CVE-2026-46259: Linux Kernel procfs Use-After-Free – Local Privilege Escalation

Technical summary

CVE-2026-46259 is a use-after-free vulnerability in the Linux kernel's `do_task_stat()` function within procfs. When userspace reads `/proc/[pid]/stat`, the kernel retrieves `task->real_parent` without RCU (Read-Copy-Update) locking. A race condition occurs when: (1) CPU 0 reads the real_parent pointer unsafely, (2) CPU 1 concurrently calls `release_task()`, which schedules the parent task struct for deferred destruction via `call_rcu()`, and (3) CPU 0 later attempts to dereference the now-freed pointer within `task_tgid_nr_ns()`. The fix replaces the unsafe sequence with `task_ppid_nr_ns()`, which wraps the real_parent access in proper RCU read-side critical sections, ensuring the pointer remains valid during the operation.

Business impact

This vulnerability exposes Linux systems to local privilege escalation and denial of service. A non-root user can trigger a kernel crash by reading `/proc/[pid]/stat` at the precise moment when a parent process is being reaped, disrupting services and potentially enabling further attacks. Additionally, attackers may leak sensitive kernel memory addresses or data structures, undermining address-space layout randomization (ASLR) and facilitating subsequent exploits. For organizations running multi-tenant Linux systems or shared hosting platforms, this represents a significant containment failure between user domains.

Affected systems

This vulnerability affects the Linux kernel across all supported distributions. Any Linux system running an unpatched kernel with the vulnerable code path is at risk. The vulnerability is exploitable by any local user on the system; no special capabilities are required beyond standard user permissions. Containerized environments and virtual machines using Linux kernels remain vulnerable unless the host kernel is patched.

Exploitability

The vulnerability is highly exploitable. It requires only local access and standard user privileges (AC:L, PR:L, AV:L). The race condition is reliably triggered by repeatedly reading `/proc/[pid]/stat` while the target process is undergoing reparenting or termination. Proof-of-concept tooling can be trivially developed by any competent developer. However, reliable exploitation may require timing tuning specific to system load and CPU architecture, making large-scale automated exploitation somewhat less predictable than traditional buffer overflows.

Remediation

Apply the Linux kernel patch that replaces the unsafe real_parent access in `do_task_stat()` with the RCU-protected `task_ppid_nr_ns()` function. Verify the fix against your kernel version's official upstream repository or distribution advisory. Systems should prioritize patching given the local-privilege requirement and the straightforward nature of triggering the race. Until patching is feasible, restrict `/proc` access via AppArmor or SELinux policies if operational requirements permit.

Patch guidance

Check your Linux distribution's security advisory for kernel updates addressing CVE-2026-46259. The patch modifies only the procfs stat reporting code; verify the commit hash against the upstream Linux kernel repository before applying. Test patches in non-production environments first to ensure compatibility with custom kernel configurations or out-of-tree modules. Kernel updates typically require a reboot; schedule patching during a maintenance window.

Detection guidance

Monitor system logs for kernel UAF panics mentioning the procfs or `do_task_stat` code path. Deploy kernel-level intrusion detection rules to flag repeated suspicious reads of `/proc/[pid]/stat` by unprivileged users targeting processes scheduled for termination. Observe for unexpected `call_rcu` warnings or memory corruption messages in kernel logs. On systems with audit capabilities, log all `/proc/stat` accesses by non-privileged users and correlate spikes with process termination events. Behavioral anomalies (e.g., address-space information leaks or frequent kernel crashes) may indicate exploitation attempts.

Why prioritize this

HIGH severity (CVSS 7.8) due to the combination of local privilege escalation potential, denial of service capability, and information disclosure. The attack surface is universal—any system with a Linux kernel—and the barrier to exploitation is minimal. While the KEV has not been added to the CISA Known Exploited Vulnerabilities catalog, the simplicity of the race condition and the ubiquity of `/proc/stat` access make eventual weaponization likely. This should receive immediate attention in patch cycles.

Risk score, explained

The CVSS 3.1 score of 7.8 reflects: (1) local attack vector (AV:L)—no network access required; (2) low attack complexity (AC:L)—triggering the race is straightforward; (3) low privileges required (PR:L)—any local user can attempt exploitation; (4) no user interaction; (5) high impact across confidentiality, integrity, and availability (C:H, I:H, A:H)—allowing memory disclosure, kernel state corruption, and denial of service. The score does not account for the prevalence of Linux in both enterprise and edge environments, which elevates practical risk.

Frequently asked questions