Interesting and potentially useful.
Some more info here:
Using the Erlang bit syntax it’s an easy task to unpack the tags of an SWF file. With this thought in mind erlswf has been specifically designed to analyse SWF Tags and ActionScript ByteCode for security issues such as the previously mentioned oversized branch offset or pattern matching against URLs loaded during runtime. The toolkit could also be used to implement a transparent proxy filter for exchanging pictures inside Flash files on the fly. Or if you had no choice but to accept prebuilt SWFs from a third party (e.g. ad hosters), it would still be possible to check for arbitrary conditions or restrictions respectively prior to delivery.
The other pure erlang SWF library eswf places emphasis on SWF construction and related data formats (AMF, ABC).
Source code is available on Google Code, here (thanks, fukami).