CVE-2024-10934: OpenBSD Use-After-Free Vulnerability

CVE-2024-10934 Overview

CVE-2024-10934 is a critical memory corruption vulnerability affecting OpenBSD's Network File System (NFS) implementation. The vulnerability exists in both NFS client and server components, where improper memory buffer (mbuf) handling can lead to a double free condition. Additionally, the NFS server's error handling code uses an uninitialized variable, which can result in unpredictable behavior and potential security implications.

Double free vulnerabilities occur when memory is freed more than once, leading to corruption of memory management structures. In the context of a network-accessible service like NFS, this flaw could potentially be exploited by remote attackers to cause denial of service or achieve arbitrary code execution.

Critical Impact

Network-accessible double free vulnerability in OpenBSD NFS implementation could allow remote attackers to corrupt memory structures, potentially leading to system crashes or arbitrary code execution.

Affected Products

• OpenBSD 7.5 (prior to errata 008)
• OpenBSD 7.4 (prior to errata 021)
• OpenBSD versions 7.4 through errata 020

Discovery Timeline

• 2024-11-15 - CVE-2024-10934 published to NVD
• 2025-10-02 - Last updated in NVD database

Technical Details for CVE-2024-10934

Vulnerability Analysis

This vulnerability involves two distinct but related memory safety issues within OpenBSD's NFS subsystem. The primary issue is a double free condition affecting mbuf (memory buffer) structures, which are fundamental data structures in BSD network stacks used for storing network packets and related data.

When an mbuf is freed twice, the memory allocator's internal data structures become corrupted. This can lead to several dangerous conditions: the same memory region may be allocated to two different callers, leading to data corruption; the allocator's free list may become circular, causing infinite loops; or carefully crafted exploitation could potentially allow an attacker to gain control over memory allocation and achieve code execution.

The secondary issue involves the use of an uninitialized variable in the NFS server's error handling path. When errors occur during NFS operations, the server may reference a variable that was never properly initialized, leading to undefined behavior that could leak sensitive information or cause unexpected state transitions.

Root Cause

The root cause of this vulnerability stems from improper memory lifecycle management in the NFS implementation. Specifically:

1. Double Free Issue: The mbuf reference counting or ownership tracking is flawed, allowing the same buffer to be freed by multiple code paths. This typically occurs when error handling paths free a buffer that is also freed by the normal cleanup path, or when asynchronous operations create race conditions around buffer ownership.

2. Uninitialized Variable: The NFS server's error handling code declares a variable but fails to initialize it before use in certain error conditions. This represents a failure to follow defensive programming practices where all variables should be initialized before use.

Attack Vector

The vulnerability is exploitable over the network, as NFS is a network file system protocol. An attacker could potentially trigger these conditions by:

1. Sending specially crafted NFS requests designed to trigger the error handling paths where the double free or uninitialized variable issues occur
2. Exploiting timing windows in NFS operations to create race conditions that lead to the double free
3. Targeting both NFS clients and servers, as both implementations contain the vulnerable code

The attack does not require authentication in scenarios where NFS exports are configured with permissive access controls, though some attack conditions may require the attacker to be on a network with access to NFS services.

Detection Methods for CVE-2024-10934

Indicators of Compromise

• Unexpected NFS service crashes or kernel panics on OpenBSD systems
• Memory corruption errors in system logs related to nfs or mbuf subsystems
• Unusual NFS traffic patterns or malformed NFS requests in network captures
• System instability following NFS operations or during high NFS workload

Detection Strategies

• Monitor OpenBSD system logs for kernel panics or crashes referencing NFS components
• Implement network intrusion detection rules to identify anomalous NFS protocol traffic
• Track OpenBSD version and errata levels across infrastructure to identify vulnerable systems
• Use memory debugging tools during testing to detect double free conditions

Monitoring Recommendations

• Enable comprehensive logging for NFS services and monitor for error conditions
• Implement network monitoring at NFS service boundaries to detect exploitation attempts
• Establish baseline NFS performance metrics to identify anomalous behavior that may indicate exploitation
• Monitor system stability metrics on OpenBSD hosts running NFS services

How to Mitigate CVE-2024-10934

Immediate Actions Required

• Apply OpenBSD errata 008 for version 7.5 or errata 021 for version 7.4 immediately
• Consider temporarily disabling NFS services on vulnerable systems if patching cannot be performed immediately
• Restrict network access to NFS services to trusted hosts only using firewall rules
• Review NFS export configurations to ensure minimum necessary access permissions

Patch Information

OpenBSD has released official patches to address this vulnerability:

• OpenBSD 7.5: Apply errata 008 NFS patch
• OpenBSD 7.4: Apply errata 021 NFS patch

The patches correct the mbuf handling to prevent double free conditions and properly initialize variables in NFS server error handling paths.

Workarounds

• Disable NFS services (nfsd and related daemons) if not required for operations
• Implement strict network segmentation to limit exposure of NFS services
• Configure host-based firewalls to restrict NFS access to specific trusted IP addresses
• Consider using alternative file sharing protocols until patches can be applied

# Disable NFS services temporarily on OpenBSD
# Stop NFS daemons
rcctl stop nfsd
rcctl stop mountd
rcctl stop portmap

# Disable NFS services from starting on boot
rcctl disable nfsd
rcctl disable mountd

# Verify NFS services are stopped
rcctl check nfsd
rcctl check mountd