During a concurrent fork scenario on the Linux kernel, the scheduler registers a newly created task as an MMCID user before the task becomes visible in the thread and global task lists. Because the per‑CPU threshold handling runs before this visibility, the scheduler may fail to fix the task’s CID, causing a stall when it later attempts to acquire a CID. This classic race condition is identified as CWE‑821 and additionally labeled NVD‑CWE‑noinfo, which can freeze the system. The flaw is analogous to the exit() symmetry issue where the task is removed as a CID user before the task is removed from the thread and task lists.

The flaw is present in the Linux kernel itself; any kernel version that contains the old MMCID handling logic is potentially vulnerable. No specific distribution or version numbers are provided, but the patch that corrects the issue is available in recent kernel releases that reorder sched_mm_cid_fork to execute after the new task is made visible.

Risk remains significant because a failure to acquire a CID can halt the scheduler and freeze the machine. The CVSS score is 5.5, indicating medium severity. The EPSS score is below 1% and the vulnerability is not listed in KEV, suggesting a low exploitation likelihood. Nevertheless, the denial‑of‑service potential justifies prompt remediation.