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Posted Jul 26, 2006
Authored by Amit Klein

Whitepaper titled "Forging HTTP Request Headers With Flash".

tags | paper, web
SHA-256 | ea05b3536fe449fc3fedd3dda363fbd5f77eefea62b709a6e4e00a23c016c940


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           Forging HTTP request headers with Flash

Amit Klein, July 2006

Flash - Introduction

Flash player is a very popular browser add-on from Adobe
(actually, Flash was invented by Macromedia, which was acquired
by Adobe). This write-up covers mostly Flash 7 and Flash 8,
together covering more than 94% of the Internet-enabled desktops
(according to NPD Online Survey conducted April 2006, quoted in
Adobe's website [1], [2]). Flash movies are delivered as SWF
(ShockWave File) files. Adobe developed a rich Javascript-like
language called ActionScript to provide scripting capabilities to
Flash. One of the interesting features of ActionScript is its
ability to send HTTP requests to 3rd party sites through the
browser which invoked it. This is where Flash becomes interesting
security-wise. With Flash it is possible to shape an outgoing
request to a 3rd party site in ways not available from within
"standard" Javascript. Specifically of interest is Flash's
ability to send arbitrary HTTP request headers with outgoing HTTP

Sending arbitrary HTTP request headers with Flash

The following is ActionScript 2.0 syntax for sending out a GET
request (in this example, to
http://www.vuln.site/some/page.cgi?p1=v1&p2=v2) with an arbitrary
HTTP header (Foo: Bar). This code works with Flash 7 and Flash 8
(probably with Flash 6 as well):

var req:LoadVars=new LoadVars();

A similar syntax will send POST request (with the same header, to
the same URL, and with body a=b&c=d):

var req:LoadVars=new LoadVars();

(note: the LoadVars.decode() method was added in Flash 7, yet
it's probably possible to compose an arbitrary POST body without
it, so Flash 6 may be covered as well by this variant).

The request is sent from the browser invoking the Flash object.
Any cookies the browser normally sends, will be sent in those
cases as well. The browser's User-Agent is sent, as well as all
browser standard headers. HTTPS links are supported.

This was successfully demonstrated with Microsoft IE 6.0,
Microsoft IE 6.0 SP2 and FireFox, running Flash
and Flash

In IE, it is possible to overwrite some "normal" browser headers
by simply calling addRequestHeader with the new value. This is
applicable to both Referer and User-Agent. In FireFox,
such headers, when used in addRequestHeader() will be appended to
the HTTP request header section.

// One UA in IE 6.0 SP2, two UAs in FF

// One Referer in IE 6.0 SP2, two Referers in FF

In IE, it is also possible to overwrite some more sensitive
headers (e.g. Host and Content-Length) by appending colon to the
header name (this technique was described in [3] in the context
of XmlHttpRequest):


This technique doesn't appear to work in FireFox

Note: when the target URL is in the same domain with the Flash
movie, LoadVars may be used to read the HTTP response data, thus
making LoadVars the basis for many Flash-based AJAX-like
frameworks (in analogy to Javascript's XmlHttpRequest object).

The security implications

The ability of an attacker to force a victim's browser to send
HTTP requests to 3rd party sites with arbitrary HTTP request
headers has impact on our understanding of web application
security - both on assessment of security-related phenomena, and
on the validity of some security mechanisms.

It is important to understand that the attacks described here are
(in themselves) not cross-site scripting attacks, neither are
they (strictly speaking) any breach of cross-domain trust in the
Flash object or between the Flash object and the embedding HTML
page. They merely make use of the fact that it's possible to send
requests from a Flash object to any URL, with almost any HTTP
headers the attacker needs. This in itself is the problem, as it
enables an attacker to send a link (to an HTML page embedding a
Flash object, or directly to a Flash object residing at the
attacker's website) that will cause a Flash object to be executed
in the victim's browser. This Flash object will send the HTTP
request (with HTTP headers chosen by the attacker) to a target
website, and this in turn will compromise the security of the
browser (victim).

