CVE-2026-89878: Linux Kernel s2255 Buffer Overflow Flaw

CVE-2026-89878 Overview

CVE-2026-89878 is an out-of-bounds read vulnerability in the Linux kernel s2255 media driver. The flaw resides in s2255_probe(), which reads a 4-byte marker and version identifier from the last 8 bytes of a firmware blob. When the firmware file is shorter than 8 bytes, the fw_size - 8 calculation underflows because fw_size is unsigned, producing a very large index. The driver then accesses fw->data[fw_size - 8] and fw->data[fw_size - 4] outside the allocated buffer. The upstream fix validates the firmware size before indexing into the buffer.

Critical Impact
An undersized firmware blob loaded by the s2255 driver causes the kernel to perform an out-of-bounds read, potentially leaking kernel memory or triggering a fault during device probe.

Affected Products

Linux kernel s2255 media (Sensoray 2255) USB video capture driver
Multiple stable kernel branches referenced by the upstream fix commits
Distributions shipping the s2255drv module until the corresponding stable backport is applied

Discovery Timeline

2026-09-16 - CVE-2026-89878 published to NVD
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-89878

Vulnerability Analysis

The s2255 driver loads firmware through the standard kernel firmware API, receiving a struct firmware pointer with a data buffer and a size field. During s2255_probe(), the driver inspects the trailing bytes of that buffer to confirm a marker value and extract a version number. The two reads target fw->data[fw_size - 8] and fw->data[fw_size - 4], assuming the blob is at least 8 bytes long.

The driver did not verify that assumption. A firmware image shorter than 8 bytes causes fw_size - 8 to wrap around, since size_t is unsigned. The subsequent array access reads memory well outside the firmware allocation. Depending on adjacent memory contents, the outcome ranges from a page fault to a silent read of unrelated kernel data. This classifies as an out-of-bounds read [CWE-125] driven by an integer underflow in size arithmetic.

Root Cause

The root cause is missing input validation on attacker- or administrator-supplied firmware content. The probe path trusted the length of the firmware blob without a lower bound check before performing fixed-offset reads from the end of the buffer.

Attack Vector

Exploitation requires the ability to influence which firmware file the s2255 driver loads during device probe. That typically involves local privileged access to place a crafted file at the firmware search path, or physical attachment of a Sensoray 2255 USB device that triggers loading of a manipulated firmware image. Remote exploitation is not indicated in the advisory. The realistic impact is denial of service via kernel fault or limited kernel memory disclosure through subsequent driver behavior.

No verified proof-of-concept code is published. See the upstream commits, including Kernel Git Commit 330f293 and Kernel Git Commit 7d22185, for the exact patched logic.

Detection Methods for CVE-2026-89878

Indicators of Compromise

Kernel log entries from the s2255 driver referencing firmware load failures or unexpected probe errors during USB device enumeration.
Unexpected s2255_fw.bin or similarly named firmware files in /lib/firmware/ with sizes smaller than 8 bytes.
KASAN or slab-out-of-bounds reports pointing to s2255_probe on kernels built with memory sanitizers.

Detection Strategies

Inventory hosts loading the s2255drv module using lsmod and package management data, then correlate with kernel version to identify unpatched systems.
Monitor dmesg and journal output for firmware-related warnings tied to media subsystem probe routines.
Audit firmware directories for tampered or truncated blobs, comparing hashes against distribution-provided images.

Monitoring Recommendations

Alert on modifications to files under /lib/firmware/ originating from non-package-manager processes.
Track USB device attach events for Sensoray 2255 hardware and correlate with subsequent kernel oops or panic events.
Ingest kernel audit logs into a central analytics platform to surface repeated probe failures across the fleet.

How to Mitigate CVE-2026-89878

Immediate Actions Required

Apply the stable kernel update that includes the s2255 firmware size validation patch on all systems running the affected driver.
Restrict write access to /lib/firmware/ so that only trusted package management processes can add or modify firmware blobs.
Where the s2255 device is not required, blacklist the s2255drv module to eliminate the affected code path.

Patch Information

The fix adds a length check before indexing the trailing bytes of the firmware buffer. It is distributed across multiple stable branches. Reference commits include Kernel Git Commit 342632a, Kernel Git Commit 3e03f12, Kernel Git Commit 5626785, Kernel Git Commit 6f6a5b0, Kernel Git Commit 8eca0f8, and Kernel Git Commit ffc2741. Track your distribution's kernel security advisories for the matching backport version.

Workarounds

Unload and blacklist the s2255drv module on hosts that do not use Sensoray 2255 capture hardware.
Enforce read-only mounts or strict file integrity monitoring on the firmware directory to prevent unauthorized blob replacement.
Limit physical USB access on servers and workstations to reduce the exposure of the driver's probe path.

bash

# Blacklist the s2255 driver on systems that do not need it
echo 'blacklist s2255drv' | sudo tee /etc/modprobe.d/blacklist-s2255.conf
sudo rmmod s2255drv 2>/dev/null || true
sudo update-initramfs -u