CVE-2025-21756: Linux Kernel Use-After-Free Vulnerability

CVE-2025-21756 Overview

In the Linux kernel, a vulnerability related to socket binding in the vsock component has been resolved. This issue prevented proper preservation of socket bindings during a transport reassignment, leading to a use-after-free vulnerability. This could allow an attacker to potentially execute arbitrary code or cause a denial of service on Linux systems.

Critical Impact

Use-after-free vulnerability leading to potential code execution.

Affected Products

• Linux Kernel

Discovery Timeline

• Not Available - Vulnerability discovered by Not Available
• Not Available - Responsible disclosure to Linux
• Not Available - CVE CVE-2025-21756 assigned
• Not Available - Linux releases security patch
• 2025-02-27 - CVE CVE-2025-21756 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-21756

Vulnerability Analysis

The vsock vulnerability stems from improper handling of socket bindings in the Linux kernel. Sockets could be unbound incorrectly during transport reassignments, leading to a use-after-free condition, as demonstrated by the Kernel Address Sanitizer (KASAN) logs. The vulnerability affects the binding and life cycle management of sockets.

Root Cause

The root cause is the failure to check if a socket was bound and moved to the bound list correctly before releasing it, resulting in premature release and use-after-free scenarios.

Attack Vector

Local attackers can exploit this vulnerability to escalate privileges or cause denial of service by crafting malicious inputs that trigger the erroneous socket handling.

// Example exploitation code (sanitized)
int main() {
    int sockfd;
    struct sockaddr_vm sa_vm;
    
    // Create a socket
    sockfd = socket(AF_VSOCK, SOCK_STREAM, 0);
    
    // Attempt to bind to the vsock address
    bind(sockfd, (struct sockaddr *)&sa_vm, sizeof(sa_vm));
    
    // Trigger conditions leading to use-after-free
    // ... (exploit code)

    return 0;
}

Detection Methods for CVE-2025-21756

Indicators of Compromise

• Abnormal socket binding operations
• KASAN logs indicating use-after-free
• Unusual vsock errors in syslogs

Detection Strategies

Implement monitoring for kernel logs that indicate use-after-free conditions and anomalies in socket operations, particularly those involving vsock.

Monitoring Recommendations

• Use tools to analyze syscalls related to socket operations
• Monitor for KASAN alerts in kernel logs

How to Mitigate CVE-2025-21756

Immediate Actions Required

• Apply the latest security patches for the Linux kernel
• Monitor for suspicious socket activity
• Disable vsock if not needed

Patch Information

A comprehensive patch is available addressing the issue by correcting the socket binding logic. Patch details can be found in the Linux kernel repositories.

Workarounds

While waiting for the patch application in production environments, consider restricting the use of vsock to trusted users or disabling it if not needed.

# Example system configuration
chmod 700 /path/to/vsock/sockets