Re: XSS (in 20 chars) in Microsoft IIS 7.5 error message

[Mark Steward <marksteward () gmail com>]

I've spotted this before and ignored it because it's all HTML-escaped. You
can actually put as much as you like before the equals, presumably
including script tags. You can also include enough after the equals to
write something like "<iframe src=//xy.co>".

Where are you seeing it unescaped? Is it some third-party handler? Try on a
clean install with just an empty .aspx and a web.config with an empty
configuration element.

Mark
On 29 Nov 2014 01:51, "A Z" <kryptos.gnostikos () gmail com> wrote:

> Hello everyone,
>
>
> I found some weird HTML code injection in an IIS error message. IIS spits
> out some part of the user input that generated the error message, but will
> only display 20 characters at most.
> My question is: is it possible to actually exploit an XSS with this ?
>
> Here is an example:
>
> HTTP Request: mypage?search=%3cb%20onclick%3dalert(1)>%3e
> HTTP Response (real):
>
> <p>An error has occured.</p>
>     <p>Exception HttpRequestValidationException occurred while attempting
> <b>mypage</b></p>
>     <p>Exception message is: <b>A potentially dangerous Request.QueryString
> value was detected from the client (search="<b
> onclick=alert(1)>...").</b></p>
>     <p>Stack trace:</p>
>     <pre>
> Server stack trace:
> [..]
>
> My payload was: <b onclick=alert(1)>> and it works (after clicking).
> However, can this actually be exploited in real life ? I tried stuff in 20
> characters like: <embed src=http://x> or <img src=http://x/z> but no luck.
> Has anyone ever tried this before ?
>
> Thanks,
>
> P.S. This might be a silly question with an obvious answer. If so, I'd be
> grateful to have some extra information (links, docs etc.).
>
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