No they're perfectly legal.
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All Runescape private servers are Illegal, if caught Jagex may take it seriously.
Please note, Runescape private servers are illegal and are shut down frequently. Jagex has the right to sue/put in jail the owners of the private servers. Users of private servers will not be affected though, although, may be subject to an IP block.
Runescape private servers are private servers where players can play the browser MMO with a select few. Sometimes the servers will have mods added to them which make gameplay different.
I don't think it is ILLEGAL to play Private Servers. However, to own a Private Server is illegal.
Technically, private servers are illegal for World of Warcraft. Nevertheless, many people still play on private servers.