    async def customize(self, ctx, command: str.lower, *, response: str = None):
        """
        Customize the response to an action.

        You can use {0} or {user} to dynamically replace with the specified target of the action.
        Formats like {0.name} or {0.mention} can also be used.
        """
        if not response:
            await self.config.guild(ctx.guild).clear_raw("custom", command)
        else:
            await self.config.guild(ctx.guild).set_raw("custom", command, value=response)
        await ctx.tick()

The application deserializes untrusted data without sufficiently verifying that the resulting data will be valid.