CVE-2026-57842 in NetBSD
Summary
by MITRE • 09/11/2026
NetBSD contains a use-after-free and double-free vulnerability in msg_recv_copyin() within the COMPAT_NETBSD32 compatibility layer due to a missing return statement before the cleanup label on the success path. Any local user able to execute a 32-bit binary on a 64-bit NetBSD system can trigger a kernel panic or memory corruption by calling recvmsg() with msg_iovlen between 9 and IOV_MAX, causing the kernel to access a freed iovec buffer and subsequently free the same allocation a second time.
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Analysis
by VulDB Data Team • 09/11/2026
The vulnerability identified in NetBSD represents a critical flaw within the COMPAT_NETBSD32 compatibility layer, specifically residing in the msg_recv_copyin() function. This component is responsible for handling system calls that allow 64-bit kernels to execute and interact with legacy 32-bit binaries. The core technical defect stems from an improper control flow structure where a necessary return statement is omitted before reaching a cleanup label on the success path of execution. In standard software engineering practices, particularly within kernel development, ensuring correct exit paths for all conditional branches is paramount to maintaining memory integrity. When this specific branch condition is met during normal operation, the function fails to terminate immediately as intended and instead proceeds to execute code that was meant only for error handling or cleanup scenarios.
This logical error directly leads to a use-after-free vulnerability combined with a double-free condition. During the processing of recvmsg() system calls where the msg_iovlen parameter is set between 9 and IOV_MAX, the kernel allocates an iovec buffer to handle input/output vector data. Due to the missing return statement, after successfully copying data into user space, the execution flow continues to a cleanup routine that attempts to free this same iovec buffer. Because the function does not exit immediately upon success, it proceeds to deallocate memory that may still be referenced or is in an inconsistent state depending on subsequent operations. More critically, if the error path logic also triggers under certain race conditions or specific kernel states, the same allocation might be freed a second time. Double-free vulnerabilities are particularly dangerous as they allow attackers to manipulate heap metadata structures, potentially leading to arbitrary code execution with kernel privileges.
The operational impact of this vulnerability is severe for any local user capable of executing 32-bit binaries on a 64-bit NetBSD system. By carefully crafting syscalls that trigger the recvmsg() function with specific vector lengths, an attacker can induce memory corruption within the kernel address space. This corruption typically manifests as a kernel panic, resulting in a denial of service where the entire system becomes unresponsive and requires a reboot. However, given the nature of double-free exploits, there is also a significant risk that sophisticated attackers could leverage this flaw to achieve arbitrary read or write primitives. Such capabilities would allow for privilege escalation from an unprivileged user account to root level access, compromising the confidentiality, integrity, and availability of all data and services hosted on the affected system.
From a classification perspective, this vulnerability aligns with CWE-416 Use After Free and CWE-415 Double Free, which are categorized as memory corruption issues within the Common Weakness Enumeration framework. In terms of tactical behavior, it relates to ATT&CK technique T1059 Command and Scripting Interpreter if leveraged for initial execution, but more accurately maps to privilege escalation vectors such as T1068 Exploitation for Privilege Escalation when an attacker utilizes the memory corruption to gain higher-level access. The vulnerability highlights the risks associated with compatibility layers that bridge different architecture instruction sets, where subtle logic errors in system call translation can have profound security implications.
Mitigation strategies must focus on both immediate patching and long-term architectural reviews. System administrators should apply vendor-provided patches as soon as they become available to correct the control flow within msg_recv_copyin(). Until patched, restricting the execution of 32-bit binaries for non-essential users can reduce the attack surface. Furthermore, developers involved in maintaining compatibility layers must enforce strict code review processes that verify all exit paths return correctly before reaching cleanup labels. Implementing static analysis tools configured to detect missing returns and double-free patterns during the development lifecycle is essential to prevent similar defects from being introduced into other parts of the kernel or related subsystems.