In other words, the implicit assumption made by many software
developers (and probably also by many security researchers) that
most HTTP headers cannot be forced to have arbitrary values by an
attacker who serves data to the victim browser is shown to be in
error in this write-up.

Example 1 - The "Expect" header

In [4], an injection problem was described wherein Apache 1.3.34,
2.0.57 and 2.2.1 are vulnerable to injecting HTML data (including
malicious Javascript code) through the Expect header. In [5],
yours truly commented, with respect to this issue, that
"Regarding XSS This phenomenon is not XSS per-se. Unless someone
can show me how it is possible to force a browser to send the
Expect header to the target site". But using a Flash object, the
attack is trivial (the author is therefore answering his own
question here...). Consider a victim (browser) that clicks the
following link:


This URL represents a Flash object that runs the following
ActionScript code:

var req:LoadVars=new LoadVars();

This ActionScript sends a request from the victim's browser, to
the target website (www.target.site) with an Expect header
containing malicious HTML (Javascript) code. If the target
website runs a vulnerable version of Apache, the net result is
cross site scripting. So to be clear - there's a working XSS
attack against Apache 1.3.34, 2.0.57 and 2.2.1 (as long as the
client browser is IE or Firefox, and it supports Flash 6/7+). As
noted in [5], for Apache 2.0/2.2 the XSS response is returned by
the server only after the request timeout elapses (typically few
minutes). Please note though that a fix for the Apache server is
available at all 3 branches (as Apache 1.3.35, 2.0.58 and 2.2.2

Example 2 - CSRF and Referer

CSRF (Cross Site Request Forgery) attack is in essence the
ability of an attacker to force a victim (browser) to effectively
perform an action (send HTTP request) in a target site. This
concept emerged several times, the first one probably on the Zope
mailing list (as "Client Side Trojans", [6], and another time on
BugTraq, under its now de-facto standard name, CSRF [7]. Both
references suggest, among other measures, to rely on the HTTP
Referer header as evidence that the browser emits the HTTP
request from a link originating in the website. Indeed,
considering the capabilities of HTML+Javascript, effectively
spoofing the Referer is nearly impossible (one exception is [8],
but it is effective only in a limited subset of scenarios; e.g.
it is not effective when HTTPS is used). However, as can clearly
be understood, with Flash this no longer holds, and Referer can
be spoofed for requests to HTTPS resources, all the while having
the browser send the site's cookies with the request. As
demonstrated above, both GET requests (with arbitrary host and
query parts) and POST requests (with arbitrary host and query
parts, and body in the standard Content-Type format of
"application/x-www-form-urlencoded") can be sent.

Note: there are many other reasons not to rely on the Referer
header, and this text is by no means the first one to warn
against this practice.

It should be obvious that any reasonable header (and combinations
thereof) can be spoofed. In IE's case, the header provided by the
attacker can replace the browser-provided header (e.g. Referer).
In Firefox's case, the Referer spoofing technique may fail
because Firefox adds the header at the bottom of the HTTP request
headers. Still, some web applications may use the last value of
the header, and as such be vulnerable to this technique.

Plainly put, all this means that (reflective) cross site
scripting attacks that make use of HTTP request headers (e.g.
Referer, User-Agent, Expect, Host, Content-Type) to send the
payload are now possible.

Flash 9

Flash 9 was announced June 28th, 2006 [9] (i.e. less than a month
ago). In Flash 9, the techniques described above (for the
LoadVars class) do not work for any browser-provided header (e.g.
User-Agent, Host and Referer), nor probably for many "protected"
headers such as Content-Length. Still, headers like Expect can be
sent, so some attacks (e.g. Example 1 above) are still effective
with Flash 9.

Limitations of the technique

- The URL and the body part will always be URL-encoded. That
is, it is impossible (so it seems) to force SP, HT, CR and
LF (and probably many other characters) to appear in their
raw form in the request URL and body.

- Only GET and POST methods can be used.

- In IE, only one instance of each header can be sent.

- At large, the header section cannot be completely
controlled, e.g. an attacker may have problems when
attempting to send special characters inside headers.

