Re: [xml-dev] RE: XML parsers use what computational power?

["Simon St.Laurent" <simonstl@simonstl.com>]

I think we had this conversation at least a dozen years ago, in 
conversation about the million-laughs attack.

As attack surface area goes, entity resolution isn't exactly drastic. 
It's not hard to apply a minor amount of defensive programming to halt 
this attack.  It's also an extremely dull attack, a denial of service at 
best.

While I recognize that you are compiling rules for an extremely 
conservative audience, I often worry that they and their experts fear 
their own shadows most of all.  Perhaps they should stick to plain 
unformatted text 'processed' by humans with security clearances?

Thanks,
Simon

On 4/10/13 5:43 AM, Costello, Roger L. wrote:
> Hi Folks,
>
> Resolving these entities:
>
> <!ENTITY ha1 "ha"> <!ENTITY ha2 "&ha1;&ha1;"> ... <!ENTITY ha128
> "&ha127;&ha127;">
>
> requires an amount of memory that is exponential to the number of
> entities.
>
> Therefore an XML parser requires more computational power than a
> linear bounded automata [1]. Furthermore:
>
> Linear bounded automata are acceptors for the class of
> context-sensitive languages. [1]
>
> Therefore XML is more powerful than a context-sensitive language.
>
> Experts recommend [2] that an input language not require more
> computational power than a deterministic pushdown automata. The XML
> language requires much more computational power than this.
>
> Therefore, XML as an input language is too complex. It requires too
> much computational power. It's attack surface is too large (as
> evidenced by the dozens (hundreds) of vulnerabilities exposed in the
> last decade).
>
> /Roger
>
> [1] Linear automata:
> http://en.wikipedia.org/wiki/Linear_bounded_automaton
>
> [2] Input languages should not require a computational power greater
> than D-PDA:
> http://www.cs.dartmouth.edu/~sergey/langsec/papers/Sassaman.pdf
>
>
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-- 
Simon St.Laurent
http://simonstl.com/