Rapid7 Advisory R7-0026: HTTP Header Injection Vulnerabilities in the Flash Player Plugin
Oct 17, 2006 - Two HTTP Header Injection vulnerabilities have been discovered by Rapid7 in the Flash Player plugin. They allow attackers to perform arbitrary HTTP requests while controlling most of the HTTP headers. This can make it easier to perform CSRF attacks  in some cases. When the HTTP server implements Keep-Alive connections and when Firefox is used, these Flash vulnerabilities can even be used to perform totally arbitrary HTTP requests where every part is controlled by the attacker: HTTP method, URI, HTTP version, headers, and data. Such attacks make use of the HTTP Request Splitting method.
- Flash Player plugin 9.0.16 (for Windows)
- Flash Player plugin 7.0.63 (for Linux)
- Earlier 9.0.x and 7.0.x versions
- 8.0.x versions
- Flash Player plugin BETA version 9.0.18d60 (for Windows)
Adobe Systems, Inc.
Sep 18, 2006
Adobe acknowledges reception of the vulnerability details.
Sep 29, 2006
Adobe responds with proposed dates for a fix later this year.
Oct 5, 2006
Adobe releases a fixed BETA version of Flash 9 for Windows (version 9.0.18d60, release files are named beta_100406).
Oct 17, 2006
Advisory is published after expiration of the 30-day grace period granted to Adobe to fix and disclose the vulnerabilities.
The vulnerabilities described hereafter have been successfully tested with the latest versions of Flash available for various platforms as of 2006/09/06, and with multiple combinations of browser/OS:
- IE6 SP2 (aka IE6 SV1) for Windows, with Flash plugin 9.0.16
- o Firefox 18.104.22.168 for Windows, with Flash plugin 9.0.16
- o Firefox 22.214.171.124 for Linux, with Flash plugin 7.0.63
Flash features a scripting language called ActionScript. ActionScript comes with a certain number of standard classes available to Flash developers. In particular, the send() method of the XML object can be used to send XML document trees to arbitrary URLs using, by default, a POST request. This, in itself, is not a vulnerability; the XML.send() method definitely complies with the Flash security model .
However another method defined in the XML class, addRequestHeader(), can be used to add arbitrary HTTP headers to the request performed by Flash. Its intended usage is:
var req:XML=new XML('test');
When calling req.send("http://host/path"), such a POST request would be submitted to 'host' (common HTTP headers that do not matter to us in this example have been removed for brevity):
POST /path HTTP/1.1
For security reasons, Flash 9 does not let developers use addRequestHeader() to set headers such as Host, Referer, or Content-Length.
But there is a way to get around this security restriction: the addRequestHeader() method does not sufficiently sanity check its two arguments. This makes it possible to inject arbitrary headers:
With IE, a request containing only the fake Referer is sent:
POST /path HTTP/1.1
With Firefox, a request containing boths the real Referer and the fake one is sent:
POST /path HTTP/1.1
Referer: (real referer)
For this attack to work, the first argument of addRequestHeader() must not contain any space (ASCII 0x20) else the Flash plugin appears to ignore the addRequestHeader() call. This is absolutely not a problem in real-world attack scenarios, because the space character usually present before the Referer value is optional (see RFC 2616 , section "4.2 Message Headers").
It is interesting to note that IE seems to post-process the headersgenerated by Flash before sending them to the HTTP server. Indeed, IE diligently removes the real Referer to use the Flash-generated one, and it even automatically adds the optional space character before the fakeReferer value.
Of course any cookie that would be associated with 'host' would beautomatically sent along with the request, which is another good thing for attackers.
For total control of the generated request, when the server supports keep-alive connections and when Firefox is used, it is possible to use the HTTP Request Splitting method to insert another HTTP request:
This generate what Firefox thinks is one request, while in fact the server interprets it as two separate requests because of the fake "Content-Length:0" header:
POST /path HTTP/1.1
Referer: (real referer)
POST /anotherpath HTTP/1.1
Tabs (ASCII 0x09) have to be used around the URI ("/anotherpath") instead of spaces (ASCII 0x20) else, as explained above, Flash ignores the addRequestHeader() call. But other than this minor inconvenient, pretty much any other character can be sent, absolutely nothing is URL-encoded, which gives plenty of freedom to attackers.
The unwanted data after "foo" (": bar\r\nContent-length: 4\r\n\r\ntest") do not create any problem at all either. Because the attacker-controlled "Content-Length:3" header takes care of instructing the server to ignore any subsequent data.
