I was pointing out the dangers of user-permitted HTML markup many years ago when I talked about how I was including Barney the Dinosaur into guestbooks on my home page.

I don't think there's a completely secure solution. tilly wrote a fairly nice strongbox solution that permits very carefully a subset of HTML, but it's more in the "only permitted things allowed" realm than the "look for bad things and prevent them" realm. Given the risk, I'd say this is the right approach.

-- Randal L. Schwartz, Perl hacker

update: Thanks to mdillon for the link for tilly's node.

In reply to •Re: Re: Preventing Cross-site Scripting Attacks by merlyn
in thread Preventing Cross-site Scripting Attacks by grep

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.