When outputting user input, do you only use htmlspecialchars() or are there are functions/actions/methods you also run? I'm looking for something that will also deal with XSS.
I'm wondering if I should write a function that escapes user input on output or just use htmlspecialchars(). I'm looking for the generic cases, not the specific cases that can be dealt with individually.
I usually use
htmlspecialchars($var, ENT_QUOTES)
on input fields. I created a method that does this because i use that a lot and it makes the code shorter and more readable.
Basically, any output that has been escaped in this manner can no longer directly execute/inject code into the page. htmlspecialchars does this for you, no need for custom functions. However, notice that it sometimes does too much, i.e. it escapes some things that need not be escaped in all instances. I therefore advise using the ENT_NOQUOTES argument. This results in slightly shorter HTML code more closely resembling code that would be written manually: people will not usually write " instead of ", since this isn't required.
Compare:
<pre>string s = "Hello world";</pre>
vs.
<pre>string s = "Hello world";</pre>
The exception is of course when the output is used in tag attributes, where a runaway quote would demolish the markup.
Lets have a quick review of WHY escaping is needed in different contexts:
If you are in a quote delimited string, you need to be able to escape the quotes. If you are in xml, then you need to separate "content" from "markup" If you are in SQL, you need to separate "commands" from "data" If you are on the command line, you need to separate "commands" from "data"
This is a really basic aspect of computing in general. Because the syntax that delimits data can occur IN THE DATA, there needs to be a way to differentiate the DATA from the SYNTAX, hence, escaping.
In web programming, the common escaping cases are: 1. Outputting text into HTML 2. Outputting data into HTML attributes 3. Outputting HTML into HTML 4. Inserting data into Javascript 5. Inserting data into SQL 6. Inserting data into a shell command
Each one has a different security implications if handled incorrectly. THIS IS REALLY IMPORTANT! Let's review this in the context of PHP:
Text into HTML: htmlspecialchars(...)
Data into HTML attributes htmlspecialchars(..., ENT_QUOTES)
HTML into HTML Use a library such as HTMLPurifier to ENSURE that only valid tags are present.
Data into Javascript
I prefer json_encode. If you are placing it in an attribute, you still need to use #2, such as
Inserting data into SQL Each driver has an escape() function of some sort. It is best. If you are running in a normal latin1 character set, addslashes(...) is suitable. Don't forget the quotes AROUND the addslashes() call:
"INSERT INTO table1 SET field1 = '" . addslashes($data) . "'"
Data on the command line escapeshellarg() and escapeshellcmd() -- read the manual
-- Take these to heart, and you will eliminate 95%* of common web security risks! (* a guess)
You shouldn't be cleansing text on output, it should happen on input. I use a filter that filters all input to the application. It is configurable so that it can allow specific tags/data through when needed (say for a wysiwig editor).
You should do as little processing of text on output as possible so that you ensure speed. Processing it once creates a lot less strain then processing it 500,0000 times.