Freeciv

The game seed, used among others to generate the map, is generated using time(0) when the game is started (or maybe the server, I didn't check as there's little difference for longturn.net). If one knows at what time a game has been started, it is trivial to generate a map with the corresponding seed on a local server. Even if the knowledge of the start time is approximate, one can generate several maps and match them against the area surrounding a starting position. Since time(0) is in seconds, the number of maps one needs to generate is small.

One suggestion is to use the current time in microseconds instead of seconds, but this might not be sufficient if a high-resolution clock is not available. In C++ I would use std::random_device, but I see no portable equivalent in C.

Another possibility is to salt the game seed with a machine-specific ID. This is less secure because a leak of the machine ID would compromise every game running on the server.

This is a very easy exploit that also leaves no traces. Once it is revealed, I wouldn't be surprised to see it in use. It affects all current games; at longturn.net, this is LearningLT and LT54 to LT58. Mitigation will be applied from LT59 onward, but the last affected game isn't expected to end before February 2021 and could last for at least one more month. Please take this into account when revealing the issue to a wider audience.