Mysterious Facebook Code

var org_str = "j8ck72di";
var session_str = "4734a9fc27f7fee1aa58f66046af6c49";
var base_str = "https://ct-m-fbx.fbsbx.com/fp";
var page_id = "1";
var ip_addr_str = "820139e7306525d7";
var tarpitting_param = "";
var carrier_id_enabled = "false";
var flash_tags = "true";

var xx0=unescape('var%20thm_tags%3d%7binjectIframe%3afunction%28org_id%2csession_id%2cpage_id%29%7bvar%20dom%2cdoc%2cwhere%2ciframe%3ddocument%2ecreateElement%28%27iframe%27%29%3biframe%2esrc%3d%22javascript%3afalse%22%3b%28iframe%2eframeElement%7c%7ciframe%29%2estyle%2ecssText%3d%22width%3a100px%3bheight%3a100px%3bborder%3a0%3bposition%3aabsolute%3btop%3a%2d5000px%3b%22%3bwhere%3ddocument%2egetElementById%28%22thm_iframe_loc%22%29%3bif%28%21where%29%7bdocument%2ebody%2eappendChild%28iframe%29%3b%7delse%7bwhere%2eparentNode%2einsertBefore%28iframe%2cwhere%29%3b%7dtry%7bdoc%3diframe%2econtentWindow%2edocument%3b%7dcatch%28e%29%7bdom%3ddocument%2edomain%3biframe%2esrc%3d%22javascript%3avar%20d%3ddocument%2eopen%28%29%3bd%2edomain%3d%27%22%2bdom%2b%22%27%3bvoid%280%29%3b%22%3btry%7bdoc%3diframe%2econtentWindow%2edocument%3b%7dcatch%28e%29%7biframe%2esrc%3dbase_str%2b%22%2ftags%3fjs%3d1%26org_id%3d%22%2borg_id%2b%22%26session_id%3d%22%2bsession_id%2b%22%26pageid%3d%22%2bpage_id%3breturn%3b%7d%20%7ddoc%2eopen%28%29%2e_l%3dfunction%28%29%7bif%28typeof%20this%2ereadyState%3d%3d%3d%22undefined%22%7c%7ctypeof%20this%2ereadyState%3d%3d%3d%22unknown%22%29%7bthis%2ereadyState%3d%22complete%22%3b%20%7dif%28dom%29%7bthis%2edomain%3ddom%3b%7dvar%20divx%2cparam1%2cparam2%2cobj%2cswf_url%2cwin%2cimg%2cjs%2cu%2cp%2cdiv%3dthis%2ecreateElement%28%27p%27%29%3bthis%2ebody%2eappendChild%28div%29%3bdiv%2estyle%2ebackground%3d%22url%28%22%2bbase_str%2b%22%2fclear%2epng%3forg_id%3d%22%2borg_id%2b%22%26session_id%3d%22%2bsession_id%2b%22%26m%3d1%26w%3d%22%2bip_addr_str%2btarpitting_param%2b%22%29%22%3bimg%3dthis%2ecreateElement%28%22img%22%29%3bimg%2esrc%3dbase_str%2b%22%2fclear%2epng%3forg_id%3d%22%2borg_id%2b%22%26session_id%3d%22%2bsession_id%2b%22%26m%3d2%22%2btarpitting_param%3bthis%2ebody%2eappendChild%28img%29%3bjs%3dthis%2ecreateElement%28%22script%22%29%3bjs%2esrc%3dbase_str%2b%22%2fcheck%2ejs%3forg_id%3d%22%2borg_id%2b%22%26session_id%3d%22%2bsession_id%2btarpitting_param%2b%22%26pageid%3d%22%2bpage_id%3bthis%2ebody%2eappendChild%28js%29%3bif%28carrier_id_enabled%3d%3d%3d%22true%22%29%20%7bvar%20ciddoc%2ccidframe%3ddocument%2ecreateElement%28%22iframe%22%29%3bcidframe%2esrc%3d%22javascript%3afalse%22%3b%28cidframe%2eframeElement%7c%7ccidframe%29%2estyle%2ecssText%3d%22width%3a100px%3bheight%3a100px%3bborder%3a0%3bposition%3aabsolute%3btop%3a%2d5000px%3b%22%3bdocument%2ebody%2eappendChild%28cidframe%29%3btry%7bciddoc%3dcidframe%2econtentWindow%2edocument%3b%7dcatch%28e%29%7bcidframe%2esrc%3d%22javascript%3avar%20d%3ddocument%2eopen%28%29%3bd%2edomain%3d%27%22%2bdocument%2edomain%2b%22%27%3bvoid%280%29%3b%22%3bciddoc%3dcidframe%2econtentWindow%2edocument%3b%7dciddoc%2eopen%28%29%2e_l%3dfunction%28%29%7bvar%20cidscript%3dthis%2ecreateElement%28%22script%22%29%3bcidscript%2esrc%3dbase_str%2b%22%2fcheckcid%2ejs%3forg_id%3d%22%2borg_id%2b%22%26session_id%3d%22%2bsession_id%3bthis%2ebody%2eappendChild%28cidscript%29%3b%7d%3bciddoc%2ewrite%28%27%3cbody%20onload%3d%22document%2e_l%28%29%3b%22%3e%27%29%3bciddoc%2eclose%28%29%3b%7dif%28flash_tags%3d%3d%3d%22true%22%29%20%7bu%3dnavigator%2euserAgent%2etoLowerCase%28%29%3bp%3dnavigator%2eplatform%2etoLowerCase%28%29%3bwin%3dp%3f%2fwin%2f%2etest%28p%29%3a%20%2fwin%2f%2etesft%28u%29%3btry%7bie%3d%21%2b%22%5cv1%22%3bif%28%21ie%29%7bie%3d%2f%2a%40cc_on%21%40%2a%2ffalse%3b%7d%20%7dcatch%28e%29%7b%7dif%28%21ie%29%7bie%3d%28navigator%2euserAgent%2eindexOf%28%27MSIE%27%29%3e%20%2d1%29%3b%7dswf_url%3dbase_str%2b%22%2ffp%2eswf%3forg_id%3d%22%2borg_id%2b%22%26session_id%3d%22%2bsession_id%2btarpitting_param%3bif%28ie%26%26win%29%7bdivx%3dthis%2ecreateElement%28%22div%22%29%3bdivx%2einnerHTML%3d%27%3cobject%20type%3d%22application%2fx%2dshockwave%2dflash%22data%3d%22%27%2bswf_url%2b%27%22width%3d%221%22height%3d%221%22%3e%3cparam%20name%3d%22movie%22value%3d%22%27%2bswf_url%2b%27%22%2f%3e%3cparam%20name%3d%22wmode%22value%3d%22transparent%22%2f%3e%3c%2fobject%3e%27%3bthis%2ebody%2eappendChild%28divx%29%3b%7delse%7bobj%3dthis%2ecreateElement%28%27object%27%29%3bobj%2esetAttribute%28%22type%22%2c%22application%2fx%2dshockwave%2dflash%22%29%3bobj%2esetAttribute%28%22data%22%2cswf_url%29%3bobj%2esetAttribute%28%22width%22%2c%221%22%29%3bobj%2esetAttribute%28%22height%22%2c%221%22%29%3bparam1%3dthis%2ecreateElement%28%27param%27%29%3bparam1%2esetAttribute%28%22name%22%2c%22movie%22%29%3bparam1%2esetAttribute%28%22value%22%2cswf_url%29%3bparam2%3dthis%2ecreateElement%28%27param%27%29%3bparam2%2esetAttribute%28%22name%22%2c%22wmode%22%29%3bparam2%2esetAttribute%28%22value%22%2c%22transparent%22%29%3bobj%2eappendChild%28param1%29%3bobj%2eappendChild%28param2%29%3bthis%2ebody%2eappendChild%28obj%29%3b%7d%20%7d%7d%3bdoc%2ewrite%28%27%3cbody%20onload%3d%22document%2e_l%28%29%3b%22%3e%27%29%3bdoc%2eclose%28%29%3b%7d%2cgo%3afunction%28%29%7bif%28session_str%26%26org_str%29%7bvar%20isWebkit%3d%27WebkitAppearance%27in%20document%2edocumentElement%2estyle%3bif%28document%2ebody%26%26%28document%2ereadyState%3d%3d%3d%27complete%27%7c%7c%21isWebkit%29%29%7bthis%2einjectIframe%28org_str%2csession_str%2cpage_id%29%3breturn%3b%7dvar%20waittime%3d200%3bvar%20node%3bif%28typeof%20window%21%3d%3d%22undefined%22%26%26typeof%20window%21%3d%3d%22unknown%22%26%26window%21%3d%3dnull%29%7bnode%3dwindow%3b%7delse%7bnode%3ddocument%2ebody%3b%7dif%28node%2eaddEventListener%29%7bnode%2eaddEventListener%28%22load%22%2cfunction%28%29%7bthm_tags%2einjectIframe%28org_str%2csession_str%2cpage_id%29%3b%7d%2cfalse%29%3b%7delse%7bif%28node%2eattachEvent%29%7bnode%2eattachEvent%28%22onload%22%2cfunction%28%29%7bthm_tags%2einjectIframe%28org_str%2csession_str%2cpage_id%29%3b%7d%29%3b%7delse%7bvar%20oldonload%3dnode%2eonload%3bnode%2eonload%3dnew%20function%28%29%7bvar%20r%3dtrue%3bif%28oldonload%21%3d%3dnull%26%26typeof%20oldonload%3d%3d%3d%22function%22%29%7br%3doldonload%28%29%3b%7dsetTimeout%28function%28%29%7bthm_tags%2einjectIframe%28org_str%2csession_str%2cpage_id%29%3b%7d%2cwaittime%29%3bnode%2eonload%3doldonload%3breturn%20r%3b%7d%3b%7d%20%7d%7d%20%7d%7d%3bthm_tags%2ego%28%29%3b%20');eval(xx0);

