CVE-2026-12234 in Zephyr
Summary
by MITRE • 08/12/2026
The userspace syscall verifiers z_vrfy_zsock_sendmsg() and z_vrfy_zsock_recvmsg() in subsys/net/lib/sockets/sockets.c snapshot the caller-supplied struct net_msghdr into a kernel-side copy with k_usermode_from_copy(), but then re-read the still-live user struct for subsequent decisions. The kernel iovec shadow buffer is sized from one read of msg->msg_iovlen, while the population loop is bounded by a second, live read of the same field.
Because msg points into ordinary user memory, a cooperating second thread in the same memory domain can inflate msg->msg_iovlen in the window between the sizing read and the loop test (a classic double-fetch / TOCTOU). The population loop then iterates past the number of net_iovec slots actually allocated, writing attacker-influenced iov_base/iov_len values beyond the end of the kernel-heap shadow buffer. The recvmsg verifier has the same defect on both its inbound and result write-back loops.
The code is reachable from an unprivileged user thread whenever CONFIG_USERSPACE is enabled and the zsock_sendmsg/zsock_recvmsg syscalls are available. A successful race corrupts kernel-managed heap memory across the user-to-kernel privilege boundary, yielding a local privilege-escalation primitive or, at minimum, a kernel-fault denial of service. The fix copies the header once and derives every size, bound, and gate from the snapshot, copying each iovec entry atomically so its base and length can no longer be raced apart.
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Analysis
by VulDB Data Team • 08/12/2026
The vulnerability exists within the userspace syscall verifiers z_vrfy_zsock_sendmsg() and z_vrfy_zsock_recvmsg() in the Zephyr kernel's networking subsystem. These functions handle socket communication operations by verifying user-supplied parameters before processing them in kernel space. The implementation employs a snapshotting mechanism using k_usermode_from_copy() to copy the user-provided struct net_msghdr into kernel memory, but this approach introduces a critical timing window where subsequent reads of the original user data can be manipulated by concurrent threads.
The technical flaw manifests as a double-fetch vulnerability or time-of-check to time-of-use (TOCTOU) race condition. During the initialization phase, the kernel reads msg->msg_iovlen from user memory to determine the size of the internal iovec shadow buffer. However, during the actual population loop, the same field is read again from live user memory to control iteration bounds. This creates an exploitable window where a cooperating thread can modify msg->msg_iovlen between these two reads, causing the kernel to write beyond the allocated buffer boundaries.
The operational impact of this vulnerability extends across multiple attack vectors within the kernel's memory management system. When the race condition succeeds, it results in heap memory corruption that crosses the user-to-kernel privilege boundary, creating a local privilege escalation primitive. The corruption occurs because the kernel allocates a fixed-size shadow buffer based on the first read value but then iterates using a potentially inflated second value. This vulnerability affects both sendmsg and recvmsg operations, with recvmsg being particularly dangerous as it suffers from the same defect in both its inbound processing and result write-back loops.
The vulnerability directly maps to CWE-367 which addresses Time-of-Check to Time-of-Use race conditions, and aligns with ATT&CK technique T1068 for Local Privilege Escalation through kernel exploits. The exploit requires only an unprivileged user thread and the presence of CONFIG_USERSPACE configuration option, making it particularly concerning as it can be triggered from standard user processes without special privileges. The memory corruption could lead to either arbitrary code execution with kernel privileges or a denial of service condition that crashes the kernel.
The recommended fix implements a comprehensive snapshot strategy that eliminates all external dependencies during critical operations. Rather than performing multiple reads of the same user data, the solution copies the entire header structure once and derives all subsequent size calculations, loop bounds, and validation checks from this single snapshot. Additionally, each iovec entry is copied atomically to prevent race conditions between base and length fields, ensuring that no attacker-influenced values can be used to corrupt kernel memory beyond intended boundaries. This approach follows the principle of minimizing data exposure during critical sections and provides a robust defense against similar timing-based vulnerabilities in kernel code.
This vulnerability represents a classic example of how seemingly benign parameter validation can create serious security implications when concurrent access patterns are not properly considered. The fix demonstrates proper defensive programming techniques that should be applied to all kernel subsystems handling user-supplied data structures, particularly those operating across privilege boundaries where memory corruption can lead to complete system compromise.