I have an old home that has several outlets in the exterior wall. These outlets are not secured to the block, they just dangle in the opening and are only secure (or appear to be firmly in place) due to the tightness of the cover plate. Are there fasteners or some type of bracing that can be used to correct the loose outlet? Is there another method of "mounting" these outlets? I can't seem to see any way to secure these outlets since the only available structure is the block itself, which is only an inch deep, so drilling tap cons right near the interior wall surface doesn't seem to be the right course of action. Any thoughts?
You may be able to secure the boxes with something like these supports
This image shows two supports that need to be separated in the middle.
The support is grasped by one of the short, perpendicular legs. The longer end of the long leg is slid on an angle into the cavity vertically between the box and the vertical edge of the opening. Then the bottom end of the support (the shorter end of the long piece) is pushed in, slipped down and hooked behind the lower lip of the opening. The short perpendicular legs then stick out and are wrapped around from the outside edge of the box to the inside. The same is done on the other side. The box is then locked in place.
This assumes the box has "ears" at top and bottom that prevent the box from being pulled below the face of the wall.
This answer assumed a different condition than what the OP has. It is left as a matter of record though it does not answer the question.
The NEC only requires equipment to be securely fastened and prohibits wood plugs in holes. Any other method is subject to interpretation by the local authority. So the best answer is confirm with your inspector before doing anything.
That said, here's some ideas you might propose. Tapcons are not listed for hollow masonry, but you could start drilling for one and if you find the cell is solid grouted, go ahead and use the tapcon. If you drill into a void, tapcons will not work. In such a case you could use a toggle bolt or a screw into a plastic or soft metal expansion anchor.
You might even be able to simply glue it to the wall with construction adhesive, though some sort of mechanical connection in addition would be preferable. Any of these might be rejected by an inspector... or accepted. I don't know. I would consider these securely fastened, but I'm not your inspector.