falcon-linux-uninstall.sh

#!/bin/bash

print_usage() {
    cat <<EOF

Usage: $0 [-h|--help]

Uninstalls the CrowdStrike Falcon Sensor from Linux operating systems.
Version: $VERSION

This script recognizes the following environmental variables:

Authentication:
    - FALCON_CLIENT_ID                  (default: unset)
        Your CrowdStrike Falcon API client ID.

    - FALCON_CLIENT_SECRET              (default: unset)
        Your CrowdStrike Falcon API client secret.

    - FALCON_ACCESS_TOKEN               (default: unset)
        Your CrowdStrike Falcon API access token.
        If used, FALCON_CLOUD must also be set.

    - FALCON_CLOUD                      (default: unset)
        The cloud region where your CrowdStrike Falcon instance is hosted.
        Required if using FALCON_ACCESS_TOKEN.
        Accepted values are ['us-1', 'us-2', 'eu-1', 'us-gov-1'].

Other Options:
    - FALCON_REMOVE_HOST                (default: unset)
        Determines whether the host should be removed from the Falcon console after uninstalling the sensor.
        Requires API Authentication.
        NOTE: It is recommended to use Host Retention Policies in the Falcon console instead.
        Accepted values are ['true', 'false'].

    - GET_ACCESS_TOKEN                  (default: unset)
        Prints an access token and exits.
        Requires FALCON_CLIENT_ID and FALCON_CLIENT_SECRET.
        Accepted values are ['true', 'false'].

    - FALCON_APH                        (default: unset)
        The proxy host for the sensor to use when communicating with CrowdStrike.

    - FALCON_APP                        (default: unset)
        The proxy port for the sensor to use when communicating with CrowdStrike.

This script recognizes the following argument:
    -h, --help
        Print this help message and exit.

EOF
}

VERSION="1.7.2"

# If -h or --help is passed, print the usage and exit
if [ "$1" = "-h" ] || [ "$1" = "--help" ]; then
    print_usage
    exit 0
fi

main() {
    if [ "$GET_ACCESS_TOKEN" = "true" ]; then
        get_oauth_token
        echo "$cs_falcon_oauth_token"
        exit 0
    fi

    # Check if Falcon sensor is installed
    cs_sensor_installed
    echo -n 'Removing Falcon Sensor  ... '
    cs_sensor_remove
    echo '[ Ok ]'
    if [ "${FALCON_REMOVE_HOST}" = "true" ]; then
        echo -n 'Removing host from console ... '
        get_oauth_token
        cs_remove_host_from_console
        echo '[ Ok ]'
    fi
    echo 'Falcon Sensor removed successfully.'
}

cs_sensor_remove() {
    remove_package() {
        pkg="$1"

        if type dnf >/dev/null 2>&1; then
            dnf remove -q -y "$pkg" || rpm -e --nodeps "$pkg"
        elif type yum >/dev/null 2>&1; then
            yum remove -q -y "$pkg" || rpm -e --nodeps "$pkg"
        elif type zypper >/dev/null 2>&1; then
            zypper --quiet remove -y "$pkg" || rpm -e --nodeps "$pkg"
        elif type apt >/dev/null 2>&1; then
            DEBIAN_FRONTEND=noninteractive apt purge -y "$pkg" >/dev/null 2>&1
        else
            rpm -e --nodeps "$pkg"
        fi
    }

    remove_package "falcon-sensor"
}

cs_remove_host_from_console() {
    if [ -z "$aid" ]; then
        echo 'Unable to find AID. Skipping host removal from console.'
    else
        payload="{\"ids\": [\"$aid\"]}"
        url="https://$(cs_cloud)/devices/entities/devices-actions/v2?action_name=hide_host"

        curl_command -X "POST" -H "Content-Type: application/json" -d "$payload" "$url" >/dev/null