When trying to use this HTTP Request Splitting method with IE, it fails with a generic "The page cannot be displayed" error. This is probably due to the fact that when IE post-processes the headers (as explained previously), it messes up the manually built HTTP request and ends up with something that doesn't look like 2 valid requests at all. In particular, what seems to trigger this error is the CR LF CR LF sequence separating the headers from the body. This issue has not been investigated any more. More research is necessary to determine if exploiting IE this way is possible.
It should be pointed out that this new Flash 9 XML.addRequestHeader() vulnerability is similar to other, previously reported vulnerabilities in Flash 7 & 8 affecting the LoadVars class, as explained in this Bugtraq posting from Amit Klein . So, in a certain way, this new vulnerability re-opens a path of exploitation that was available to attackers in Flash 7 & 8.
The XML class defines the contentType attribute, which can be set by Flash developers, e.g.:
req.contentType = "text/plain";
The exact same vulnerability than the one described in the previous section also exists for this attribute: Flash does not check the validity of its value before building the HTTP request. It is possible to exploit it in the same way that addRequestHeader() is used:
req.contentType = "text/plain\r\nReferer: anything";
Contrary to addRequestHeader(), Flash allows spaces (ASCII 0x20) to be used in this string.
The consequence of these vulnerabilities is that attackers have a lot more control on the headers that get sent along with HTTP requests, compared to what is commonly thought possible. In particular the Host, Referer, and User-Agent headers can be spoofed. It is even possible to voluntarily generate malformed HTTP requests too, when the HTTP Request Splitting method is used.
But when combined with other flaws, such as XSS or CSRF, these Flash vulnerabilities become a handy tool to exploit them. The next section describes a CSRF attack scenario.
This vulnerability was discovered by Marc Bevand of Rapid7.
Description of CSRF Attacks
A Cross-Site Request Forgery (CSRF) is a form of attack where a malicious site A exploits the trust a site B has in a user by forging an HTTP request and sending it to site B, which sees it as coming from its trusted user. See  for a more detailled description of CSRF attacks.
The Flash vulnerabilities presented above can help attackers forge such HTTP requests.
Multiple approaches exist to prevent CSRF attacks, offering varying levels of protection. For example requiring POST requests instead of GET requests is a very poor way to protect against them. Other times, the protection chosen by Web site developers is to check for the HTTP Referer. The assumption is that spoofing the Referer header is much harder, but not impossible. These Flash vulnerabilities are able to spoof it.
A much better way to protect against CSRF attacks is to require the use of a security hash, as described in .
Let's say a malicious attacker is able to convince a user to visit his malicious Web site http://malicious. The intent of the attacker is to perform a CSRF attack against the user's bank Web site https://bank to silently transfer money from the user's account to the attacker's account. This attack assumes that (1) the bank uses SSL/TLS (HTTPS) combined with cookie-based authentication, (2) checks the Referer header to prevent CSRF attacks (hopefully no bank is doing this), and (3) that a money transfer order can be initiated via such an HTTP request:
POST /xfer.cgi HTTP/1.1
The attack also assumes the user is using Firefox, because we are going to use the CR LF CR LF sequence with addRequestHeader() to insert the HTTP body data (as explained in section 1.1 this sequence does not work with IE for an unknown reason).
We have to insert the HTTP body using addRequestHeader() because this way the body doesn't get URL-encoded by Flash (it is URL-encoded when passed as a string to the XML() constructor).
And we have to prevent URL-encoding in order to preserve the ampersands ("&") present in the body ("from=...&to=...").
So, in order to perform the attack, the attacker would have to place a Flash movie on http://malicious containing this ActionScript code:
var req:XML=new XML('x');
Then, when the user would visit http://malicious, assuming his browser still owns an unexpired cookie from the bank, this request would be sent via HTTPS (XML.send() allows an HTTP site to send data over HTTPS):
POST /xfer.cgi HTTP/1.1
Cookie: (bank cookie)
And the attack would succeed, despite SSL/TLS, despite the cookie auth, and despite the Referer check (if the application server takes into account the second Referer).
If URL-encoding of the HTTP body is acceptable, then IE can be targetted by not using the CR LF CR LF sequence, then only one Referer would be present in the headers.
Used the fixed BETA version (9.0.18d60). Only allow trusted Web sites to use Flash. Disable or uninstall the Flash plugin. Use alternative Flash plugins (GplFlash, Gnash).
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This advisory Copyright (C) 2006 Rapid7, LLC. Permission is hereby granted to redistribute this advisory, providing that no changes are made and that the copyright notices and disclaimers remain intact.