Thought I'd jump in here. I did a little bit of clean up and renaming. I redacted and removed a lot of non-operative or obfuscated path code. It's a little more comprehensible, but not wonderful. I pointed the original Facebook and Online Metrix collection URLs to 127.0.0.1 and ran an echo web server to inspect the payloads (a little easier to just let it run, than to probe the code line-by-line).

https://pastebin.com/5wnVZHbK

Some fun stuff I noticed that most of you may have already:

• The hex junk is more or less just obfuscated string data utilized by way of a method call parent.td_f(offset, numChars). It contains things like MIME type strings, some JavaScript keywords, and a s sprinkle of human readable error messages that are never logged (unless you define a logger callback where possible).

• Neat obfuscation tricks:
  Number(890830).toString(31) === 'true'
Number(103873).toString(18) === 'head'

• Most payloads are hashed with MD5 before egress. MD5 was implemented in the raw in the original JavaScript code, and I extracted the implementation here:

  https://pastebin.com/0eFX7ba1

• The WebSocket "port scanner" is really interesting! Appears to glean based on whether onError or onClose with reason was fired.

• The system font signature is generated by looking for discrepancies in the Canvas 2D rendering context's metrics of the default mono and serif fonts with a giant list of possible system fonts for each of Windows, Linux, and OS X. Fonts that don't render with equal widths are added to a list that is eventually hashed.

Contacted Facebook about this years ago, and received prebaked "We appreciate your feedback" response.