[Cellar] Security considerations: recursive elements

[Michael Bradshaw <[e-mail]>]

The EBML and Matroska specs currently don't mention the possibility of a
stack overflow due to deeply nested recursive elements. Currently, there's
no limit on the recursion depth (unless I've overlooked it somewhere).

I think it would be worth adding to the security section of EBML that one
type of attack on an EBML Reader could include deep element recursion.

Additionally, I would like to see what people think about potentially
adding/suggesting an upper limit on recursion (either as a MUST or a MAY).
This could also include a lower limit too. For example, something like "a
parser SHOULD handle recursion up to X levels deep, and MAY abort the parse
if it reaches Y levels deep".

Thoughts from others?