        handle_curl_error $?
    fi
}

cs_cloud() {
    case "${cs_falcon_cloud}" in
        us-1) echo "api.crowdstrike.com" ;;
        us-2) echo "api.us-2.crowdstrike.com" ;;
        eu-1) echo "api.eu-1.crowdstrike.com" ;;
        us-gov-1) echo "api.laggar.gcw.crowdstrike.com" ;;
        *) die "Unrecognized Falcon Cloud: ${cs_falcon_cloud}" ;;
    esac
}

cs_sensor_installed() {
    if ! test -f /opt/CrowdStrike/falconctl; then
        echo "Falcon sensor is already uninstalled." && exit 0
    fi
    # Get AID if FALCON_REMOVE_HOST is set to true and sensor is installed
    if [ "${FALCON_REMOVE_HOST}" = "true" ]; then
        get_aid
    fi
}

old_curl=$(
    if ! command -v curl >/dev/null 2>&1; then
        die "The 'curl' command is missing. Please install it before continuing. Aborting..."
    fi

    version=$(curl --version | head -n 1 | awk '{ print $2 }')
    minimum="7.55"

    # Check if the version is less than the minimum
    if printf "%s\n" "$version" "$minimum" | sort -V -C; then
        echo 0
    else
        echo 1
    fi
)

curl_command() {
    # Dash does not support arrays, so we have to pass the args as separate arguments
    set -- "$@"

    if [ "$old_curl" -eq 0 ]; then
        curl -s -x "$proxy" -L -H "Authorization: Bearer ${cs_falcon_oauth_token}" "$@"
    else
        echo "Authorization: Bearer ${cs_falcon_oauth_token}" | curl -s -x "$proxy" -L -H @- "$@"
    fi
}

handle_curl_error() {
    if [ "$1" = "28" ]; then
        err_msg="Operation timed out (exit code 28)."
        if [ -n "$proxy" ]; then
            err_msg="$err_msg A proxy was used to communicate ($proxy). Please check your proxy settings."
        fi
        die "$err_msg"
    fi

    if [ "$1" = "5" ]; then
        err_msg="Couldn't resolve proxy (exit code 5). The address ($proxy) of the given proxy host could not be resolved. Please check your proxy settings."
        die "$err_msg"
    fi

    if [ "$1" = "7" ]; then
        err_msg="Failed to connect to host (exit code 7). Host found, but unable to open connection with host."
        if [ -n "$proxy" ]; then
            err_msg="$err_msg A proxy was used to communicate ($proxy). Please check your proxy settings."
        fi
        die "$err_msg"
    fi
}

json_value() {
    KEY=$1
    num=$2
    awk -F"[,:}]" '{for(i=1;i<=NF;i++){if($i~/'"$KEY"'\042/){print $(i+1)}}}' | tr -d '"' | sed -n "${num}p"
}

die() {
    echo "Fatal error: $*" >&2
    exit 1
}

aws_ssm_parameter() {
    local param_name="$1"

    hmac_sha256() {
        key="$1"
        data="$2"
        echo -n "$data" | openssl dgst -sha256 -mac HMAC -macopt "$key" | sed 's/^.* //'
    }

    token=$(curl -X PUT "http://169.254.169.254/latest/api/token" -H "X-aws-ec2-metadata-token-ttl-seconds: 21600")
    api_endpoint="AmazonSSM.GetParameters"
    iam_role="$(curl -s -H "X-aws-ec2-metadata-token: $token" http://169.254.169.254/latest/meta-data/iam/security-credentials/)"
    aws_my_region="$(curl -s -H "X-aws-ec2-metadata-token: $token" http://169.254.169.254/latest/meta-data/placement/availability-zone | sed s/.$//)"
    _security_credentials="$(curl -s -H "X-aws-ec2-metadata-token: $token" http://169.254.169.254/latest/meta-data/iam/security-credentials/"$iam_role")"
    access_key_id="$(echo "$_security_credentials" | grep AccessKeyId | sed -e 's/  "AccessKeyId" : "//' -e 's/",$//')"
    access_key_secret="$(echo "$_security_credentials" | grep SecretAccessKey | sed -e 's/  "SecretAccessKey" : "//' -e 's/",$//')"
    security_token="$(echo "$_security_credentials" | grep Token | sed -e 's/  "Token" : "//' -e 's/",$//')"
    datetime=$(date -u +"%Y%m%dT%H%M%SZ")
    date=$(date -u +"%Y%m%d")
    request_data='{"Names":["'"${param_name}"'"],"WithDecryption":"true"}'
    request_data_dgst=$(echo -n "$request_data" | openssl dgst -sha256 | awk -F' ' '{print $2}')
    request_dgst=$(
        cat <<EOF | head -c -1 | openssl dgst -sha256 | awk -F' ' '{print $2}'
POST
/

