Guys I had same problem with rain water from front drain holes before and some minor rust was present way back in cavity
So what I did was clean out area and got some resin (Stuff you use with fibreglass) And painted inside of cavity back up against fire wall, had car up on ramp at the front and also laid it on thick.
The resin did partially come into cabin (Obviously thru rusted area)
now there are no leaks from this spot and maybe stop rust getting a further foothold in the future.
Check the area also boys have seen major rust in some cars here.