UPDATE: Following the 2017-04-26 Filmspeler ruling, using Showbox is probably illegal.
The case concerned the sale of a multimedia player which allowed a customer to stream directly to their TV shows/movies uploaded on third party websites without the right holder's consent. On top of that, the multimedia player was specifically advertised as having these capabilities.
The courtp. 70:
It must also be held that, as a rule, temporary acts of reproduction, on a multimedia player such as that at issue in the main proceedings, of copyright-protected works obtained from streaming websites belonging to third parties offering those works without the consent of the copyright holders are such as to adversely affect the normal exploitation of those works and causes unreasonable prejudice to the legitimate interests of the right holder, because, as the Advocate General observed in points 78 and 79 of his opinion, that practice would usually result in a diminution of lawful transactions relating to the protected works, which would cause unreasonable prejudice to copyright holders (see, to that effect, judgment of 10 April 2014, ACI Adam and Others, C‑435/12, EU:C:2014:254, paragraph 39).
With how similar Showbox is to the Filmspeler device, its difficult to see how Showbox could be anything but illegal to use. The single argument I can think of making (and I don't quite buy it myself) is that Showbox seems to be free to use (though I don't care to download it and check for myself) while the Filmspeler player was sold commercially. It could be argued that Filmspeler took money that would otherwise have gone to copyright holders whereas Showbox users would not have otherwise given the money to copyright holders thus not causing them "unreasonable prejudice."
However, the court did take the time to reaffirmp. 67 their decision in Premier League (cited by my original answer) which is a case that had been cited by the media as allowing streaming of pirated media. As such, I've left my original answer intact below, and below that I speculate where the line might be drawn for streaming online media.
I would like to further explore the decision cited by jqning. This is because in this decision, the judges did not directly considerp. 25 the following portion of Article 5(1) of the Copyright Directive which states that acts of reproduction are only exempted when their...
[...] sole purpose is to enable:
- (a) a transmission in a network between third parties by an intermediary, or
- (b) a lawful use
of a work or other subject-matter to be made, and which have no independent economic significance.
Following the citations in the decision though, leads to the 2011 judgement Football Association Premier League and Others, also decided by the CJEU.
This judgement was made in relation to two merged cases. In the most relevant one, a UK public house operator was using a foreign satellite decoder to decode a foreign broadcast of Premier league matches, and showing them in the house without authorization.
The court considered that
Mere reception as such of those broadcasts [...] in private circles does not reveal an act restricted by European Union legislation or by that of the United Kingdom [...]
and thus considered the act of reproduction was to enable a lawful usep. 170-173.
Turning to the "independent economic significance" clause, the court acknowledged that there is obvious economic value in the broadcast. However, for the clause to not be redundant, they considered that "independent" has to be interpreted as having the economic significance go
[...] beyond the economic advantage derived from mere reception of a broadcast containing protected works [...]
The court thus found that there was no additional economic significance derived from the act of reproductionp. 174-178.
Overall, they ruled that the copyright holder's Article 2 rights were exempted in this case under Article 5. However, the court found a violation of Article 3, "Communication to the public." The display of this broadcast in the public house was illegal, though not the copying itself.
The purpose of using Showbox to me seems the same as the purpose in Premier League, except for the displaying in public part. Article 3 would thus not be relevant, and Showbox would be legal to use with respect to these provisions in Article 5.
Online streaming after Filmspeler (speculation)
Disclaimer: I'm just some bloke on the internet who's not quite satisfied with any of the analyses he's read.
First, I am not convinced that the court found a blanket violation of Article 5(1). The section where they discuss whether the temporary reproductions enabled a lawful use, as required by Article 5(1) in this case, concludes as followsp. 69:
[...] it must be held that it is, as a rule, deliberately and in full knowledge of the circumstances that the purchaser of such a player accesses a free and unauthorised offer of protected works.
Note that it's not a general decision and they tie it specifically to the particular Filmspeler player. The court states that it is presumed a purchaser of such a device knows that they are viewing unauthorized works. So is it knowledge that the material is unauthorized that makes it illegal to view and so causes an Article 5(1) violation?
Additionally, while the court citedp. 70 the Advocate General's opinion on a violation of Article 5(5)p. 78-79, they did not cite his opinion on a violation of Article 5(1)p. 70-71 but instead took the time to reaffirmp. 67 their decision in Premier League.
Granted, the significant difference in Premier League is that the source was an authorized one, it was just the method of access that was questionable. That said the analysis there as to a "lawful use" seems to apply well to Filmspeler. Additionally, if it is knowledge that makes the act illegal as might be understood in Filmspeler, I fail to see how that didn't apply to Premier League as it was clear that the Premier League was geo-restricting access to the content.
Above concerns aside, what I think is the biggest takeaway in this decision is the court's application of Article 5(5). The article states that Article 5(1) should only be applied in cases that do not...
[...] conflict with a normal exploitation of the work or other subject-matter and do not unreasonably prejudice the legitimate interests of the rightholder.
The court very clearly finds that the Filmspeler player does in fact conflict with normal exploitation. This might be due to its commercial nature, but I think more important is the court's finding that the "practice would usually result in a diminution of lawful transactions relating to protected works." I think this means that the ruling can be extended to any product or service whose primary purpose is providing a method to access unauthorized sources of protected works whether directly or indirectly. And since Article 5(5) specifies how Article 5(1) can be considered, it doesn't even matter what the analysis of Article 5(1) is when Article 5(5) so clearly prohibits the action.
There's clearly a line to be drawn somewhere, because I don't think watching unauthorized content on YouTube is an infringement for example, as the site is primarily legal, but Filmspeler (and ACI Adam) shows us the line is much closer to the rights holder than was thought beforehand.
Anyways, that's my analysis, if you don't like it, here's IPKat's analysis, Fieldfisher's analysis, and European Law Blog's analysis. Note if you're going to Google this further, the Premier League case is also referred to as FAPL, or Murphy (after the other party in the case).