CVE-2026-31748: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-31748 Overview

CVE-2026-31748 is an out-of-bounds read vulnerability [CWE-787] in the Linux kernel comedi subsystem, specifically within the me_daq driver. The flaw resides in me2600_xilinx_download(), which loads firmware obtained through request_firmware(). The function reads a data stream length from the first 4 bytes of the firmware file and then reads that many bytes starting at offset 16. While the code verifies the firmware is at least 16 bytes long, it does not validate that the buffer contains enough data to satisfy the declared file_length, allowing a source buffer overrun.

Critical Impact

A local attacker with the ability to supply or influence firmware loaded by the me_daq driver can trigger an out-of-bounds memory read in kernel space, leading to information disclosure, kernel memory corruption, or denial of service.

Affected Products

• Linux Kernel (multiple stable branches prior to fixes referenced in mainline)
• Linux Kernel 7.0-rc1 through 7.0-rc6
• Systems using the comedime_daq driver with the Meilhaus ME-2600i data acquisition hardware

Discovery Timeline

• 2026-05-01 - CVE-2026-31748 published to NVD
• 2026-05-07 - Last updated in NVD database

Technical Details for CVE-2026-31748

Vulnerability Analysis

The vulnerability resides in me2600_xilinx_download(), the routine responsible for downloading FPGA bitstream firmware to Meilhaus ME-2600i acquisition cards. The function consumes a firmware blob retrieved by request_firmware() and parses a 16-byte header. The first 4 bytes are interpreted as a little-endian length field (file_length) describing how many bytes of bitstream data follow at offset 16.

The existing length check ensures the firmware blob is at least 16 bytes, which is sufficient only to read the header itself. The driver then dereferences and copies file_length bytes from the firmware buffer without confirming that 16 + file_length is within the actual buffer size returned by the firmware loader. A malformed or truncated firmware file with an inflated file_length value causes the driver to read past the end of the kernel-allocated firmware buffer.

The upstream fix introduces an explicit bounds check that compares the declared data stream length against the available buffer size. If the firmware is too short to contain the declared payload, the driver logs an error and returns -EINVAL instead of proceeding with the unsafe copy.

Root Cause

The root cause is missing input validation on attacker-influenced length metadata embedded in a firmware file. The driver trusted the file format and used file_length as a copy size without cross-checking it against the firmware buffer length, producing a classic header-declared-length versus actual-buffer-size mismatch.

Attack Vector

Exploitation requires local access with privileges sufficient to influence firmware loading for the me_daq driver. This typically means writing a crafted file to the firmware search path (such as /lib/firmware) or otherwise causing the kernel firmware loader to deliver a malformed blob when the driver probes Meilhaus hardware. The CVSS vector indicates a local attack with low complexity and low privileges, with high impact to confidentiality, integrity, and availability.

For technical details on the patched code path, see the upstream commits, including kernel commit f3f8ec00cfb8 and kernel commit cc797d4821c7.

Detection Methods for CVE-2026-31748

Indicators of Compromise

• Unexpected kernel log entries referencing me_daq, me2600_xilinx_download, or firmware load failures returning -EINVAL.
• Presence of unsigned or modified firmware files in /lib/firmware matching names consumed by the me_daq driver.
• Kernel oops, slab-out-of-bounds, or KASAN reports referencing the comedime_daq module.

Detection Strategies

• Inventory hosts that load the comedi and me_daq kernel modules using lsmod and package management telemetry.
• Monitor dmesg and journald for firmware loader errors associated with Meilhaus ME-2600i hardware.
• Use vulnerability scanners that consume NVD CPE data to identify Linux kernels in the affected ranges, including 7.0-rc1 through 7.0-rc6.

Monitoring Recommendations

• Audit file integrity on /lib/firmware and alert on creation or modification of comedi-related firmware blobs.
• Track loading of legacy or rarely used kernel modules through auditd rules on init_module and finit_module.
• Correlate kernel crash telemetry with module load events to surface exploitation attempts targeting driver-level parsing flaws.

How to Mitigate CVE-2026-31748

Immediate Actions Required

• Apply the latest stable Linux kernel update from your distribution that includes the comedi: me_daq firmware length validation fix.
• If the me_daq driver is not required, blacklist the module to remove the attack surface entirely.
• Restrict write access to /lib/firmware and any custom firmware search paths to root only and validate package provenance.

Patch Information

The vulnerability is resolved by upstream commits that add a bounds check ensuring the supplied firmware is large enough to contain both the 16-byte header and the declared data stream before copying. Fixes are referenced in the kernel stable tree, including commit 1bf8761eb59e, commit 2fc25a4c2e05, commit 9f39fa07259e, commit a47ae40339c1, commit c16ac4e173a0, and commit c8c607a77aab. Consume backports through your distribution's standard kernel update channel.

Workarounds

• Blacklist the me_daq and comedi modules on systems that do not use Meilhaus ME-2600i hardware.
• Enforce strict permissions and integrity monitoring on the firmware directory to block planting of crafted firmware files.
• Limit local and console access to trusted administrators, since exploitation requires local privileges to influence firmware loading.

# Configuration example: blacklist the vulnerable driver until patched
echo 'blacklist me_daq' | sudo tee /etc/modprobe.d/blacklist-me_daq.conf
echo 'install me_daq /bin/true' | sudo tee -a /etc/modprobe.d/blacklist-me_daq.conf
sudo update-initramfs -u
# Verify the module is not loaded after reboot
lsmod | grep -E 'me_daq|comedi'