Elgg 1.0, meet kses. Kses, Elgg 1.0.

git-svn-id: https://code.elgg.org/elgg/trunk@1344 36083f99-b078-4883-b0ff-0f9b5a30f544

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+kses attribute value checks
+===========================
+
+As you've probably already read in the README file, an $allowed_html array
+normally looks like this:
+
+$allowed = array('b' => array(),
+                 'i' => array(),
+                 'a' => array('href' => 1,
+                              'title' => 1),
+                 'p' => array('align' => 1),
+                 'br' => array());
+
+This sets what elements and attributes are allowed.
+
+From kses 0.2.0, you can also perform some checks on the attribute values. You
+do it like this:
+
+$allowed = array('b' => array(),
+                 'i' => array(),
+                 'a' => array('href' =>
+                                array('maxlen' => 100),
+                              'title' => 1),
+                 'p' => array('align' => 1),
+                 'font' => array('size' =>
+                                array('maxval' => 20)),
+                 'br' => array());
+
+This means that kses should perform the maxlen check with the value 100 on the
+<a href=> value, as well as the maxval check with the value 20 on the <font
+size=> value.
+
+The currently implemented checks (with more to come) are 'maxlen', 'maxval',
+'minlen', 'minval' and 'valueless'.
+
+'maxlen' checks that the length of the attribute value is not greater than the
+given value. It is helpful against Buffer Overflows in WWW clients and various
+servers on the Internet. In my example above, it would mean that
+"<a href='ftp://ftp.v1ct1m.com/AAAA..thousands_of_A's...'>" wouldn't be
+accepted.
+
+Of course, this problem is even worse if you put that long URL in a <frame>
+tag instead, so the WWW client will fetch it automatically without a user
+having to click it.
+
+'maxval' checks that the attribute value is an integer greater than or equal to
+zero, that it doesn't have an unreasonable amount of zeroes or whitespace (to
+avoid Buffer Overflows), and that it is not greater than the given value. In
+my example above, it would mean that "<font size='20'>" is accepted but
+"<font size='21'>" is not. This check helps against Denial of Service attacks
+against WWW clients.
+
+One example of this DoS problem is <iframe src="http://some.web.server/"
+width="20000" height="2000">, which makes some client machines completely
+overloaded.
+
+'minlen' and 'minval' works the same as 'maxlen' and 'maxval', except that they
+check for minimum lengths and values instead of maximum ones.
+
+'valueless' checks if an attribute has a value (like <a href="blah">) or not
+(<option selected>). If the given value is a "y" or a "Y", the attribute must
+not have a value to be accepted. If the given value is an "n" or an "N", the
+attribute must have a value. Note that <a href=""> is considered to have a
+value, so there's a difference between valueless attributes and attribute
+values with the length zero.
+
+You can combine more than one check, by putting one after the other in the
+inner array.