The Anatomy of Cross Site Scripting

LogError writes "Many documents discuss the actual insertion of HTML into a vulnerable script, but stop short of explaining the full ramifications of what can be done with a successful XSS attack. While this is adequate for prevention, the exact impact of cross site scripting attacks has not been fully appreciated. This paper will explore those possibilities."

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Text Version for People Who Hate PDFs

[akedia]

The Anatomy of Cross Site Scripting
Anatomy, Discovery, Attack, Exploitation
by Gavin Zuchlinski (gav@libox.net )
http://libox.net/
November 5, 2003

Introduction
Cross site scripting (XSS) flaws are a relatively common issue in web
application security, but they are still extremely lethal. They are
unique in that, rather than attacking a server directly, they use a
vulnerable server as a vector to attack a client. This can lead to
extreme difficulty in tracing attackers, especially when requests are
not fully logged (such as POST requests). Many documents discuss the
actual insertion of HTML into a vulnerable script, but stop short of
explaining the full ramifications of what can be done with a successful
XSS attack. While this is adequate for prevention, the exact impact of
cross site scripting attacks has not been fully appreciated. This paper
will explore those possibilities.

Anatomy of a Cross Site Scripting Attack
A cross site scripting attack is typically done with a specially crafted
URI that an attacker provides to their victim. The XSS attack could be
considered analogous to a buffer overflow, where the injected script is
similar to overwriting an EIP. In both techniques, there are two options
once a successful attack has occurred: insert funny data or jump to
another location. Insertion of funny data during a buffer overflow
typically results in a denial of service attack. In the case of a XSS
attack, it allows the attacker to display arbitrary information, and
suppress the display of the original webpage. When jumping to
another location during a buffer overflow attack, the new location is
another location in memory where shellcode or other important data
resides - allowing the attacker to take control of the flow of the
program. Within the XSS context, the attacker instead jumps the
victim to another location on the Internet (typically under the
attacker's control), hijacking the victim's web browsing session.

Discovery
But how do cross site scripting attacks occur? XSS attacks are the
result of flaws in server- side web applications and are rooted in user
input which is not properly sanitized for HTML characters. If the
attacker can insert arbitrary HTML then they could control execution of
the page under permissions of the site. A simple page vulnerable to
cross site scripting looks like:

Once the page is accessed, the variable sent via the GET method is
placed directly on the rendered page. Since the input is not marked as
variable input , the user- supplied input is interpreted exactly as its
metacharacters command, very similar to SQL injection. Passing
"Gavin Zuchlinski" as an argument outputs the content in correct form:
Sending input with HTML metacharacters allows for unexpected output:
The input is not validated by the script before rendering by the victim's
web browser. This allows for user controlled HTML to be inserted on to
the vulnerable page. Occasionally user input not directly parsed by the
script it is sent to. Rather, the data is inserted into a file or database
and retrieved later to be reinserted on the page.
Common points where cross site scripting exists are confirmation
pages (such as search engines which echo back user input in the event
of a search) and error pages that help the user by filling in parts of the
form which were correct. Commonly in the latter case (and sometimes
the former) the containment of the form text box must be escaped
with a quote and a greater than sign ("> - the quote closes the value
property and the greater than closes the tag).

Attack
Once a vulnerable input is identified the valid HTTP methods must be
determined. The way in which the variables are sent to the target
script is an important consideration; are variables sent by GET, POST,
or will either work? Some scripts are specific, but several which use
canned methods (like PHP and Perl scr

Re:Text Version for People Who Hate PDFs

[Anonymous Coward]

I, for one, welcome our Karma loving Whorverloads.

Re:Text Version for People Who Hate PDFs

[samjam]

Many people consider cut-and-pasting the article to be inherently redundant. I generally agree with them

An accurate judgement, no doubt, but the point is this:
Is there any value in moderating the post as redundant - redundant it may be, but useful, and arguably more useful than its moderation as redundant

XSS Protection

[mnmlst]

Cross Site Scripting attack protection is a standard feature of many network security products these days. Check Point NG with Application Intelligence (Feature Pack 4 in other words) includes XSS protection as part of its' so-called SmartDefense. I am curious if anyone has run into situations where SmartDefense is screwing up legitimate traffic, especially traffic that resembles an XSS attack.

BTW, does anybody have some good recommendations for cheaper alternatives with pretty comparable protection to Check Point? I would like something that is as defensive, but not as configurable or extensible.

