A severe vulnerability in the Linux kernel’s Virtual Socket (vsock) implementation, identified as CVE-2025-21756, has been discovered. This flaw enables local attackers to escalate privileges to root level. Researchers have confirmed the vulnerability’s high exploitability, with a CVSS v3.1 Base Score of 7.8. The flaw arises from improper handling of socket bindings during transport reassignment in the vsock subsystem, specifically causing a use-after-free condition.
The issue occurs when the socket’s reference counter is incorrectly decremented, which can cause the system to mistakenly treat the socket as unbound.
This leads to further errors in socket handling, allowing attackers to exploit the situation. The kernel’s developers have implemented a patch to resolve this issue by adding a simple check to preserve socket bindings until socket destruction. This patch should mitigate the risks associated with this vulnerability, which is critical for Linux-based systems.
Exploitation of CVE-2025-21756 involves triggering the use-after-free bug, followed by reclaiming the freed memory with controlled data. This sophisticated attack bypasses Linux Security Module protections like AppArmor. Attackers can exploit this bug using techniques like leaking memory addresses and executing a Return-Oriented Programming (ROP) chain to escalate privileges.
Once successful, attackers can gain root privileges, which allows for system compromise, data theft, or service disruption.
Linux distributions have released patches to fix the vulnerability, and users are urged to update systems immediately. In cases where patches cannot be applied immediately, limiting local access and monitoring suspicious activity related to vsock is recommended. CVE-2025-21756 represents a critical risk to systems, especially those in cloud or containerized environments, and should be addressed as a priority by system administrators.
