CVE-2026-98753: Linux Kernel TTY Race Condition Vulnerability

CVE-2026-98753 Overview

CVE-2026-98753 is a Linux kernel vulnerability in the TTY subsystem. The flaw resides in the tty_port activation logic, where the TTY_IO_ERROR flag is not restored after an activation failure. This leaves a window in which drivers mishandle an ioctl-vs-hangup race condition.

The missing flag restoration allows I/O attempts against a port that has been removed or powered down. Exploitation of the race can trigger a kernel panic through unclocked hardware accesses or NULL-pointer dereferences when the port is deregistered.

Critical Impact
Local attackers or faulty userspace can trigger a kernel panic by racing a TTY ioctl with hangup and open on a powered-down or deregistered port.

Affected Products

Linux kernel (TTY core subsystem, drivers/tty/tty_port.c)
Stable kernel branches referenced in the upstream fix commits
Distributions shipping affected kernel versions prior to the backported patches

Discovery Timeline

2026-10-18 - CVE-2026-98753 published to the National Vulnerability Database (NVD)
2026-10-21 - Last updated in NVD database

Technical Details for CVE-2026-98753

Vulnerability Analysis

The Linux kernel TTY layer uses the TTY_IO_ERROR flag on a tty_struct to short-circuit I/O operations when the underlying port is unavailable. During port open, the flag is cleared before invoking the driver-specific activation routine. If activation fails, the fix ensures the flag is restored so subsequent I/O paths bail out safely.

Without the restoration, the TTY remains in a state that appears writable while the backing hardware is gone or unpowered. Concurrent paths such as a hangup followed by an ioctl or re-open can then reach driver code that assumes the port is active. The result is dereferencing freed or NULL pointers, or performing memory-mapped I/O against hardware whose clock has been disabled.

The commit note explicitly identifies kernel panic as the observed impact. The flag is reset in tty_port_close(), but relying on close is insufficient because the race window opens on failed activation. This is a kernel race condition and NULL-pointer dereference class issue.

Root Cause

The root cause is incomplete state rollback in the TTY port open path. When tty_port_open() or an equivalent activation routine fails, the code did not revert TTY_IO_ERROR to its prior set state. Any thread holding or acquiring a reference to the TTY could then invoke ioctl or read/write handlers that assumed the port was activated.

Attack Vector

The attack vector is local. Triggering the condition requires concurrent userspace operations against a TTY device whose underlying hardware can disappear, including removable serial adapters, hot-unplug paths, or runtime-power-managed UARTs. An attacker races an ioctl on an open file descriptor with hangup and re-open to reach the unsafe driver path. Successful races produce a denial of service through kernel panic.

No verified public exploit is available. See the upstream patch commits for implementation details: Kernel Code Change 1, Kernel Code Change 2, Kernel Code Change 3, Kernel Code Change 4, and Kernel Code Change 5.

Detection Methods for CVE-2026-98753

Indicators of Compromise

Unexpected kernel panics referencing TTY driver functions, tty_port, or uart_ symbols in the stack trace.
NULL-pointer dereference oops messages originating in serial or TTY driver code after device removal or runtime power-down.
Kernel logs showing TTY activation failures followed closely by ioctl or hangup events on the same device.

Detection Strategies

Collect and parse dmesg and /var/log/kern.log for panic signatures involving the TTY subsystem after hot-unplug or suspend-resume cycles.
Audit installed kernel versions across endpoints and servers against the patched stable commits listed in the upstream references.
Correlate crash dumps from kdump or pstore to identify repeated TTY-related faults suggesting active race exploitation or latent bugs.

Monitoring Recommendations

Forward kernel log telemetry to a central analytics platform and alert on panic patterns in TTY and serial driver code paths.
Track package inventory for linux-image and vendor kernel builds to confirm patch deployment status.
Monitor removable serial device events alongside subsequent kernel errors to flag environments where exploitation preconditions exist.

How to Mitigate CVE-2026-98753

Immediate Actions Required

Apply the latest stable kernel update from your Linux distribution vendor containing the tty_port activation fix.
Reboot affected systems after kernel upgrade to activate the patched image.
Prioritize patching on systems that expose serial, USB-serial, or other hot-pluggable TTY devices to untrusted local users.

Patch Information

The fix restores the TTY_IO_ERROR flag when the TTY port activation callback returns an error. Upstream commits containing the fix are published in the Linux stable tree. Reference the mainline and stable patches at Kernel Code Change 1, Kernel Code Change 2, Kernel Code Change 3, Kernel Code Change 4, and Kernel Code Change 5. Confirm your distribution has backported the fix to the kernel series you run.

Workarounds

Restrict local user access to TTY device nodes using strict udev permissions and group membership on affected hosts.
Avoid runtime power management for UART devices on systems where kernel upgrade is not yet possible.
Limit hot-plug of USB serial adapters on multi-user systems until patched kernels are deployed.

bash

# Verify running kernel and check for distribution update
uname -r
# Debian/Ubuntu
sudo apt update && sudo apt upgrade linux-image-$(uname -r | cut -d- -f3-)
# RHEL/Fedora
sudo dnf update kernel
# Reboot to activate the patched kernel
sudo systemctl reboot