Short circuit protection for my voltage source
The most professional solution requires access to internals of the supply [so needs to be designed in it], specifically access its feedback pin/loop. See the application schematics for LMP8646 for instance.

Since you probably don't have access to that with your DSL modem DC adapter (unless you open it and mod it, which requires good understanding of the mains-voltage safety issues), you'll have to use a pass element (BJT or FET) for the overcurrent comparator to shut off instead. Look at LTC4361 for the general idea; that one is for 5V though. I don't know an IC off the top of my head for 12V. Something probably exists. A hotswap controller would also have this current limiting function, but may be overkill for what you need.