CVE-2025-40323 fbcon: Set fb_display[i]->mode to NULL when the mode is released
A use-after-free vulnerability in the Linux kernel's framebuffer console allows local attackers to potentially execute code or escalate privileges via crafted ioctl calls.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-40323?
CVE-2025-40323 is a high-severity use after free vulnerability in Linux, published on 8 Dec 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
fbcon: Set fb_display[i]->mode to NULL when the mode is released
Recently, we discovered the following issue through syzkaller:
BUG: KASAN: slab-use-after-free in fb_mode_is_equal+0x285/0x2f0 Read of size 4 at addr ff11000001b3c69c by task syz.xxx ... Call Trace: <TASK> dump_stack_lvl+0xab/0xe0 print_address_description.constprop.0+0x2c/0x390 print_report+0xb9/0x280 kasan_report+0xb8/0xf0 fb_mode_is_equal+0x285/0x2f0 fbcon_mode_deleted+0x129/0x180 fb_set_var+0xe7f/0x11d0 do_fb_ioctl+0x6a0/0x750 fb_ioctl+0xe0/0x140 __x64_sys_ioctl+0x193/0x210 do_syscall_64+0x5f/0x9c0 entry_SYSCALL_64_after_hwframe+0x76/0x7e
Based on experimentation and analysis, during framebuffer unregistration, only the memory of fb_info->modelist is freed, without setting the corresponding fb_display[i]->mode to NULL for the freed modes. This leads to UAF issues during subsequent accesses. Here's an example of reproduction steps: 1. With /dev/fb0 already registered in the system, load a kernel module to register a new device /dev/fb1; 2. Set fb1's mode to the global fb_display[] array (via FBIOPUT_CON2FBMAP); 3. Switch console from fb to VGA (to allow normal rmmod of the ko); 4. Unload the kernel module, at this point fb1's modelist is freed, leaving a wild pointer in fb_display[]; 5. Trigger the bug via system calls through fb0 attempting to delete a mode from fb0.
Add a check in do_unregister_framebuffer(): if the mode to be freed exists in fb_display[], set the corresponding mode pointer to NULL.
Which versions are affected by CVE-2025-40323?
How do I fix CVE-2025-40323?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2025-40323 being exploited?
There is no known exploitation of CVE-2025-40323 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2025-40323, DEBIAN-CVE-2025-40323, GHSA-vv6w-x7vv-rv35, ROOT-OS-DEBIAN-11-CVE-2025-40323, ROOT-OS-DEBIAN-12-CVE-2025-40323, ROOT-OS-DEBIAN-13-CVE-2025-40323 and 3 more.