CVE-2026-84543 in macOS
Summary
by MITRE • 09/15/2026
An out-of-bounds access issue was addressed with improved bounds checking. This issue is fixed in macOS Golden Gate 27, macOS Sequoia 15.8, macOS Tahoe 26.7. Connecting to a malicious SMB server may cause unexpected system termination or corrupt kernel memory.
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Analysis
by VulDB Data Team • 09/15/2026
The vulnerability described involves an out-of-bounds access flaw within the Server Message Block protocol implementation in Apple's operating systems, specifically affecting versions prior to macOS Golden Gate 27, macOS Sequoia 15.8, and macOS Tahoe 26.7. This type of memory corruption issue typically arises when the software fails to properly validate input data before writing it to a buffer or accessing array elements. In this context, the flaw is located in the SMB client component, which handles communication with remote file servers. When an attacker controls the server side of the connection, they can craft specific network packets containing malformed headers or payload structures that exceed the expected boundaries of allocated memory regions within the kernel space.
From a technical perspective, out-of-bounds access represents a critical failure in input validation and boundary checking mechanisms. The operating system's SMB stack likely allocates fixed-size buffers for processing incoming protocol messages without sufficiently verifying that the length fields provided by the client match the actual data size or do not exceed pre-allocated limits. When such invalid lengths are processed, the kernel attempts to read from or write to memory addresses outside the intended scope. This can lead to overwriting adjacent memory structures, corrupting critical system data, or triggering illegal memory access exceptions that force an immediate termination of the affected process or the entire operating system instance.
The operational impact of this vulnerability is severe due to its potential for remote exploitation and kernel-level execution. An attacker positioned on a network segment can connect to a vulnerable macOS device using a malicious SMB server configured to send specially crafted packets. The primary consequence observed is unexpected system termination, which manifests as a crash or reboot, resulting in a denial-of-service condition that disrupts availability for users relying on the affected systems. Furthermore, if the memory corruption allows for arbitrary code execution rather than just a crash, it could lead to privilege escalation, allowing an attacker to gain root-level access and compromise the integrity of the entire system. This aligns with Common Weakness Enumeration CWE-125, which describes out-of-bounds read vulnerabilities that can be leveraged for information disclosure or further exploitation steps.
In terms of threat modeling, this vulnerability maps directly to MITRE ATT&CK technique T1496, Remote File Share Intrusion, where adversaries use legitimate file sharing protocols like SMB to move laterally within a network or execute malicious code. The specific mechanism falls under the broader category of memory corruption vulnerabilities often associated with CWE-787, out-of-bounds write, if the exploit involves writing data beyond buffer limits. Security analysts should treat this as a high-severity issue requiring immediate attention because SMB is frequently exposed to untrusted networks in enterprise environments, and kernel-level exploits bypass many user-space security controls.
Mitigation strategies primarily involve applying the provided software updates that introduce improved bounds checking logic into the SMB client implementation. These patches ensure that all incoming data lengths are validated against buffer capacities before processing occurs, thereby preventing the out-of-bounds access condition. In addition to patching, organizations should enforce strict network segmentation policies to limit exposure of macOS devices to untrusted networks or external internet-facing SMB services where possible. Implementing deep packet inspection solutions capable of detecting anomalous SMB traffic patterns can also provide an additional layer of defense against exploitation attempts while updates are being deployed across the infrastructure. Regular auditing of SMB configurations and disabling unnecessary file sharing features further reduces the attack surface associated with this protocol stack.