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Linux Kernel Hacked: CVE-2025-21756 – Exploiting the Vsock UAF for Root Access
- The Linux kernel’s Vsock subsystem has a privilege escalation vulnerability known as CVE-2025-21756, caused by incorrect reference count decrease leading to a Use After Free condition.
- Exploiting the UAF allows attackers to recover a freed vsock object, gain control of execution flow, bypass kASLR, and leak kernel memory.
- The vulnerability required bypassing AppArmor security checks, using side channels like vsock_diag_dump, and employing a ROP chain for root access.
- CVE-2025-21756 affects the Linux kernel's management of virtual sockets, impacting systems reliant on virtual machine connectivity.
- The vulnerability stems from incorrect reference counting on vsock objects, leading to a UAF situation and potential system instability or remote code execution.
- A test case provided by kernel maintainers demonstrates how the UAF vulnerability can be activated.
- Exploitation involves prematurely releasing a vsock object, enabling memory reuse for malicious code insertion and potential privilege escalation.
- An attacker could manipulate kernel memory by taking advantage of a freed vsock object, hence affecting the kernel's internal state.
- The vulnerability could allow attackers to execute arbitrary code in the kernel space and achieve root access, posing a high risk.
- The fix involved enhancing memory management procedures, implementing memory validation checks, and applying hardening strategies to prevent future similar vulnerabilities.
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