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Cross-Site Request Forgeries (Re: The Dangers of Allowing Users to Post Images)
From: Peter W <peterw () usa net>
Date: Fri, 15 Jun 2001 01:15:42 -0400

        Cross-Site Request Forgeries
                (CSRF, pronounced "sea surf")

I hope you don't mind if I expand on this a bit. You've come across the
tip, in my opinion, of a rather large iceberg. It's another
Web/trust-relationship problem. Many Web applications are fairly good at
identifying users and understanding requests, but terrible at verifying
origins and intent.

The problem isn't the IMG tag on the message board, it's the backend app
you seek to attack via the IMG tag. And I suspect lots of Web apps are
vulnerable. Lots. I've been to training on highly-regarded, widely-used,
expensive Web app development frameworks, and none of the classes taught
how to avoid the problems I will attempt to describe. In fact, they all
seem to teach the "easy way" of handling what look like user requests,
which is, of course, the vulnerable way.

Anyway, let's look at how your post relates to what I call CSRF.

On Wed, Jun 13, 2001 at 07:33:04PM +0100, John Percival wrote:

This exploit shows how almost any script that uses cookie session/login data
to validate CGI forms can be exploited if the users can post images.

What is the problem? Well, by using an [img] (or HTML <img> or <iframe> or
<script src="">) tag, the user is having anyone who views the thread access
that image - that is perform an HTTP GET on the URL specified for the image.
Even if its not an image, it still can be accessed, but will display a
broken image. 

Depending on what's allowed, height/width and CSS/visibility tags can be 
used to hide the broken image icon.

This means that the user can put a CGI script inside [img]

** Learning from Randal's purple dinosaur?

The problem you describe is not uploading images, it's allowing users to
post code that's inserted in an appropriate HTML tag attribute. This is
something of a variation on Randal Schwartz's purple dinosaur hack,[2] but
much more interesting and dangerous than even what you describe.

This script will be called by whoever views that thread. When used
maliciously, it could force the user to: unknowingly update their profile,
respond to polls in a certain way, post new messages or threads, email a
user with whatever text they want, the list goes on. This would be
particularly worrying for a 'worm' to spread through a forum, filling it
with rubbish posts.

** The difference between XSS and CSRF

Right. There's something much larger going on here. Darnit, I wanted to
make a nice formal paper out of this, but you're forcing my hand. :-) The
problem is what I call CSRF (Cross-Site Request Forgeries, pronounced "sea
surf"). Any time you can get a user to open an HTML document, you can use
things like IMG tags to forge requests, with that user's credentials, to
any Web site you want -- the one the HTML document is on, or any other.

This looks somewhat similar to Cross-Site Scripting (XSS), but is not the
same. XSS aimed at inserting active code in an HTML document to either abuse
client-side active scripting holes, or to send privileged information (e.g.,
authentication/session cookies) to a previously unknown evil collection site.

CSRF does not in any way rely on client-side active scripting, and its aim
is to take unwanted, unapproved actions on a site where the victim has some
prior relationship and authority.

Where XSS sought to steal your online trading cookies so an attacker could
manipulate your portfolio, CSRF seeks to use your cookies to force you to
execute a trade without your knowledge or consent (or, in many cases, the
attacker's knowledge, for that matter). [Just an extreme example there; I
do not have any idea if any trading sites are vulnerable. I have not 
tested *any* applications or sites that I don't have some personal 
involvement in the design and maintenance of. Don't ask me to.]
 <img src="https://trading.example.com/xfer?from=MSFT&to=RHAT&confirm=Y";>
 <img src="https://books.example.com/clickbuy?book=ISBNhere&quantity=100";>

** Ubiquity of attack channels

Since HTML documents are popping up everywhere (even in corporate email 
systems!!!), and it's impossible to discern what IMG or HREF values might be 
direct CSRF attacks, or redirect users to unwittingly do dangerous things 
via CSRF redirects, the fix has to be in the applications that do the 
interesting things.

For example, if a user posted something along these lines:
Then the post would go through, under the name of whoever viewed the image.
This is of particular danger when an administrator views an image, which
then calls a page in an online control panel - thus granting the user access
to the control panel.

