Critical Linux Kernel Vulnerability Exposes Systems to Privilege Escalation Attacks

A significant vulnerability in the Linux kernel’s Virtual Socket (vsock) implementation, designated as CVE-2025-21756, has been identified that could allow local attackers to escalate privileges to root level. 

Security researchers have confirmed that this flaw, which received a CVSS v3.1 Base Score of 7.8 (HIGH), can be reliably exploited on affected systems.

Critical Linux Kernel Vulnerability- CVE-2025-21756

According to the Hoefler report, the vulnerability stems from improper handling of socket bindings during transport reassignment in the vsock subsystem. 

Specifically, the issue occurs in a sequence where the socket’s reference counter is incorrectly decremented, leading to a use-after-free condition.

The core of the vulnerability lies in the following code path in the Linux kernel:

When a transport reassignment occurs, this function decrements the reference counter without verifying if the socket was bound and moved to the bound list. 

This can create a scenario where subsequent calls to vsock_bind() assume the socket is in the unbound list and call __vsock_remove_bound(), leading to the use-after-free condition.

The patch implemented by Linux kernel developers addresses this issue by adding a simple check to preserve socket bindings until socket destruction:

Risk Factors	Details
Affected Products	Linux kernel with vsock (Virtual Socket) implementation (notably versions before 6.6.79, 6.12.16, 6.13.4, and 6.14-rc1)
Impact	Privilege escalation possible
Exploit Prerequisites	Local access with ability to create and manipulate vsock sockets; low attack complexity; no user interaction required; attacker must have local privilege
CVSS 3.1 Score	7.8 (High)

Exploitation Method

A detailed exploitation method has emerged in the security community. The attack involves triggering the use-after-free bug, then reclaiming the freed memory with controlled data. 

One particularly sophisticated approach leverages pipe backing pages to overwrite critical kernel structures.

The exploit bypasses Linux Security Module (LSM) protections, specifically AppArmor, by finding 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 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 and Patch Released

This vulnerability affects all Linux distributions running vulnerable kernel versions. 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.

Major Linux distributions have released patches addressing this vulnerability. Users should update their systems immediately with the latest kernel versions.

For systems that cannot be immediately patched, limiting access to local users and monitoring for suspicious activities related to the vsock subsystem is recommended.

CVE-2025-21756 represents a significant security risk for Linux systems. While requiring local access limits its immediate impact, the reliability of known exploit methods makes this vulnerability particularly dangerous in multi-user or container environments. 

System administrators should prioritize patching affected systems to mitigate this threat.

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