LCGs are used in web applications to create quick and easy pseudo-random numbers. They are by nature broken and can be easily made to be predictable with enough data. The algorithm for an LCG is:
Here, X is the current value, a is a fixed multiplier, c is a fixed increment, and m is a fixed modulus. If any data is leaked, such as the multiplier, modulus, and increment in this example, it is possible to calculate the seed and thus the next values.