As it was officially published today on a few communication channels, an old "backdoor" through which computer-savvy players could have gained privileged information about a game they are playing has just been shut and welded. The "backdoor" wasn't deemed too relevant for single-player games, but with the expansion of Longturn multiplayer communities, it started to become a problem.
Due to the nature of the map generator, it was possible to recreate the map you are playing at if you managed to find out the exact second the game was started because the generator used the computer time in seconds as a seed for generating the map. Even if the exact time was not known, it was possible to create several maps and compare them to the known area around your units.
The "backdoor" is removed starting from Freeciv version 2.6.4 and all Longturn games from LT59 onwards have already been starting with a patch.
It is not completely certain if this will affect Freeciv Web. They have been informed about this a week before publication of the existence of the "backdoor" and the fix, but so far they have been quiet about this.