CVE-2026-80580: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-80580 Overview

CVE-2026-80580 is a Linux kernel vulnerability in the framebuffer device (fbdev) subsystem. The flaw resides in show_modes() and mode_string(), which expose display mode information through sysfs. mode_string() calls snprintf(), which can return a value larger than the remaining buffer. show_modes() accumulates that return value into an offset without verifying the buffer is full. A sufficiently long modelist advances the write offset past the sysfs PAGE_SIZE buffer, causing an out-of-bounds write in kernel memory.

Critical Impact
A local authenticated user can trigger kernel memory corruption through sysfs, threatening confidentiality, integrity, and availability of the host.

Affected Products

Linux kernel fbdev subsystem prior to the fix commit d15d51fb26e830af58f3f21964f1c09c239077ea
Distributions shipping kernels containing the vulnerable mode_string() and show_modes() implementation
Systems exposing framebuffer sysfs attributes such as /sys/class/graphics/fb*/modes

Discovery Timeline

2026-08-26 - CVE-2026-80580 published to NVD
2026-08-27 - Last updated in NVD database

Technical Details for CVE-2026-80580

Vulnerability Analysis

The fbdev sysfs interface exposes the list of supported display modes through the modes attribute. The kernel builds this attribute output using mode_string(), a helper that formats one mode entry into the caller-supplied page buffer with snprintf(). snprintf() returns the number of bytes it would have written, not the bytes actually written when the buffer is truncated.

show_modes() walks the mode list and adds each mode_string() return value to an accumulator i. The loop does not check whether i has reached PAGE_SIZE. When the modelist is long enough, the offset advances past the end of the sysfs page buffer, and subsequent snprintf() calls write into memory adjacent to that buffer.

The fix, commit d15d51fb26e830af58f3f21964f1c09c239077ea, adds a size parameter to mode_string(), switches to scnprintf() so only bytes actually written are returned, and short-circuits when the offset already exceeds the buffer. show_modes() stops accumulating once the buffer is full.

Root Cause

The root cause is improper handling of the snprintf() return value, an out-of-bounds write pattern common when snprintf() is misused as if it were scnprintf(). The kernel treated the theoretical length as bytes consumed, allowing the write cursor to escape the fixed-size sysfs page.

Attack Vector

Exploitation requires local access with low privileges and no user interaction. An attacker who can influence or populate a long framebuffer modelist and then read the modes sysfs attribute triggers the overflow in kernel context. The impact ranges from denial of service through kernel panic to potential privilege escalation depending on adjacent memory layout.

No verified public proof-of-concept is currently referenced in the CVE data. See the kernel commit fixing the issue and the related change for source-level details.

Detection Methods for CVE-2026-80580

Indicators of Compromise

Unexpected kernel oops or panic messages referencing show_modes, mode_string, or fb_sysfs in dmesg and /var/log/kern.log
KASAN reports flagging out-of-bounds writes in the fbdev code path when kernel address sanitizer is enabled
Repeated reads of /sys/class/graphics/fb*/modes from unprivileged processes on multi-user systems

Detection Strategies

Audit kernel version and patch level against the fix commit d15d51fb26e830af58f3f21964f1c09c239077ea on all Linux hosts
Enable and monitor kernel hardening telemetry such as KASAN, panic_on_oops, and lockdown events in test and staging environments
Correlate process execution with reads of framebuffer sysfs attributes to surface anomalous local activity

Monitoring Recommendations

Forward kernel logs to a centralized logging platform and alert on new fbdev stack traces
Track deployment status of kernel updates across the fleet using configuration management inventory
Watch for local processes accessing /sys/class/graphics/ that are not part of expected display or session management workflows

How to Mitigate CVE-2026-80580

Immediate Actions Required

Apply the upstream kernel patch containing commit d15d51fb26e830af58f3f21964f1c09c239077ea or upgrade to a distribution kernel that includes the fix
Restrict local shell and interactive access on multi-tenant Linux systems until patched kernels are deployed
Reboot hosts after installing the patched kernel to load the fixed fbdev module

Patch Information

The fix is available in the upstream stable tree. Review the kernel commit fixing the issue and the related commit. Distribution vendors will backport these commits into their supported kernel branches.

Workarounds

Restrict read access to framebuffer sysfs attributes where operationally acceptable using udev rules or filesystem permissions
Blacklist or unload the fbdev module on servers that do not require framebuffer console support
Limit local user accounts on affected systems and enforce least-privilege access controls

bash

# Verify running kernel and check for fbdev sysfs exposure
uname -r
ls -l /sys/class/graphics/fb*/modes 2>/dev/null

# Optional: restrict read access to framebuffer mode listings
chmod 600 /sys/class/graphics/fb*/modes