CVE-2024-10934

Executive Summary

CVE-2024-10934 is a critical vulnerability in OpenBSD's NFS (Network File System) implementation, affecting both client and server components. The flaw involves a double-free of an mbuf (memory buffer) and the use of an uninitialized variable in error handling paths within the NFS server. This can lead to memory corruption, denial of service, or potentially remote code execution. The CVSS score of 9.8 reflects the ease of exploitation (network-based, low complexity, no privileges required) and the severe impact on confidentiality, integrity, and availability. No public proof-of-concept or exploit code is available at this time.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – Exploitable remotely over the network.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – Exploit does not affect other components.
• Confidentiality (C): High (H) – Full data compromise possible.
• Integrity (I): High (H) – Full data modification possible.
• Availability (A): High (H) – Full system denial of service possible.

Threat Level: Critical. Remote, unauthenticated exploitation with high impact.

Deep Dive & Mechanics

The vulnerability resides in the NFS client and server implementation within the OpenBSD kernel. Specifically, it involves:

1. Mbuf Double Free: In certain error conditions, the NFS code path may free an mbuf (memory buffer used for network data) more than once. This corrupts the kernel memory allocator, leading to crashes or potential exploitation.
2. Uninitialized Variable: In the NFS server's error handling, a variable is used without being properly initialized. This can lead to undefined behavior, including memory corruption or information disclosure.

The root cause is improper error handling and resource management in the NFS code. An attacker can trigger these conditions by sending specially crafted NFS requests, causing the kernel to enter error paths that mishandle memory.

Vulnerable vs. Patched Code

The patch addresses the double-free by ensuring mbufs are freed only once, and initializes the variable before use in error handling.

Vulnerable Code (Conceptual):

// In NFS error handling path
struct mbuf *m;
// ... some operations ...
if (error) {
    m_freem(m); // First free
    // ... more error handling ...
    m_freem(m); // Double free (m already freed)
}

Patched Code (Conceptual):

// In NFS error handling path
struct mbuf *m = NULL; // Initialize to NULL
// ... some operations ...
if (error) {
    if (m != NULL) {
        m_freem(m);
        m = NULL; // Set to NULL after free
    }
    // ... more error handling ...
    // No second free because m is NULL
}

The exact patch is available in OpenBSD's source code repository (CVS or git) as part of errata 008 for 7.5 and errata 021 for 7.4.

Exploitation & Lab Setup

Red Team

Note: No public exploit code is available. The following is a conceptual approach.

1. Setup: Deploy a vulnerable OpenBSD 7.4 or 7.5 system with NFS server enabled.
2. Reconnaissance: Scan for open NFS ports (default 2049/TCP and 2049/UDP).
3. Trigger: Send malformed NFS requests (e.g., crafted RPC calls) that trigger error paths in the NFS server. This may involve manipulating NFS procedure arguments or causing resource exhaustion.
4. Exploitation: The double-free can be leveraged to corrupt kernel memory. A skilled attacker could potentially achieve arbitrary code execution by controlling freed memory chunks.

Example (Conceptual - Not Functional):

import socket

# Craft a malformed NFS request (simplified)
payload = b"\x00" * 100  # Placeholder for malformed data
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.sendto(payload, ("target_ip", 2049))

Blue Team

1. Detection: Monitor for unusual NFS traffic patterns, especially malformed RPC calls or repeated error responses.
2. Hunting: Look for kernel panics or crashes related to NFS or mbuf allocation.
3. Mitigation: Immediately apply the vendor patch (errata). If patching is not possible, disable NFS services or restrict access via firewall rules.
4. Forensics: Collect memory dumps and logs from affected systems to analyze the attack vector.

Indicators of Compromise (IOCs)

• Network: Unusual NFS traffic on port 2049 (TCP/UDP) with malformed RPC headers.
• System: Kernel panics or crashes with messages related to "mbuf", "m_freem", or "NFS".
• Logs: System logs showing repeated NFS errors or unexpected NFS client disconnections.
• Memory: Corrupted kernel memory allocator structures (requires kernel debugging).

Detection Engineering

YARA Rule

rule CVE_2024_10934_NFS_DoubleFree {
    meta:
        description = "Detects potential exploitation of CVE-2024-10934 via malformed NFS packets"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2024-10934"
    strings:
        $nfs_mbuf = "m_freem" ascii wide nocase
        $nfs_error = "nfsrv" ascii wide nocase
    condition:
        $nfs_mbuf and $nfs_error
}

Sigma Rule

title: CVE-2024-10934 NFS Double-Free Exploitation Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects malformed NFS packets that may trigger CVE-2024-10934
author: Valters IT
logsource:
    category: network_connection
    product: suricata
detection:
    selection:
        dest_port: 2049
        event_type: alert
        alert.signature_id: 3000001
    condition: selection
falsepositives:
    - Legitimate NFS traffic with unusual patterns
level: high

Suricata Rule

alert udp any any -> any 2049 (msg:"CVE-2024-10934 NFS Double-Free Attempt"; content:"|00 00 00 00|"; depth:4; sid:3000001; rev:1;)

Remediation & Mitigation

1. Immediate: Apply the vendor patch (OpenBSD errata 008 for 7.5, errata 021 for 7.4). Reboot the system after patching.
2. Workaround: If patching is not possible, disable NFS services (rcctl stop nfsd and rcctl stop mountd) or restrict access to trusted networks only using firewall rules (e.g., pf).
3. Monitoring: Enable detailed logging for NFS and monitor for suspicious activity.
4. Long-term: Keep systems updated with the latest security patches. Consider using NFSv4 with stronger authentication (Kerberos) if possible.

Valters IT
Securing Digital Frontiers