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Cross-origin brute-forcing of Github SAML and 2FA recovery codes

Yesterday while reading my Twitter stream I found this interesting article about  downloading GitHub SSO bypass codes. Same as Yasin Soliman I was invited to a Github pre-release of the organisation SAML single sign-on (SSO) private program. And same as him I found an issue in the same endpoint. So I thought to write a quick blog post about it.
Github already published a tl;dr about this,




 I will try to fill the blanks here.

As mentioned by Yasin, Github offers an endpoint where privileged users can recover bypass codes. These recovery codes were accessible for download as plaintext and had the content-type as text/plain , something like:




What immediately caught my attention was that the format of the code forms (with some exceptions) a valid JavaScript file with lines in the format of XXXXX-XXXXX, ten hex digits separated by a hyphen. This is interpreted in JavaScript as the subtraction of two variables! This remember an old blog post of mine where I could possibly exfiltrate information from properties file formatted in a peculiar way. And another great blog post: Plain text considered harmful: A cross-domain exploit.
So I thought I could do something similar here. It did not take long until I found the right approach.

Caveat #1: also in this case Github sets the X-Content-Type-Options: nosniff to prevent browsers from interpreting this content as valid JavaScript or other file types. But while Firefox now added support for nosniff the browser compatibility is still spotty (I am looking at you Safari!!).

But without waiting any further HERE is the live POC. The nut of the trick is to define a valueOf function for the corresponding variable:




and enumerate them all!!

Caveat #2: We are talking about enumerating/brute-forcing 5 hex digit variables that requires a considerable effort, but is far from be unfeasible. A rough calculation tells us that we need to define about 16^5 variables that are about 1048576!

Caveat #3: not all the codes are valid Javascript variable (e.g. the one starting with a number are not). For a random hexadecimal digit that's six out of sixteen, thus a 37.5% chance.

Disclosure timeline 

06-03-2017 - Reported the issue via Hackerone.
07-03-2017 - Github triaged the issue. 
16-03-2017 - Bounty awarded

Acknowledgement

I would like to thank the Github security  team, you guys rock, really!!


Well that's all folks. For more Javascript trickery follow me on Twitter.







Comments

Unknown said…
Nice Find !

I Wonder why Safari Doesn't Support's X-Content-Sniffing Header ?

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