I use Markdown for provide a simple way for write posts to my users in my forum script.
I'm trying to sanitize every user inputs, but I've a problem with Markdown's inputs.
I need to store in database the markdown text, not the HTML converted version, because users are allowed to edit their posts.
Basically I need something like what StackOverflow does.
I read this article about XSS vulnerability of Markdown. And the only solution I found is to use HTML_purifier before every output my script provides.
I think this can slowdown my script, I imagine output of 20 posts and running HTML_purifier for each one...
So I was trying to find a solution for sanitize from XSS vulnerabilities sanitizing the input instead of the output.
I can't run HTML_purifier on the input because my text is Markdown, not HTML. And if I convert it for get HTML I can't convert back for turn into Markdown.
I already remove (I hope) all HTML code with:
htmlspecialchars(strip_tags($text));
I've thinked about another solution:
When an user is trying to submit a new post: Convert the input from Markdown to HTML, run HTML_purifier, and if it find some XSS injection it simply return an error. But I don't know how to make this nor I know if HTML_purifier allows it.
I've found lot of questions about the same problem there, but all solutions was to store the input as HTML. I need to store as Markdown.
Someone has any advice?
strip_tags()
, 2. sanitize input which are used in attributes like the [link](javascript:alert('xss')): 3. consider runninghtmlspecialchars()
on the input before turning converting into HTML and output. Should be fairly safe no? I mean only you control what HTML tags are going to be used so as long as you strip them all in the input and sanitize attributes you got full control over XSS. – kjetilh