Out-of-bounds write in Linux kernel - CVE-2026-63799

Vulnerability identifier: #VU138854

CSH Severity: Low

CVSS v4: 5.8 [CVSS:4.0/AV:L/AC:L/AT:P/PR:L/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N]

CVE-ID: CVE-2026-63799

CWE-ID: CWE-787

Exploitation vector: Local access

Exploit availability: No public exploit available

The vulnerability allows a local user to corrupt kernel memory.

The vulnerability exists due to an out-of-bounds write in mm_cid_schedout() via mm_cid_fixup_cpus_to_tasks() when handling a task with MM_CID_UNSET in the per-CPU to per-task fixup path. A local user can trigger task state transitions such as fork() or execve() during this window to corrupt kernel memory.

The issue occurs only in per-CPU CID mode during the window before the task's next schedule-in assigns a real CID, and the resulting corruption is a deterministic single-bit clear at a fixed offset rather than an arbitrary write.

Linux kernel
Ubuntu
linux (Ubuntu package)
linux-azure (Ubuntu package)
linux-gcp-7.0 (Ubuntu package)
linux-oracle-7.0 (Ubuntu package)
linux-oem-7.0 (Ubuntu package)
linux-azure-7.0 (Ubuntu package)

Install security update from vendor's repository.

Linux kernel - update to 7.0 rc3
linux (Ubuntu package) - addressed in versions 7.0.0-31.31, 7.0.0-31.31.1, 7.0.0-31.31~24.04.1, 7.0.0-1006.7, 7.0.0-1011.11~24.04.1, 7.0.0-1012.12, 7.0.0-1012.12~24.04.1
linux-azure (Ubuntu package) - addressed in versions 7.0.0-1011.11, 7.0.0-1014.14
linux-gcp-7.0 (Ubuntu package) - update to 7.0.0-1011.11~24.04.1
linux-oracle-7.0 (Ubuntu package) - update to 7.0.0-1011.11~24.04.1
linux-oem-7.0 (Ubuntu package) - update to 7.0.0-1013.13
linux-azure-7.0 (Ubuntu package) - update to 7.0.0-1014.14~24.04.1

• https://git.kernel.org/stable/c/8d32856fb72ba976d9c87ba405fd17e80419934c
• https://git.kernel.org/stable/c/de3ab9bd3133899efb92e4cd05ba4203e58fc0a3

• Out-of-bounds write in Linux kernel sched
• Ubuntu update for linux-oem-7.0
• Ubuntu update for linux
• Ubuntu update for linux-gcp-7.0
• Ubuntu update for linux-oracle-7.0
• Ubuntu update for linux-azure
• Ubuntu update for linux-azure-7.0