A significant security flaw has been identified in the Linux kernel’s Virtual Socket (vsock) implementation, designated as CVE-2025-21756. This vulnerability allows local attackers to escalate their privileges to root level, posing a substantial risk to affected systems. Security researchers have assigned it a CVSS v3.1 Base Score of 7.8, indicating a high severity level.
Understanding CVE-2025-21756
The vulnerability originates from improper handling of socket bindings during transport reassignment within the vsock subsystem. Specifically, the issue arises when the socket’s reference counter is incorrectly decremented, leading to a use-after-free condition. This flaw can be exploited by local users to gain elevated privileges, potentially compromising the entire system.
Technical Details
The core of the vulnerability lies in the following code path in the Linux kernel:
“`c
void vsock_remove_bound(struct vsock_sock vsk)
{
if (!hlist_unhashed(&vsk->bound_table))
hlist_del_init(&vsk->bound_table);
}
“`
In this function, during a transport reassignment, the reference counter is decremented without verifying if the socket was bound and moved to the bound list. This oversight can lead to a scenario where subsequent calls to `vsock_bind()` assume the socket is in the unbound list and call `__vsock_remove_bound()`, resulting in a use-after-free condition.
Exploitation Methodology
Exploiting this vulnerability involves triggering the use-after-free bug and then reclaiming the freed memory with controlled data. One sophisticated approach leverages pipe backing pages to overwrite critical kernel structures. Attackers can bypass Linux Security Module (LSM) protections, such as AppArmor, by identifying functions not protected by these security mechanisms. By using `vsock_diag_dump()` as a side channel, attackers can leak the memory address of `init_net`, effectively defeating Kernel Address Space Layout Randomization (KASLR). With these capabilities, attackers can construct a Return-Oriented Programming (ROP) chain that calls `commit_creds(init_cred)` to elevate privileges. The final exploit redirects execution through a function pointer overwrite at `sk->sk_error_report`, triggered by calling the socket’s `release()` function.
Affected Systems
This vulnerability affects all Linux distributions running vulnerable kernel versions, notably those before 6.6.79, 6.12.16, 6.13.4, and 6.14-rc1. The issue is particularly concerning for cloud environments and virtualized systems that rely heavily on the vsock functionality for guest-host communications. If exploited, attackers can gain root privileges, potentially leading to complete system compromise, data theft, or service disruption.
Mitigation and Patching
In response to this vulnerability, Linux kernel developers have released patches addressing the issue. The patch adds a simple check to preserve socket bindings until socket destruction:
“`c
void vsock_remove_bound(struct vsock_sock vsk)
{
if (!hlist_unhashed(&vsk->bound_table))
hlist_del_init(&vsk->bound_table);
}
“`
Major Linux distributions have incorporated these patches into their latest kernel versions. Users are strongly advised to update their systems immediately to mitigate the risk associated with CVE-2025-21756. For systems that cannot be immediately patched, limiting access to local users and monitoring for suspicious activities related to the vsock subsystem is recommended.
Broader Context of Linux Kernel Vulnerabilities
The discovery of CVE-2025-21756 underscores the ongoing challenges in maintaining the security of the Linux kernel. Over the years, several critical vulnerabilities have been identified, each highlighting different aspects of kernel security.
Historical Perspective: Dirty COW
One of the most notorious Linux kernel vulnerabilities is Dirty COW (CVE-2016-5195), a privilege escalation bug that exploits a race condition in the kernel’s copy-on-write mechanism. This vulnerability allowed local attackers to gain root access by modifying read-only files. The widespread impact of Dirty COW led to significant efforts to patch and secure affected systems.
Recent Vulnerabilities: StackRot and Others
More recently, vulnerabilities like StackRot (CVE-2023-3269) have been discovered. StackRot is a privilege escalation issue affecting Linux kernel versions 6.1 through 6.4, stemming from a use-after-free condition in the memory management subsystem. Exploiting this flaw could allow local users to escalate privileges, posing a significant security risk.
Another example is CVE-2024-1086, a vulnerability in the netfilter subsystem of the Linux kernel. This flaw allows local attackers to escalate privileges due to improper handling of packet filtering rules. The active exploitation of such vulnerabilities highlights the importance of timely patching and system monitoring.
The Importance of Vigilance
The recurring discovery of such vulnerabilities emphasizes the need for continuous vigilance in system security. System administrators and users must stay informed about potential threats and apply patches promptly to protect their systems. Regular system updates, monitoring for unusual activities, and adherence to security best practices are essential in mitigating the risks associated with kernel vulnerabilities.
Conclusion
CVE-2025-21756 represents a significant security risk for Linux systems, particularly in environments that utilize the vsock functionality. While the requirement for local access limits its immediate impact, the potential for privilege escalation to root level makes it imperative for users to apply the available patches without delay. This incident serves as a reminder of the critical importance of maintaining up-to-date systems and the need for ongoing vigilance in the face of evolving cybersecurity threats.
