CVE-2026-68340 Overview
CVE-2026-68340 is a boundary validation flaw in the Linux kernel's hwmon OCC (On-Chip Controller) driver. The OCC poll response parser walks a counted list of sensor data blocks using the static backing-array capacity as its parse boundary. A truncated transport response, where only data_length bytes are valid, causes the parser to consume a block header or block extent beyond the current response buffer. This results in an out-of-bounds read of adjacent kernel memory.
Critical Impact
A malformed OCC poll response can trigger out-of-bounds reads in kernel memory, leading to kernel information disclosure or denial of service on affected Linux systems.
Affected Products
- Linux kernel hwmon subsystem, occ driver
- Systems using IBM POWER OCC hardware monitoring via the affected driver
- Distributions shipping vulnerable Linux kernel versions prior to the referenced stable commits
Discovery Timeline
- 2026-08-10 - CVE-2026-68340 published to NVD
- 2026-08-13 - Last updated in NVD database
Technical Details for CVE-2026-68340
Vulnerability Analysis
The vulnerability resides in the OCC poll response parser inside the Linux kernel hwmon/occ driver. The parser iterates a counted list of sensor data blocks returned by the OCC transport layer. It relies on the static capacity of the backing array to bound parsing operations rather than the actual length of the response.
When the transport delivers a truncated response, only data_length bytes of the buffer contain valid data. The parser can then read a block header or block payload that extends past data_length, dereferencing adjacent kernel memory that was not populated by the transport.
The fix uses data_length as the parent boundary, validates the fixed poll header and each subsequent block header before dereferencing, and validates the complete block extent before advancing the cursor. Parsed sensor metadata is held locally until the full response passes validation, and malformed-response errors propagate before publishing the OCC as active.
Root Cause
The root cause is improper input validation combined with an out-of-bounds read condition. The parser trusted a structural size derived from a fixed array capacity rather than the runtime-attested data_length field, allowing header and payload accesses beyond the valid response region.
Attack Vector
Exploitation requires local access with the ability to influence or supply an OCC poll response, such as from a compromised or emulated OCC transport path. No authentication or user interaction is required. Successful triggering yields kernel memory disclosure or a driver-side crash affecting system availability.
No public exploit code is available for CVE-2026-68340. The Linux kernel commits referenced in the kernel.org stable tree contain the corrective patch series.
Detection Methods for CVE-2026-68340
Indicators of Compromise
- Kernel log entries from the hwmon/occ driver reporting malformed poll responses or parse failures after patch application
- Unexpected KASAN or slab out-of-bounds warnings referencing occ_poll or sensor block parsing routines
- OCC driver failing to publish as active following a poll response error
Detection Strategies
- Compare running kernel versions against the fixed commits listed in the kernel.org stable references to identify unpatched hosts
- Monitor dmesg and journalctl -k output for occ driver errors indicating truncated or malformed responses
- Audit systems using OCC hardware monitoring (typically IBM POWER-based servers) for exposure
Monitoring Recommendations
- Enable kernel address sanitizer (KASAN) or slab debugging in test environments to surface out-of-bounds reads in the OCC parser
- Forward kernel logs to a centralized SIEM and alert on driver initialization failures on hwmon devices
- Track kernel package versions across the fleet to confirm patch deployment coverage
How to Mitigate CVE-2026-68340
Immediate Actions Required
- Apply the Linux kernel update containing the OCC poll response validation fix as soon as vendor packages are available
- Inventory all systems running the hwmon/occ driver, focusing on POWER-architecture hosts using OCC telemetry
- Restrict local access to trusted administrators until patched kernels are deployed
Patch Information
The fix is available in the Linux kernel stable tree. Refer to the following commits: commit 112525534ab5, commit 538d862cc0db, commit 54cb78eceb4e, commit 70e76e700fc6, and commit b042e538e98b. Rebuild or update to a kernel that includes these changes.
Workarounds
- Unload or blacklist the occ_hwmon kernel module on systems that do not require OCC telemetry until the patched kernel is installed
- Limit local shell and driver-interaction privileges to reduce the local attack surface
- Isolate management interfaces used to communicate with OCC transport paths from untrusted users
# Blacklist the OCC hwmon module until patched
echo "blacklist occ_hwmon" | sudo tee /etc/modprobe.d/blacklist-occ.conf
sudo update-initramfs -u
# Verify installed kernel version against fixed stable release
uname -r
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
