CVE-2024-43102: FreeBSD Use-After-Free Vulnerability

CVE-2024-43102 Overview

CVE-2024-43102 is a critical Use-After-Free vulnerability affecting the FreeBSD operating system's user-level mutex (UMTX) subsystem. The flaw exists in how the kernel handles concurrent removals of anonymous shared memory mappings through the UMTX_SHM_DESTROY sub-request of UMTX_OP_SHM. When exploited, this race condition causes the reference count of the memory mapping object to be decremented excessively, resulting in premature memory deallocation.

A malicious actor exercising the UMTX_SHM_DESTROY sub-request in parallel can trigger a kernel panic or enable further Use-After-Free attacks. The potential impact includes arbitrary code execution with kernel privileges and Capsicum sandbox escape, making this a particularly severe vulnerability for FreeBSD systems.

Critical Impact

This vulnerability enables kernel panic, arbitrary code execution, and Capsicum sandbox escape through race condition exploitation in the FreeBSD UMTX subsystem.

Affected Products

• FreeBSD 13.3 (all patch levels through p5)
• FreeBSD 14.0 (all versions including beta5, rc3, rc4-p1, and patch levels through p9)
• FreeBSD 14.1 (all versions through p3)

Discovery Timeline

• September 5, 2024 - CVE-2024-43102 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2024-43102

Vulnerability Analysis

The vulnerability resides in the FreeBSD kernel's UMTX subsystem, which provides user-level mutex support for threading and inter-process synchronization. The specific weakness occurs when multiple threads or processes simultaneously invoke the UMTX_SHM_DESTROY sub-request to remove anonymous shared memory mappings.

The kernel maintains reference counts on shared memory objects to track active users and determine when memory can be safely freed. Under concurrent destruction requests, a race condition allows the reference count to be decremented multiple times for what should be a single operation. This leads to the reference count reaching zero prematurely while other parts of the kernel still hold references to the object.

Once the memory is freed while still being referenced elsewhere, subsequent accesses result in Use-After-Free conditions. Attackers can potentially manipulate the freed memory to achieve arbitrary code execution within the kernel context or escape the Capsicum capability-based sandbox.

Root Cause

The root cause is a classic Time-of-Check Time-of-Use (TOCTOU) race condition in the reference counting mechanism for anonymous shared memory objects. The UMTX_SHM_DESTROY operation lacks proper synchronization primitives to prevent concurrent access during the destruction sequence. This allows multiple execution contexts to pass the validity check and proceed to decrement the reference count, causing a double-free or Use-After-Free scenario when the count underflows or reaches zero prematurely.

Attack Vector

The vulnerability is exploitable over the network with low attack complexity and requires no privileges or user interaction. An attacker can craft a malicious program that spawns multiple threads, each invoking the UMTX_OP_SHM system call with the UMTX_SHM_DESTROY sub-request targeting the same shared memory object. By timing these requests carefully, the attacker can trigger the race condition.

The exploitation pathway involves creating an anonymous shared memory mapping, then racing multiple UMTX_SHM_DESTROY requests against it. Upon successful exploitation, the attacker can achieve a kernel panic for denial of service, or with more sophisticated techniques, can reclaim the freed memory with attacker-controlled data to hijack kernel execution flow. The Capsicum sandbox escape is particularly concerning for systems relying on this mechanism for privilege separation and sandboxing of untrusted code.

Detection Methods for CVE-2024-43102

Indicators of Compromise

• Unexpected kernel panics with stack traces referencing the UMTX subsystem or shared memory operations
• System logs showing repeated crashes with memory corruption indicators near umtx_shm_destroy or related functions
• Evidence of processes creating and rapidly destroying shared memory mappings in an unusual pattern
• Core dumps indicating Use-After-Free conditions in kernel memory allocations

Detection Strategies

• Monitor for abnormal system call patterns involving UMTX_OP_SHM with UMTX_SHM_DESTROY sub-requests at high frequency
• Implement kernel-level auditing to track shared memory lifecycle operations and detect potential race condition exploitation attempts
• Deploy endpoint detection solutions capable of identifying exploitation signatures associated with UMTX race conditions
• Enable FreeBSD kernel debugging options such as INVARIANTS and WITNESS to catch use-after-free conditions during testing

Monitoring Recommendations

• Configure system monitoring to alert on unexpected kernel panics or system reboots
• Review audit logs for processes making unusual numbers of shared memory system calls
• Monitor for new processes attempting sandbox escape or unexpected privilege escalation after UMTX operations
• Implement SentinelOne Singularity Platform for real-time kernel-level threat detection and automated response to exploitation attempts

How to Mitigate CVE-2024-43102

Immediate Actions Required

• Apply the security patches referenced in FreeBSD Security Advisory SA-24:14 immediately
• Upgrade to the latest patched versions: FreeBSD 14.1-p4 or later, FreeBSD 13.3-p6 or later
• Review systems for signs of compromise, including unexpected crashes or sandbox escape attempts
• Limit access to untrusted code execution on affected FreeBSD systems until patches are applied

Patch Information

FreeBSD has released security patches addressing this vulnerability as documented in FreeBSD Security Advisory SA-24:14. The fix introduces proper synchronization to prevent concurrent destruction of shared memory objects from causing reference count corruption. Additionally, NetApp has published an advisory (NTAP-20240916-0001) for affected NetApp products running FreeBSD.

System administrators should apply the binary patch or upgrade to a corrected release:

• FreeBSD 14.1 users should update to 14.1-p4 or later
• FreeBSD 14.0 users should update to 14.0-p10 or later
• FreeBSD 13.3 users should update to 13.3-p6 or later

Workarounds

• Restrict the ability of untrusted users and processes to execute arbitrary code on FreeBSD systems
• Implement additional access controls to limit which processes can use shared memory operations
• Consider running high-risk workloads in isolated environments until patches can be applied
• Monitor systems closely for exploitation attempts while implementing patches during maintenance windows

# Apply FreeBSD security patches
freebsd-update fetch
freebsd-update install

# Verify patch installation and current version
freebsd-version -k

# Reboot to load the patched kernel
shutdown -r now