CVE-2026-98434: Linux Kernel Sysctl Range Check Vulnerability

CVE-2026-98434 Overview

CVE-2026-98434 is a Linux kernel vulnerability in the sysctl subsystem. The function proc_dointvec_ms_jiffies_minmax is missing a range check that was incorrectly removed by commit d174174c6776 ("sysctl: replace SYSCTL_INT_CONV_CUSTOM macro with functions"). Without the min/max bounds enforcement, values written through affected sysctl entries can bypass intended validation. The upstream Linux kernel maintainers have resolved the issue by restoring the range check in do_proc_int_conv_ms_jiffies_minmax.

Critical Impact
Missing range validation in a sysctl handler allows unvalidated millisecond-to-jiffies values to be written through /proc/sys entries that rely on proc_dointvec_ms_jiffies_minmax for bounds enforcement.

Affected Products

Linux kernel versions containing commit d174174c6776 prior to the fix
Distributions shipping the affected mainline/stable kernels
Systems exposing sysctl entries that use proc_dointvec_ms_jiffies_minmax

Discovery Timeline

2026-10-18 - CVE-2026-98434 published to NVD
2026-10-19 - Last updated in NVD database

Technical Details for CVE-2026-98434

Vulnerability Analysis

The Linux kernel sysctl interface exposes tunables through /proc/sys. Several of these tunables accept time values specified in milliseconds and are converted internally to jiffies. The handler proc_dointvec_ms_jiffies_minmax is intended to enforce caller-defined minimum and maximum values on these writes.

A refactoring commit, d174174c6776 ("sysctl: replace SYSCTL_INT_CONV_CUSTOM macro with functions"), replaced the custom conversion macro with equivalent functions. During that refactor, the explicit min/max range check inside do_proc_int_conv_ms_jiffies_minmax was dropped. As a result, writes through the affected handler no longer enforce the bounds declared by the sysctl table entry.

The fix restores the range check in do_proc_int_conv_ms_jiffies_minmax so that out-of-range values are rejected before conversion. See the upstream fixes in Kernel Git Commit 1c23dba and Kernel Git Commit 8243187.

Root Cause

The root cause is a regression introduced during a code cleanup. The replacement function do_proc_int_conv_ms_jiffies_minmax omitted the min/max comparison that the original SYSCTL_INT_CONV_CUSTOM macro performed, leaving values unchecked before they were committed to the target variable.

Attack Vector

Exploitation requires the ability to write to the affected /proc/sys entries. On a default Linux system this generally requires CAP_SYS_ADMIN or root privileges, so the practical risk is limited to local, privileged contexts, container escapes with excessive capabilities, or configuration tools that pass unvalidated input to sysctl. The impact is a configuration-integrity issue: subsystems reading the tunable may receive values outside their intended operating range, with downstream behavior dependent on each specific sysctl consumer.

No verified public exploit code is available. Technical details are documented in the referenced upstream commits.

Detection Methods for CVE-2026-98434

Indicators of Compromise

Unexpected values present in /proc/sys entries backed by proc_dointvec_ms_jiffies_minmax that fall outside the documented min/max range
Audit records (auditd) showing writes to affected sysctl paths from non-administrative workflows
Kernel subsystem behavior anomalies tied to timer or timeout tunables after sysctl writes

Detection Strategies

Compare running kernel version against the fixed commits 1c23dba51dbf7e86c7c2e617c964be1411329cb4 and 82431877d837a6c2593efd8e66ba28b35a220ca4 and distribution backports
Enable Linux audit rules on /proc/sys writes and alert on values outside documented ranges
Use configuration-drift tooling to detect sysctl values that deviate from a known baseline

Monitoring Recommendations

Centralize auditd and kernel log data in a SIEM for correlation of sysctl writes with privilege use
Monitor container runtimes for workloads granted CAP_SYS_ADMIN that could reach /proc/sys
Track package and kernel update status across the fleet to confirm the fix is deployed

How to Mitigate CVE-2026-98434

Immediate Actions Required

Update to a Linux kernel build that includes the upstream fix commits 1c23dba and 8243187
Apply the latest stable kernel updates from your distribution vendor
Review which sysctl entries in your environment use proc_dointvec_ms_jiffies_minmax and verify current values are within expected ranges

Patch Information

The fix restores the min/max range check inside do_proc_int_conv_ms_jiffies_minmax. Patched sources are available in Kernel Git Commit 1c23dba and Kernel Git Commit 8243187. Consult your distribution's security tracker for backported stable kernel packages.

Workarounds

Restrict write access to /proc/sys by limiting CAP_SYS_ADMIN and avoiding privileged containers
Enforce sysctl values through managed configuration (for example, /etc/sysctl.d/) and reconcile drift on a schedule
Add audit rules to log and alert on writes to affected sysctl paths until the kernel is patched

bash

# Configuration example: audit writes to /proc/sys and pin sysctl values
# 1) Audit all writes under /proc/sys
auditctl -w /proc/sys/ -p wa -k sysctl_write

# 2) Pin a known-good value for an affected tunable (example path)
echo 'kernel.example_ms_tunable = 1000' | sudo tee /etc/sysctl.d/99-cve-2026-98434.conf
sudo sysctl --system

# 3) Verify the running kernel includes the fix commits
uname -r