Flasm is a free command line assembler/disassembler of Flash ActionScript bytecode. It lets you make changes to any SWF. Flasm fully supports SWFs produced by Macromedia Flash 8 and earlier Flash versions. You can
download Flasm from their website.
In this video, we see a demo of how to disassemble a Flash file using Flasm. In this specific example, the developer has made the mistake of embedding credentials in the flash swf and doing local authentication. What most developers don't realize is that flash binaries can be easily disassembled, and any strings embedded in them can be easily seen.