CVE-2026-12234 Overview
CVE-2026-12234 is a double-fetch time-of-check to time-of-use (TOCTOU) vulnerability in the Zephyr RTOS userspace syscall verifiers z_vrfy_zsock_sendmsg() and z_vrfy_zsock_recvmsg() located in subsys/net/lib/sockets/sockets.c. The verifiers snapshot the caller-supplied struct net_msghdr with k_usermode_from_copy(), but then re-read the still-live user structure for subsequent decisions. A cooperating second thread in the same memory domain can inflate msg->msg_iovlen between the sizing read and the loop test, causing the population loop to write past the allocated net_iovec shadow buffer. The flaw is classified as [CWE-367] Time-of-check Time-of-use Race Condition.
Critical Impact
A local unprivileged user thread can corrupt kernel-heap memory across the user-to-kernel privilege boundary, yielding a local privilege escalation primitive or a kernel-fault denial of service.
Affected Products
- Zephyr RTOS builds with CONFIG_USERSPACE enabled
- Systems exposing the zsock_sendmsg and zsock_recvmsg syscalls to unprivileged threads
- Zephyr networking stack component subsys/net/lib/sockets/sockets.c prior to the fix commit 2e0f9cf05be61056ed1a94dab51b832fbad27e2c
Discovery Timeline
- 2026-08-12 - CVE-2026-12234 published to NVD
- 2026-08-12 - Last updated in NVD database
Technical Details for CVE-2026-12234
Vulnerability Analysis
The verifiers z_vrfy_zsock_sendmsg() and z_vrfy_zsock_recvmsg() are designed to validate user-supplied struct net_msghdr arguments before the kernel acts on them. They correctly copy the header into kernel memory using k_usermode_from_copy() to prevent tampering. However, subsequent code paths dereference the still-live user pointer msg rather than the trusted kernel copy. The kernel iovec shadow buffer is sized based on one read of msg->msg_iovlen, while the population loop is bounded by a second, independent read of the same user-controlled field. The recvmsg verifier repeats the defect on both its inbound and result write-back loops.
Root Cause
The root cause is a classic double-fetch pattern. Because msg points into ordinary user memory, its contents remain mutable throughout the verifier's execution. The verifier trusts the field's stability across two separate reads instead of deriving every size, bound, and gate from a single atomic snapshot. This violates the fundamental syscall verifier contract that user input must be validated once and never re-read.
Attack Vector
Exploitation requires two cooperating threads in the same memory domain. Thread A invokes zsock_sendmsg or zsock_recvmsg with a small msg_iovlen value that passes the initial sizing read. Thread B, running concurrently, inflates msg->msg_iovlen in the window between the sizing read and the loop bound check. The population loop then iterates past the allocated net_iovec slots, writing attacker-influenced iov_base and iov_len values beyond the end of the kernel-heap shadow buffer. This corrupts adjacent heap metadata or object contents, providing a local privilege escalation primitive at minimum.
The vulnerability manifests only when CONFIG_USERSPACE is enabled and the affected socket syscalls are exposed to unprivileged threads. See the GitHub Security Advisory GHSA-fcp3-vrr2-xfjv for technical details.
Detection Methods for CVE-2026-12234
Indicators of Compromise
- Unexpected kernel faults or panics originating from network socket syscall paths on Zephyr-based devices
- Heap corruption diagnostics referencing net_iovec allocations or the sockets subsystem
- Unprivileged user threads issuing rapid, paired zsock_sendmsg or zsock_recvmsg calls sharing overlapping msg_hdr memory
Detection Strategies
- Audit Zephyr build configurations for CONFIG_USERSPACE=y combined with exposed socket syscalls
- Enable kernel heap sanitizers or guard pages during pre-production testing to surface out-of-bounds writes from iovec population loops
- Review firmware images for the vulnerable version of subsys/net/lib/sockets/sockets.c predating commit 2e0f9cf05be61056ed1a94dab51b832fbad27e2c
Monitoring Recommendations
- Instrument affected devices to capture and forward kernel oops or fault logs to a centralized telemetry pipeline
- Monitor for repeated abnormal terminations of user threads that invoke socket syscalls
- Track firmware inventory to confirm all Zephyr deployments run patched builds
How to Mitigate CVE-2026-12234
Immediate Actions Required
- Apply the upstream fix from commit 2e0f9cf05be61056ed1a94dab51b832fbad27e2c and rebuild affected Zephyr firmware
- Rebuild and redeploy any downstream products that include the Zephyr networking stack with CONFIG_USERSPACE enabled
- Restrict which user threads may invoke zsock_sendmsg and zsock_recvmsg until patched firmware is deployed
Patch Information
The fix copies the net_msghdr header once and derives every size, bound, and gate from the snapshot. Each iovec entry is copied atomically so its base and length can no longer be raced apart. Review the GitHub Commit 2e0f9cf for the corrected implementation.
Workarounds
- Disable CONFIG_USERSPACE if the deployment does not require user/kernel privilege separation
- Remove or gate access to the zsock_sendmsg and zsock_recvmsg syscalls in application memory domains
- Isolate untrusted application threads into single-thread memory domains to prevent a cooperating attacker thread from mutating the header mid-verification
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
