK_SEQPACKET, 0);
if (client_socket < 0) {
perror("socket failed");
return -1;
}
addr.svm_port = 1234;
if (connect(client_socket, (struct sockaddr *)&addr, sizeof(addr)) == 0) {
printf("Successfully connected using reused (freed) vsock object!\n");
// Here, the attacker could manipulate the kernel memory
}
close(client_socket);
return 0;
}The attacker binds a vsock object, shuts it (causing the object to be freed), and then tries to reconnect using the freed vsock object in this simple hack. This “reused” object could be used by the attacker to take over the kernel memory if the memory reuse mechanism of the kernel permits it.
The problem was resolved by improving memory management procedures to avoid similar vulnerabilities and implementing appropriate memory validation checks to make sure that objects in the VSOCK_STATE_FREE state are not reused. To reduce the probability of exploitation, hardening strategies including stack protection and usercopy hardening were advised, and kernel fixes were made available for impacted versions.
