The other posters are addressing a potential safety issue pertaining to the strut tower, which (if I remember) would be part of the uni-body. At this point, keep in mind that my experience with Civics was extensive, but they were models preceding yours by 22 years. If your rust is occurring on the fender and not the uni-body, read on. Otherwise, good advice has already been posted.
When I bought my '79 Civic, I purchased rust-proofing for doors and other parts as an option. Later, my front quarter-panels began to rust along a weld seam toward the rear of the tire -- a cosmetic problem, mostly.
Here's a link to JC Whitney, where I'd once purchased a driver-side fender/quarter-panel for about the same price specific to a '79 CVCC Civic:
2001 Civic DX fender
Of course, it means a paint-job, if only for the fender. Look for welded seams on the inside of the new fender, and treat it with the rubberized-undercoating you can buy in a can before putting the fender on the car.