content-type:application/x-amz-json-1.1
host:ssm.$aws_my_region.amazonaws.com
x-amz-date:$datetime
x-amz-security-token:$security_token
x-amz-target:$api_endpoint

content-type;host;x-amz-date;x-amz-security-token;x-amz-target
$request_data_dgst
EOF
    )
    dateKey=$(hmac_sha256 key:"AWS4$access_key_secret" "$date")
    dateRegionKey=$(hmac_sha256 "hexkey:$dateKey" "$aws_my_region")
    dateRegionServiceKey=$(hmac_sha256 "hexkey:$dateRegionKey" ssm)
    hex_key=$(hmac_sha256 "hexkey:$dateRegionServiceKey" "aws4_request")

    signature=$(
        cat <<EOF | head -c -1 | openssl dgst -sha256 -mac HMAC -macopt "hexkey:$hex_key" | awk -F' ' '{print $2}'
AWS4-HMAC-SHA256
$datetime
$date/$aws_my_region/ssm/aws4_request
$request_dgst
EOF
    )

    response=$(
        curl -s "https://ssm.$aws_my_region.amazonaws.com/" \
            -x "$proxy" \
            -H "Authorization: AWS4-HMAC-SHA256 \
            Credential=$access_key_id/$date/$aws_my_region/ssm/aws4_request, \
            SignedHeaders=content-type;host;x-amz-date;x-amz-security-token;x-amz-target, \
            Signature=$signature" \
            -H "x-amz-security-token: $security_token" \
            -H "x-amz-target: $api_endpoint" \
            -H "content-type: application/x-amz-json-1.1" \
            -d "$request_data" \
            -H "x-amz-date: $datetime"
    )
    handle_curl_error $?
    if ! echo "$response" | grep -q '^.*"InvalidParameters":\[\].*$'; then
        die "Unexpected response from AWS SSM Parameter Store: $response"
    elif ! echo "$response" | grep -q '^.*'"${param_name}"'.*$'; then
        die "Unexpected response from AWS SSM Parameter Store: $response"
    fi
    echo "$response"
}

check_aws_instance() {
    local aws_instance

    # Check if running on EC2 hypervisor
    if [ -f /sys/hypervisor/uuid ] && grep -qi ec2 /sys/hypervisor/uuid; then
        aws_instance=true
    # Check if DMI board asset tag matches EC2 instance pattern
    elif [ -f /sys/devices/virtual/dmi/id/board_asset_tag ] && grep -q '^i-[a-z0-9]*$' /sys/devices/virtual/dmi/id/board_asset_tag; then
        aws_instance=true
    # Check if EC2 instance identity document is accessible
    else
        curl_output="$(curl -s --connect-timeout 5 http://169.254.169.254/latest/dynamic/instance-identity/)"
        if [ -n "$curl_output" ] && ! echo "$curl_output" | grep --silent -i 'not.*found'; then
            aws_instance=true
        fi
    fi

    echo "$aws_instance"
}

get_falcon_credentials() {
    if [ -z "$FALCON_ACCESS_TOKEN" ]; then
        aws_instance=$(check_aws_instance)
        cs_falcon_client_id=$(
            if [ -n "$FALCON_CLIENT_ID" ]; then
                echo "$FALCON_CLIENT_ID"
            elif [ -n "$aws_instance" ]; then
                aws_ssm_parameter "FALCON_CLIENT_ID" | json_value Value 1
            else
                die "Missing FALCON_CLIENT_ID environment variable. Please provide your OAuth2 API Client ID for authentication with CrowdStrike Falcon platform. Establishing and retrieving OAuth2 API credentials can be performed at https://falcon.crowdstrike.com/support/api-clients-and-keys."
            fi
        )

