There are many Big Mac sauce recipes on the Internet, but this is reputedly the real deal, and I must admit, it tastes like it.
The story is that McDonalds sauce used to arrive at their restaurants ready-made, but in the staff manual there was also a recipe for reproducing it in-store in case they ran out. Sometime during 1969, the recipe got out. The page was promptly removed from other manuals for fear that other burger chains would learn and use their 'top secret' sauce, but too late. The recipe was made public the following year.
This is the recipe, which doesn't have the added preservatives the pre-made stuff contains.