In the Linux kernel, the following vulnerability has been resolved:

USB: serial: quatech2: fix null-ptr-deref in qt2_process_read_urb()

This patch addresses a null-ptr-deref in qt2_process_read_urb() due to
an incorrect bounds check in the following:

   if (newport > serial->num_ports) {
           dev_err(&port->dev,
                   "%s - port change to invalid port: %i\n",
                   __func__, newport);
           break;
   }

The condition doesn't account for the valid range of the serial->port
buffer, which is from 0 to serial->num_ports - 1. When newport is equal
to serial->num_ports, the assignment of "port" in the
following code is out-of-bounds and NULL:

   serial_priv->current_port = newport;
   port = serial->port[serial_priv->current_port];

The fix checks if newport is greater than or equal to serial->num_ports
indicating it is out-of-bounds.

References

• https://nvd.nist.gov/vuln/detail/CVE-2025-21689
• https://git.kernel.org/stable/c/4b9b41fabcd38990f69ef0cee9c631d954a2b530
• https://git.kernel.org/stable/c/575a5adf48b06a2980c9eeffedf699ed5534fade
• https://git.kernel.org/stable/c/6068dcff7f19e9fa6fa23ee03453ad6a40fa4efe
• https://git.kernel.org/stable/c/6377838560c03b36e1153a42ef727533def9b68f
• https://git.kernel.org/stable/c/8542b33622571f54dfc2a267fce378b6e3840b8b
• https://git.kernel.org/stable/c/94770cf7c5124f0268d481886829dc2beecc4507
• https://git.kernel.org/stable/c/f371471708c7d997f763b0e70565026eb67cc470
• https://git.kernel.org/stable/c/fa4c7472469d97c4707698b4c0e098f8cfc2bf22