Re:XSS Protection

[Anonymous Coward]

ASP.NET 1.1 will, by default, refuse to process any forms that have fields in which the user has tried to post values that contain HTML. You can override on a per-page basis, but I think it's a reasonable default.

Re:Can someone explain?

[nestler]

X is a cross if you look at it.

The main reason Cross Site Scripting is abbreviated XSS (instead of CSS) is to avoid confusing it with Cascading Style Sheets (this confusion is likely to happen since both of these things are related to web pages).

Short solution

[Anonymous Coward]

Do not have a blacklist for denying invalid input. It's hell/impossible to maintain such blacklists

Handle all user input as it was written by satan himself, and only allow input complying to your strict specification.

This is news?

[unfortunateson]

I'm surprised this merited a news item.
Webmonkey had a similar article three and a half years ago, that provide some more solid examples of what happens.

I designed an e-commerce site over the last six years, and evaluated where there might be XSS vulnerabilities. Not having a bulletin board or guestbook removes many areas for exploitation.

So if someone types contaminated data into their address field when checking out, you'd think all it hoses is their own purchase, right?

Well, with PHP or Perl CGI, it's possible that the inputted variables could exploit server knowledge: if you know the variable names used in the PHP code for, say, the MySQL password, then embedding this in the input to be evaluated on output can open an avenue for hacking. The variable has to be evaluated in most cases, although code which generates new PHP pages could result in similar problems.

HTML encode EVERYTHING the user sends to you.

But of course

[freeweed]

have we reverted to referring to letters by the way they look?

Why yes.

You ever notice that "C" stands for "Cookie"?

It's good enough for me.

Now find me some Crescent shaped cookies.

Re:English for Geeks 101

[aminorex]

I'm not going to just lay here and take this.
Hey, if you don't like the affect of English
spelling history, you can just immigrate to
some place where they speak Canadian. Your
allusions of superiority try to make capitol
of the principals of colloquial language, but
in doing so they create a climactic change
which I find frankly unseasoned.

Free Article on Cross-Site Attacks

[shiflett]

Although it is PHP-specific, this free article explains XSS and CSRF in quite a bit of detail and might be useful for Web developers using any language:

http://www.phparch.com/sample.php?mid=16

Enjoy

Lethal !!!

[Timesprout]

Cross site scripting (XSS) flaws are a relatively common issue in web application security, but they are still extremely lethal

You better believe it. Why only last week I had one of my web developers executed for writing code vunerable to a Cross Scripting Attack. I dont want any slackers on my team.

PS I now have an opening for an experienced web developer. Sent resumes to spareme@icodetolive.com

Re:Booring

[xchino]

I completely disagree. XSS is as dangerous if not more so than a buffer overflow, im many cases. Take this for example:

Your target is one or more users of a community web site. The site itself isn't the target, only the means to your own ends. Remember, it's the users you are after, not the site itself. So you smash the stack on the server, grap the mySQL database, and open it up. Bummber, all the passwords are md5'd and basically useless. With XSS you could conceivably alter the login for that they get, and before md5($password) is executed you export $password (still in plain text) off to your little database.

Cracking isn't about what is the most "exciting and leet" way to do it, it's about using the tools you have at your disposal to get what you want done, done. Sometimes this is a buffer overflow, sometimes it a XSS attack, sometimes an emailed trojan, and sometimes even social engineering to gain physical access (even via an unwitting human proxy).

Re:Static

[orthogonal]

Static webpages aren't vunerable to this kind of attack. Yay!

Neither is my Ford Taurus, Orangutans, or bananas. What's your point?

You're sure bananas aren't vulnerable?

Now he tells me. Oh, oh, the time I have wasted.

Asp.NET 1.1 and XSS

[palad1]

Asp.Net protects users from XSS by default since version 1.1 by parsing the parameters of a page and looking for javascript/html code in the query.
Of course I was bitten by this feature when upgrading from 1.0 to 1.1, but that's just because I didn't bother reading the readme.txt :)
Automatic protection bundled with any application server is good, especially if you can turn it off [you can in asp.net , validateRequest=false et voila].

A few techniques used and more

[iceco2]

The generall tecnhique described above is with
volnerable scripts which display text which came
from URL encoded data, This is one of many methods
to display the attackers HTML in an unsuspecting
users browser.
It is very common for the 404 message on a website to contain the URL which was entered, In the past this was done mostly by copying it as is. This would allow an attack.

In order to hide the attack hex encoding is used in the URL so the victim would not notice the script in the URL.

