On 27/05/2008, at 6:08 PM, Neil Mitchell wrote:
It most certainly is a security flaw.
In the src of an img, yes, probably. In the href of a link, its a completely valid thing to do - and one that I've done loads of times. The URI is fine, its just the particular location that is dodgy.
Sure, but for other reasons (potential inaccessibility) I am quite happy to ban JavaScript from URIs. (Not all URIs, just the ones coming from untrusted users.)
whole pile of dodgy URIs. Most get culled (in my case) by the HaXml parser and/or XHTML 1.0 Strict validation, and now I hope to eliminate the rest by carefully handling the URIs.
I don't think that's possible. A URI can validly have javascript, and can validly be a lot of things which are unsafe.
Sure, I now realise my notion of allowable URI goes beyond (is an additional restriction of) the RFC. One way to show the URI is valid is to fetch what it is pointing to, and ensure it is an image or whatever.
On that topic, does anyone have any good advice for handling these things?
My advice is that you are targeting security at the wrong level. You shouldn't be cleaning the HTML to get a secure page, you should be having the level that interprets the HTML be secure regardless of the input.
I am taking comments on a web forum from arbitrary people. The interpretation of the HTML occurs at the user's browser. A lot of people will be using outdated browsers (IE 5.5 / 6), ergo security (at the source) becomes my problem. I cannot force them to upgrade their browsers.
If anyone knows of the state-of-the-art in this area, I'd appreciate a pointer.
http://htmlpurifier.org/live/smoketests/printDefinition.php
doesn't seem to think the style attribute is unsafe. Have they not been following the MySpace fiascos?
Safety is a property of the HTML viewer, not of the HTML or CSS.
Well, yes and no. I am heavily restricting the XHTML I accept (e.g. no scripts, no style attribute, ...), in an attempt to keep things visually accessible and avoid phishing attacks. I was alluding to the use of absolute positioning in CSS. If I had a CSS parser I might allow the style attribute. Safety for me involves making sure that what is displayed is trustworthy and easily identifiable as such. This is not something the HTML viewer can always help with. I think we're off-topic enough for me to stop here. Thanks for your comments. cheers peter