mirror of
https://github.com/EGroupware/egroupware.git
synced 2024-11-17 13:33:15 +01:00
69 lines
2.8 KiB
Plaintext
69 lines
2.8 KiB
Plaintext
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.
|