Partial solution

Notice the first limitation of the technique - it states that no
raw CR and LF can be placed in the body section. This means that
the technique cannot be used to send (POST) requests whose body
complies with the "multipart/form-data" content-type format (this
format uses raw CRs and LFs to mark headers and boundaries). In
other words, a (POST) request whose body is a valid
"multipart/form-data" stream is guaranteed (as far as today's
knowledge extends) not to be sent from a Flash player. Web
application authors can therefore use HTML forms whose ENCTYPE
attribute is set to "multipart/form-data", and enforce that the
submission contains a valid multipart/form-data body. Once these
mechanisms are in place, and a request passes through, it is
known not to originate from a Flash player, so the attack
described here is irrelevant.

This solution is of course intrusive - both the HTML pages and
the receiving scripts must be altered to use (and enforce)
multipart/form-data. With some web development platforms, this is
trivial, yet in others (e.g. raw Perl, and ASP) it is not.
Furthermore, in cases such as Example 1 above, the HTTP headers
are interpreted and used by the (Apache) web server, and the
control never reaches the web application layer, so in such
cases, this solution is not applicable.

Future directions

HTTP Request Splitting ([8]) and HTTP Request Smuggling ([10]) -
in IE + Flash 7/8 it is possible to send a Content-Length header
with any value. This opens up an opportunity to perform HTTP
Request Splitting attacks. Also, injecting Transfer-Encoding
header, or a second Content-Length header may open up HTTP
Request Smuggling. Further research is needed in order to
understand whether these directions lead to viable exploitation

Flash 9 - while experiments show that Flash 9 is more strict
concerning which headers can be specified through
LoadVars.addRequestHeader(), it's ActionScript 3.0 language is
much richer than ActionScript 2.0. As such, it may open up
several interesting opportunities at the HTTP level, e.g. the
ability to send various HTTP methods, not just GET and POST
(WebDAV anyone?). Flash 9 and ActionScript 3.0 should be studied
better in order to understand what is their potential with
respect to crafting HTTP requests.


Relying on the authenticity of HTTP request headers when they are
sent from a browser is not a good idea. Practically every header
can be spoofed if the client can be forced to run a malicious
Flash movie, and this is probably applicable to over 80% of the
Internet desktops (i.e. Internet desktops running IE + Flash
7/8). Consequently, the applicability of cross site scripting
attacks based on such headers, as well as cross site request
forgery attacks (against sites which protect themselves by
checking the Referer header) is higher than many people may

A partial solution to the latter case was suggested in the write-
up, however it is strongly tied to the specific technology used -
Flash, and as such may not provide any protection against
variants that make use of a different technology, or against
developments in Flash itself. For cross-site request forgery,
therefore, different solutions, not relying on Referer, should be


[1] "Technology Breakdown" (Adobe website)

[2] "Macromedia Flash Player Version Penetration" (Adobe website)

[3] "Re: 'Exploiting the XmlHttpRequest object in IE' - paper by
Amit Klein" by Anonymous, BugTraq posting, September 27th, 2005

[4] "Unfiltered Header Injection in Apache 1.3.34/2.0.57/2.2.1"
by Thiago Zaninotti, BugTraq posting, May 8th, 2006

[5] "Re: Unfiltered Header Injection in Apache
1.3.34/2.0.57/2.2.1" by Amit Klein, BugTraq posting, May 18th, 2006

[6] "Client Side Trojans", Zope developers mailing list, May 2000

[7] "Cross Site Request Forgeries" by Peter Watkins, BugTraq
posting, June 15th, 2001

[8] "Exploiting the XmlHttpRequest object in IE - Referrer
spoofing, and a lot more..." by Amit Klein, BugTraq posting,
September 24th, 2005

[9] "Adobe Flash Player 9 Leads a New Generation of Dynamic Media
and Rich Internet Applications" (Adobe website), June 28th, 2006

[10] "HTTP Request Smuggling" by Chaim Linhart, Amit Klein, Ronen
Heled and Steve Orrin, June 6th, 2005

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