Skip to main content

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


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.


Arbazz Hussain said…
Nice Find !

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

Popular posts from this blog

Slack SAML authentication bypass

tl;dr  I found a severe issue in the Slack's SAML implementation that allowed me to bypass the authentication. This has now been solved by Slack.
Introduction IMHO the rule #1 of any bug hunter (note I do not consider myself one of them since I do this really sporadically) is to have a good RSS feed list.  In the course of the last years I built a pretty decent one and I try to follow other security experts trying to "steal" some useful tricks. There are many experts in different fields of the security panorama and too many to quote them here (maybe another post). But one of the leading expert (that I follow) on SAML is by far Ioannis Kakavas. Indeed he was able in the last years to find serious vulnerability in the SAML implementation of Microsoft and Github. Usually I am more an "OAuth guy" but since both, SAML and OAuth, are nothing else that grandchildren of Kerberos learning SAML has been in my todo list for long time. The Github incident gave me the final…

CSRF in Facebook/Dropbox - "Mallory added a file using Dropbox"

tl;dr  Facebook Groups offers the option to upload files directly from the Dropbox account. This integration is done using the OAuth 2.0 protocol and suffered from a variant of the classic OAuth CSRF (defined by Egor Homakov as the the Most Common OAuth2 Vulnerability),  see video below:

Introduction  Facebook Groups offers the option to upload files directly from the Dropbox account:

This will allow to surf via browser the Dropbox account 

and post a specific file to the group.  This integration is done using a variant of the OAuth 2.0 protocol seen in this blog many many times. But once more, OAuth is an access delegation protocol standardized under the IETF umbrella. A typical OAuth flow would look like:
Usually the client initiates the OAuth flow in the following way:

then after that the resource owner has authorized the client the authorization server redirects the resource owner back to the client with an authorization code:
Then the OAuth dance continues....
Facebook/Dropbox i…

Meh : CSRF in Facebook Delegated Account Recovery

Note this is going to be a quick post.

This year, at Enigma 2017 Conference, Facebook introduced a way to move Account Recovery beyond Email and the "Secret" Question.
After the presentation the moved operationally and presented the first integration partner : Github.

These days I have seen a lot of press around this and both Facebook and Github open sourced their implementation and specification (also presented at F8).
Well it turned out that Facebook side was susceptible to Cross Site Request Forgery.
Really simple explanation:

The attacker start the integration with Github and stop the flow at the right moment. The create an attacker page as
<img src="…