CVE-2025-40304

What it actually does

Missing bounds checks in the fbdev bit_putcs function allow writes past framebuffer memory boundaries when rendering text near screen edges, resulting in out-of-bounds writes due to mismatched buffer allocation and character count parameters passed to aligned and unaligned rendering routines.

Why it matters to the business

Unpatched systems risk kernel memory corruption that can cause crashes, denial of service, or local privilege escalation on Linux servers and desktops, especially older distributions still in extended support; delayed patching increases exposure window on widely deployed kernels.

How an attack unfolds

An attacker with local user access supplies crafted text rendering operations that position output near framebuffer edges, bypassing the absent Y-position, height, and X-position checks; this triggers buffer-size mismatches that write attacker-controlled data beyond allocated regions, enabling memory corruption.

How to fix it

Upgrade the Linux kernel packages on all affected distributions to the versions containing the bounds-checking fix (for example, linux 4.4.0-142.168 on Ubuntu 16.04 LTS, linux-aws 4.4.0-1075.85 on Ubuntu 16.04 LTS, and the corresponding fixed releases listed for each 14.04 and 16.04 variant). Reboot after installation and verify the running kernel version.

How to confirm the fix worked

Confirm the installed kernel version matches or exceeds one of the listed fixed releases for the distribution; optionally build and inspect the fbdev source to verify presence of the Y-position early return, height clipping, X-position break, and character-count update logic.

Typical attack path

1. Obtain local shell or process execution on the target system
2. Interact with the framebuffer device to trigger text rendering near screen boundaries
3. Supply parameters that cause the character count to exceed the clipped width
4. Trigger the mismatched call to bit_putcs_aligned or bit_putcs_unaligned
5. Achieve out-of-bounds write for memory corruption or code execution

How to detect it

Monitor kernel logs (dmesg, /var/log/kern.log) for framebuffer-related oops or panic messages near text-rendering paths; correlate with unusual local process activity that interacts with /dev/fb*; no public exploitation artifacts are known, so focus on post-crash memory corruption indicators.

What your security team should do

• Prioritize kernel updates on all internet-facing and multi-user systems within the next patch cycle
• Inventory running kernels across the fleet to identify any remaining 14.04/16.04 LTS instances
• Enable automatic security updates for kernel packages where operationally feasible
• Review local access controls and framebuffer device permissions as a compensating control