** Impossible to filter content

Right, and as I say, the site you act against can be somewhere else 
entirely. Here's what a CSRF attack might look like:
 <img src="http://example.net/logo.gif"; height=0 width=0 alt="">
That's it. When your client requests /logo.gif -- exposing no cookies -- the
example.net server redirects you to a URL like the one you show, above. So
the end result us the same as if the attacker had embedded the more obvious
URL inside the IMG tag. 

If an attacker wants, he can also use a simple, innocent looking hyperlink
and hope the victim clicks on it (http://example.net/kyotoanalysis.htm). You
don't allow hyperlinks? Well, someone might copy/paste the link, and be
stung that way. They'd notice? Maybe not -- the URL could be a mostly useful
page, with a tiny frameset sliver that loads your attack URL.

How can it be fixed? Well, there are a couple of ways to stop it, but the
easiest (in PHP at least) seems to be to have most of the variables used by
scripts be used through $HTTP_POST_VARS. So instead of checking for $action
in a script, $HTTP_POST_VARS['action'] would be checked. This forces the
user to use a POST request, not a GET. 

which means the attacker reverts to using Javascript, or entices the victim
to click on an image that's acting as a submit control in a <form>. 
Requiring POST raises the bar, but doesn't really fix the problem.

Alternatively, the sessionid could be
required to come with the GET/POST request variables, rather than by cookie.

...thereby exposing an important piece of authentication information to
history files and proxy servers; I really don't like URL mangling for
authentication purposes, especially in non-SSL systems. A combination of
cookie + URL mangling might not be bad, though in the message board case, a
CSRF attacker could use an intermediate redirect (as described earlier) to
get the URL mangling (from the Referer), and redirect back to the
messageboard with the proper mangling as well as all cookies that might be
expected/needed. So in your example case, URL mangling would buy nothing. :-(

Finally, in the specific case of [img] tags, the use of ? or & in the img
URL can be disabled by some regexes.

Not at all adequate. Browsers follow redirects on IMG tags, so I redirect
you to http://example.net/logo.gif which in turn redirects you to the final
URL, as described earlier.

If the software that you run is not secure, we recommend that you disable
HTML and/or [img] tags, until the fixes have been implemented.

It's much worse than that.

Please see the following URLs for an introduction to the dangers of CSRF, 
and some discussion of countermeasure strategies. 


** Server-Side Countermeasures

The fix MUST be implemented on the backend that's being attacked. In your 
example, newreply.cgi needs to be intelligent enough to detect and stop CSRF 

We've talked about how an attacker can post a message to the messageboard
with innocent looking URLs. But an attacker can also simply send the
victim a piece of HTML email including the full attack IMG URL. No amount
of IMG tag filtering in your messageboard posting system can stop that.

** Three-phase tests before acting

When it comes to generic CSRF attacks, any application that uses a
two-phase approach to action approval is vulnerable (the two phases being
[1] do you possess authentication information and [2] are all the required
arguments present). What's needed is a third test: is the user really
using a proper application form to generate the request?

** The 90% solution: Referer tests

For many sites, you can achieve a high level of protection by checking the
HTTP Referer header. This would prevent things like attacks via email. But
it would also mean locking out any user whose requests did not contain
Referer information.[1] As long as the values in the allowed Referer list
are all coded with XSS and CSRF in mind, this could be adequate.  Referer
checks should be as specific as possible, e.g. you might require the
Referer to begin with "https://example.com/admin/admin.cgi"; or
"https://example.com/admin/"; instead of simply "https://example.com/";.

** The more difficult cases

Some other applications are more difficult to secure. Consider webmail
apps. So webmail.example.com decides only "message delete" requests from
webmail.example.com pages will be accepted: well, if the attacker sends a
CSRF message to your webmail account, then when you read it via webmail,
the Referer in the CSRF image request (your client thinks it's an image
request) says it's indeed from the proper webmail server (even in the case
of an intermediate redirect; check the bugtraq archives for past
discussion of anonymizing hyperlinks, redirects vs. client-pull, etc.), so
the request gets through. Basically, any application that allows posting
of URLs needs more sophisticated protection than Referer checks. This
would also include messageboards and discussion sites like Slashdot.

