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Entities can execute system level commands?

Hi Folks,

The RFC on XML Media Types (RFC2376) says this in the section on Security Considerations:

   XML entities contain
   information to be parsed and processed by the recipient's XML system.
   These entities may contain and such systems may permit explicit
   system level commands to be executed while processing the data.  To
   the extent that an XML system will execute arbitrary command strings,
   recipients of XML entities may be at risk. In general, it may be
   possible to specify commands that perform unauthorized file
   operations ...


How can the use of an entity result in "explicit system level commands to be executed while processing the data"?

For example, here is an XML document that contains an external entity reference:

<?xml version="1.0"?>
<!DOCTYPE BookCatalogue [
    <!ENTITY Book SYSTEM "Book.xml">
                <Title>Illusions The Adventures of a Reluctant Messiah</Title>
                <Author>Richard Bach</Author>
                <Publisher>Dell Publishing Co.</Publisher>
                <Title>The First and Last Freedom</Title>
                <Author>J. Krishnamurti</Author>
                <Publisher>Harper &amp; Row</Publisher>

How can this entity execute system level commands?


See Section 4 of RFC2376: http://www.ietf.org/rfc/rfc2376.txt 

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