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XDMCP is an unencrypted protocol, and therefore, presents a security risk, see e.g. XDMCP Gnome docs.

To disable XDMCP support in Gnome, set Enable to false under the [xdmcp] configuration section in /etc/gdm/custom.conf. For example:



XDMCP provides unencrypted remote access through the Gnome Display Manager (GDM) which does not provide for the confidentiality and integrity of user passwords or the remote session. If a privileged user were to login using XDMCP, the privileged user password could be compromised due to typed XEvents and keystrokes will traversing over the network in clear text.


Shell script

The following script can be run on the host to remediate the issue.


# Remediation is applicable only in certain platforms
if dpkg-query --show --showformat='${db:Status-Status}\n' 'gdm3' 2>/dev/null | grep -q installed; then

# Try find '[xdmcp]' and 'Enable' in '/etc/gdm3/custom.conf', if it exists, set
# to 'false', if it isn't here, add it, if '[xdmcp]' doesn't exist, add it there
if grep -qzosP '[[:space:]]*\[xdmcp]([^\n\[]*\n+)+?[[:space:]]*Enable' '/etc/gdm3/custom.conf'; then
    sed -i "s/Enable[^(\n)]*/Enable=false/" '/etc/gdm3/custom.conf'
elif grep -qs '[[:space:]]*\[xdmcp]' '/etc/gdm3/custom.conf'; then
    sed -i "/[[:space:]]*\[xdmcp]/a Enable=false" '/etc/gdm3/custom.conf'
    if test -d "/etc/gdm3"; then
        printf '%s\n' '[xdmcp]' "Enable=false" >> '/etc/gdm3/custom.conf'
        echo "Config file directory '/etc/gdm3' doesnt exist, not remediating, assuming non-applicability." >&2

    >&2 echo 'Remediation is not applicable, nothing was done'