Known Vulnerable: Infopop's UBB 6.04e (probably the whole 6.xx series),
ezboard 6.2, WWW Threads PHP 5.4, vBulletin 2.0.0 Release Candidate 2 and
before (later versions are safe). Probably many more bulletin boards and CGI
scripts out there, but those are the main ones that we have been tested

** One-time authorization codes

The URLs I list above outline a server-side one-use-token approach to
closing the hole. For instance, the page that users are expected to use
for drafting messages (in your newreply.cgi example) would create a
one-time use token, good for a limited time. The newreply.cgi processing
script would require this value be present, correct, and in time. So while
the attacker knows that action, subject, body, and authcode values are
required, the attacker does not know, and cannot ascertain, the proper
value needed for the authcode argument.[0] These tactics tend to introduce 
certain inconveniences (e.g., preventing use of the "back" button) so you 
may wish to analyze the various actions your application can take and 
provide varying levels of protection. For example, in a webmail system 
sending and deleting messages need more protection than displaying 

** Unpredicatable argument names?

Other tactics may be possible. For instance, consider
"action=newthread&subject=aaa&body=some+naughty+words&submit=go". On the 
server side, you could have an "argument map table" for each session, e.g. 
pick random surrogates for the normal argument names. For one user, the 
system might look for "876575665" as an argument name instead of the 
predictable "action", "9876dafd987" for "body", etc. There may be some 
tricks vis-a-vis anonymizing referers if the labels are constant 
throughout a session, but it might be possible to do something like this to 
make it more difficult to construct a valid URL for a CSRF malicious action.

** Attacking sites behind corporate firewalls

Want more fun? CSRF tactics can be used to attack servers behind corporate 
firewalls. It's not just your public Web apps that are at risk.
 <img src="http://intranet/admin/purgedatabase?rowslike=%2A&confirm=yes";>
If the attacker knows enough to make a URL and can get you to open a 
message, that's all it takes. Here we see that HTTP Referer headers can be 
a double-edged sword. Earlier we described how Referer tests can add 
security to many apps relatively easily. But Referer headers can also leak 
information about "private" sites if those sites use non-anonymized 
hyperlinks and external document references.

I'm afraid CSRF is going to be a mess to deal with in many cases. 
Like trying to tame the seas.

** Workarounds

Most of us probably depend on applications that won't be fixed
anytime soon. So what can you do to prevent a CSRF attack from making
your browser request something without your approval?
 - Do not use an email client that renders HTML
 - Do not use a newsgroup client tied to your Web browser
 - Do not allow your browser to save usernames/passwords
 - Do not ask Web sites you care about to "remember" your login
 - Be sure to "log off" before and after using any authenticated
   Web site that's important to you [or your employer ;-)], even
   if that means exiting your Web browser completely
 - Consider using something like Windows 2000's "Run As" shortcut 
   feature or my "runxas" shell script (available at the tux.org
   URL listed below) to run a Web browser for casual use

My apologies for the somewhat rambling nature of this post; I may yet
clean this up and put it in a proper paper, and do some real editing...
but I hope even in this rough form it makes some sense, and helps folks
design better, safer applications.


[0] Not unless the page that included the authcode is readable, e.g. if the 
composition page had XSS bugs that would facilitate construction of a URL 
for a CSRF attack.

[1] As discussed earlier (http://www.securityfocus.com/archive/1/41653),
client-pull pages usually result in no Referer information being sent by
the client. So if your application allows a request with no Referer, an
attacker need only direct the victim to an HTML document that uses a
client-pull META tag to send the victim to the CSRF attack URL. This might
be tricker to pull off, but remains feasible. So if you want to use
Referer checks, you really ought to go all the way and deny every request
that lacks a Referer header.

[2] http://www.stonehenge.com/merlyn/ [3]

[3] fellow cornfed users: the horror! footnotes referenced in reverse order!

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