Cis Ubuntu1804 V220 1 1 10

Disable USB Storage

Profile

Level 1 - Server / Level 2 - Workstation, Assessment: Automated

Description

USB storage provides a means to transfer and store files insuring persistence and availability of the files independent of network connection status. Its popularity and utility has led to USB-based malware being a simple and common means for network infiltration and a first step to establishing a persistent threat within a networked environment.

Rationale

Restricting USB access on the system will decrease the physical attack surface for a device and diminish the possible vectors to introduce malware.

Impact

An alternative snolution to disabling the usb-storage module may be found in USBGuard.

Use of USB storage devices to transfer work files may be disrupted.

Audit Procedure

Command Line

Run the following script to verify the usb-storage module is disabled. If the module is available in the running kernel:

• An entry including /bin/true or /bin/false exists in a file within the /etc/modprobe.d/ directory
• The module is deny listed in a file within the /etc/modprobe.d/ directory
• The module is not loaded in the kernel

#!/usr/bin/env bash
{
    l_output="" l_output2="" l_output3="" l_dl=""
    l_mname="usb-storage"
    l_mtype="drivers"
    l_searchloc="/lib/modprobe.d/*.conf /usr/local/lib/modprobe.d/*.conf /run/modprobe.d/*.conf /etc/modprobe.d/*.conf"
    l_mpath="/lib/modules/**/kernel/$l_mtype"
    l_mpname="$(tr '-' ' ' <<< "$l_mname")"
    l_mndir="$(tr '-' '/' <<< "$l_mname")"
    module_loadable_chk()
    {
        l_loadable="$(modprobe -n -v "$l_mname")"
        [ "$(wc -l <<< "$l_loadable")" -gt "1" ] && l_loadable="$(grep -P -- "(^\h*install|\b$l_mname)\b" <<< "$l_loadable")"
        if grep -Pq -- '^\h*install \/bin\/(true|false)' <<< "$l_loadable"; then
            l_output="$l_output\n - module: \"$l_mname\" is not loadable: \"$l_loadable\""
        else
            l_output2="$l_output2\n - module: \"$l_mname\" is loadable: \"$l_loadable\""
        fi
    }
    module_loaded_chk()
    {
        if ! lsmod | grep "$l_mname" > /dev/null 2>&1; then
            l_output="$l_output\n - module: \"$l_mname\" is not loaded"
        else
            l_output2="$l_output2\n - module: \"$l_mname\" is loaded"
        fi
    }
    module_deny_chk()
    {
        l_dl="y"
        if modprobe --showconfig | grep -Pq -- '^\h*blacklist\h+'"$l_mpname"'\b'; then
            l_output="$l_output\n - module: \"$l_mname\" is deny listed in: \"$(grep -Pls -- "^\h*blacklist\h+$l_mname\b" $l_searchloc)\""
        else
            l_output2="$l_output2\n - module: \"$l_mname\" is not deny listed"
        fi
    }
    for l_mdir in $l_mpath; do
        if [ -d "$l_mdir/$l_mndir" ] && [ -n "$(ls -A $l_mdir/$l_mndir)" ]; then
            l_output3="$l_output3\n  - \"$l_mdir\""
            [ "$l_dl" != "y" ] && module_deny_chk
            if [ "$l_mdir" = "/lib/modules/$(uname -r)/kernel/$l_mtype" ]; then
                module_loadable_chk
                module_loaded_chk
            fi
        else
            l_output="$l_output\n - module: \"$l_mname\" doesn't exist in \"$l_mdir\""
        fi
    done
    [ -n "$l_output3" ] && echo -e "\n\n -- INFO --\n - module: \"$l_mname\" exists in:$l_output3"
    if [ -z "$l_output2" ]; then
        echo -e "\n- Audit Result:\n  ** PASS **\n$l_output\n"
    else
        echo -e "\n- Audit Result:\n  ** FAIL **\n - Reason(s) for audit failure:\n$l_output2\n"
        [ -n "$l_output" ] && echo -e "\n- Correctly set:\n$l_output\n"
    fi
}

Expected Result

• module: "usb-storage" is not loadable
• module: "usb-storage" is not loaded
• module: "usb-storage" is deny listed

Remediation

Command Line

If the module is available in the running kernel:

• Create a file with install usb-storage /bin/false in the /etc/modprobe.d/ directory
• Create a file with blacklist usb-storage in the /etc/modprobe.d/ directory
• Unload usb-storage from the kernel

If available in ANY installed kernel:

• Create a file with blacklist usb-storage in the /etc/modprobe.d/ directory

If the kernel module is not available on the system or pre-compiled into the kernel:

• No remediation is necessary

Default Value

Not disabled by default.

References

1. NIST SP 800-53 Rev. 5: CM-7

CIS Controls

v8 - 4.8 Uninstall or Disable Unnecessary Services on Enterprise Assets and Software (IG 2, IG 3) v7 - 9.2 Ensure Only Approved Ports, Protocols and Services Are Running (IG 2, IG 3)

MITRE ATT&CK Mappings: T1005, T1005.000 (TA0005) - M1050