CVE-2023-53747: 
Linux Kernel vulnerability analysis and mitigation

CVE-2023-53747 is a use-after-free (UAF) vulnerability in the Linux kernel's virtual console screen driver (vc_screen), specifically within the vcs_write() function in drivers/tty/vt/vc_screen.c. After console_unlock() is called inside the vcs_write() while loop, the vc_data struct can be freed by vc_port_destruct() on another thread; the stale pointer is then used when vcs_size() is called, triggering a UAF. The vulnerability was disclosed on December 8, 2025, and affects the Linux kernel. It carries a CVSS v3.1 base score of 5.5 (Medium), requiring local access and low privileges with no user interaction (Red Hat Advisory, Red Hat Bugzilla).

The root cause is a use-after-free condition (CWE-416) in vcs_write() (drivers/tty/vt/vc_screen.c). When the function releases the console lock via console_unlock() mid-loop, a concurrent thread can deallocate the vc_data structure through vc_port_destruct() → kfree(). Upon re-acquiring the lock, vcs_write() continues to use the now-freed vc_data pointer when calling vcs_size(), resulting in a read of freed memory. The bug was originally discovered and reported by Syzkaller, which produced a KASAN report showing a slab-use-after-free read of 4 bytes at the stale address (Red Hat Bugzilla). The fix requires reloading the vc_data pointer from the tty structure after each re-acquisition of console_lock() within the loop (Red Hat Advisory).

Successful exploitation of this vulnerability can cause a kernel denial of service (system crash or hang) due to invalid memory access in kernel space. Because the vulnerability is triggered in a kernel driver with no confidentiality or integrity impact, the primary risk is availability — an attacker with local, low-privileged access can crash the system by racing vcs_write() against a virtual terminal deallocation operation (Red Hat Bugzilla, Red Hat Advisory).

No public exploit code or in-the-wild exploitation has been reported for CVE-2023-53747. The vulnerability was originally surfaced by the Syzkaller kernel fuzzer, and no threat actor attribution or CISA KEV catalog listing is known. The EPSS score is approximately 0.024% (0.000240), indicating a very low probability of exploitation in the near term (Red Hat Advisory). Detection plugins are available via Nessus (plugin IDs 277787, 298924) and Qualys (ID 761056).

1. Gain local access: Obtain a low-privileged local user account on a vulnerable Linux system with access to virtual console devices (e.g., /dev/vcs*).
2. Open a virtual console: Open a virtual terminal device to trigger allocation of a vc_data structure via vc_allocate() → con_install() → tty_init_dev().
3. Initiate concurrent write: In one thread, begin a write operation to the virtual console screen device (/dev/vcs*), causing vcs_write() to enter its loop and call console_unlock().
4. Race with deallocation: In a second concurrent thread, issue a VT_DISALLOCATE ioctl (or equivalent) to trigger vt_disallocate_all() → vc_port_destruct() → kfree(), freeing the vc_data struct while the first thread holds a stale pointer.
5. Trigger UAF: The first thread re-acquires console_lock() and calls vcs_size() using the freed vc_data pointer, causing a kernel crash (denial of service) (Red Hat Bugzilla).

• Logs: Kernel logs (dmesg / /var/log/kern.log) showing BUG: KASAN: slab-use-after-free in vcs_size with a call trace through vcs_write, vfs_write.
• Logs: KASAN reports referencing drivers/tty/vt/vc_screen.c:215 and drivers/tty/vt/vc_screen.c:664.
• Process: Unusual processes repeatedly opening and closing virtual terminal devices (/dev/tty*, /dev/vcs*) or issuing VT_DISALLOCATE ioctls in rapid succession.
• System: Unexpected kernel panics or system reboots on systems with virtual console activity (Red Hat Bugzilla).

The fix involves reloading the vc_data pointer after each re-acquisition of console_lock() within the vcs_write() loop, preventing use of a stale pointer. Upstream kernel patches have been committed to stable branches (git.kernel.org commits: 0deff678, 1de42e76, 8fb9ea65, 11dddfbb). Users should apply the latest stable Linux kernel updates provided by their distribution vendor (e.g., Red Hat, SUSE, Ubuntu) that include this fix. As a workaround, restricting local user access to virtual console devices (/dev/vcs*) via file permissions or access control policies can reduce exposure (Red Hat Advisory, Red Hat Bugzilla).