Still the attacker needs to minimize the length of the URL this causes him to use HTML options
such as iframe in order to insert a lot of HTML
taken from a diffrent site.

The main point of intrest is that the page appears to be comming from the (probably trusted) server, this can convince the user to do stuff he may not do on the attackers web site, say for example enter credit card info.

Also one could collect cookies this way, the cookies are likely to contain passwords or equivelent informations for sites with user login.

In some forums a user can put scripts in his signature or profile, this allows similar results,
but with out sending funny URLs.

DO NOT TRUST USER INPUT, it may harm not only you
but also the user, they must be protected.

Me.

I learnt this lesson a long time ago.

[Marak]

In high school for economics class we got to play a mock stock martket game (on the web). Well my stock market team consisted of myself and another CS student.

On the website you would enter in the amount of stock, stock symbol, and BUY or SELL in a form. That form would POST to a confirmation page and from there you would click "TRADE" and it would post to some server side page to execute the trade. The fools that designed the site thought it would be a good idea to validate all the data on the confirmation page and NOT on the server side page. We created a local version of the initial confirmation page, changed the action of the form to "http://www.tradingsite.com/cgi-bin/trade.pl". We then proceeded to buy -100000 shares of MSFT for about 40 bucks a pop.
The server had a formula of something like:

(STOCKPRICE * SHARES) + COMMISION = SUM
The sum was then checked against your accounts cash balance.
Something like:
IF (SUM > CASHBALANCE)
ERROR;
ELSE
EXECUTE TRADE;

Well we had a big negative number for our SUM so it passed.

The server then procceeded to:
CASHBALANCE = CASHBALANCE - SUM

Well anyone who has taken 5th grade math knows what happens when you subtract a negative number.
To make a long story short....we come into school about 2 weeks later and there is a big list of all the teams playing the stock market game in NY state. Our team is number 1 by about 2 million bucks, 2nd place is at about 105k. We confessed to whole the thing explained to the site what they did wrong and didn't get in any trouble.

The morale of this story:

Validate all user input before you perform ANY actions with it.

Its news to you

[samjam]

HTML encode EVERYTHING the user sends to you

*cough*

Its this kind of lack of understanding that makes the problem so prevelant.

First it doesn't make sense to htmlencode everything just as id doesn't make sense to addslashes everything (now turned off by default in all good php configurations).

Here's why: Not everything that comes in is to be displayed as html, just as not everything that comes in is destined for the database.

Unless you understand the risks, you can't guard against them though it appears some people are still able to be certain they have guarded against them.

If you do this,

sqlquery("select * from user where username='$user'") then you need to think what the problem is, its a well defined problem, it is that $user may contain a final ' mark and then some; maybe:
$user="jimjoe' or 1'"

so your preferences page now shows the first user in the db, or depending on your web page, all of them.

In php, htmlentities doesn't encode the '

If you are invoking system commands (and yes I one had to do a LOT of this from php) then be careful about shell meta characters like ` ' " and $ in certain cases.

The principle is that you need to make sure the system you are passing data on to interprets it in the literal sense that you require and you cannot do this unless you understand completely how each of the systems you will pass the data on to really does interpret data.

So if your user data is destined for the database, then escape it, something like:

sqlquery(sprintf("select * from user where username='%s'",addslashes($user)));
(yes there are other better was of doing it)

If you want to display on the web page inline:
echo htmlentities($user);
on the other hand if you want to display in an text area I think there is other encoding to use. If it is for a url you need to urlencode and htmlentities but I forget the order.

Understand the system you are communicating with.

Sam

Re:Know of a sanitizing script in PHP?

[ajs318]

You need to do something like this. Use preg_replace to change all mustang signs to < and > sequences. But that's overzealous - you need to un-mung sequences that look like HTML tags you regard as innocuous. Now you have to define an array for allowed HTML tags, indexed by their "munged" form, like this:

$allowed_tags = array('&lt;B&gt;' => '<B>',
'&lt;/B&gt;' => '</B>',
&c.);

Do a foreach ($allowed_tags as $i=>$j), and str_replace {it's supposedly quicker than preg_replace} each occurrence of the index $i with the value $j. Only permitted HTML tags will remain. You can even do a second foreach further down the page to list the permitted tags {since they're already HTML-escaped you can just display the indexes and the reader will see it rendered to look like a HTML tag}.

If you want to allow <A> or <IMG> tags, you should use preg_match expressions for elementary sanity checking.