        cs_falcon_client_secret=$(
            if [ -n "$FALCON_CLIENT_SECRET" ]; then
                echo "$FALCON_CLIENT_SECRET"
            elif [ -n "$aws_instance" ]; then
                aws_ssm_parameter "FALCON_CLIENT_SECRET" | json_value Value 1
            else
                die "Missing FALCON_CLIENT_SECRET environment variable. Please provide your OAuth2 API Client Secret for authentication with CrowdStrike Falcon platform. Establishing and retrieving OAuth2 API credentials can be performed at https://falcon.crowdstrike.com/support/api-clients-and-keys."
            fi
        )
    else
        if [ -z "$FALCON_CLOUD" ]; then
            die "If setting the FALCON_ACCESS_TOKEN manually, you must also specify the FALCON_CLOUD"
        fi
    fi
}

get_oauth_token() {
    # Get credentials first
    get_falcon_credentials

    cs_falcon_oauth_token=$(
        if [ -n "$FALCON_ACCESS_TOKEN" ]; then
            token=$FALCON_ACCESS_TOKEN
        else
            token_result=$(echo "client_id=$cs_falcon_client_id&client_secret=$cs_falcon_client_secret" |
                curl -X POST -s -x "$proxy" -L "https://$(cs_cloud)/oauth2/token" \
                    -H 'Content-Type: application/x-www-form-urlencoded; charset=utf-8' \
                    -H "User-Agent: crowdstrike-falcon-scripts/$VERSION" \
                    --dump-header "${response_headers}" \
                    --data @-)

            handle_curl_error $?

            token=$(echo "$token_result" | json_value "access_token" | sed 's/ *$//g' | sed 's/^ *//g')
            if [ -z "$token" ]; then
                die "Unable to obtain CrowdStrike Falcon OAuth Token. Double check your credentials and/or ensure you set the correct cloud region."
            fi
        fi
        echo "$token"
    )

    if [ -z "$FALCON_ACCESS_TOKEN" ]; then
        region_hint=$(grep -i ^x-cs-region: "$response_headers" | head -n 1 | tr '[:upper:]' '[:lower:]' | tr -d '\r' | sed 's/^x-cs-region: //g')

        if [ -z "${FALCON_CLOUD}" ]; then
            if [ -z "${region_hint}" ]; then
                die "Unable to obtain region hint from CrowdStrike Falcon OAuth API, Please provide FALCON_CLOUD environment variable as an override."
            fi
            cs_falcon_cloud="${region_hint}"
        else
            if [ "x${FALCON_CLOUD}" != "x${region_hint}" ]; then
                echo "WARNING: FALCON_CLOUD='${FALCON_CLOUD}' environment variable specified while credentials only exists in '${region_hint}'" >&2
            fi
        fi
    fi

    rm "${response_headers}"
}

get_aid() {
    aid="$(/opt/CrowdStrike/falconctl -g --aid | awk -F '"' '{print $2}')"
}

#------Start of the script------#
set -e

cs_falcon_cloud=$(
    if [ -n "$FALCON_CLOUD" ]; then
        echo "$FALCON_CLOUD"
    else
        # Auto-discovery is using us-1 initially
        echo "us-1"
    fi
)

response_headers=$(mktemp)

# shellcheck disable=SC2001
proxy=$(
    proxy=""
    if [ -n "$FALCON_APH" ]; then
        proxy="$(echo "$FALCON_APH" | sed "s|http.*://||")"

        if [ -n "$FALCON_APP" ]; then
            proxy="$proxy:$FALCON_APP"
        fi
    fi

    if [ -n "$proxy" ]; then
        # Remove redundant quotes
        proxy="$(echo "$proxy" | sed "s/[\'\"]//g")"
        proxy="http://$proxy"
    fi
    echo "$proxy"
)

main "$@"