In late January, I have found and reported a Server-Side Request Forgery (SSRF) vulnerability on toolbox.googleapps.com to Google’s VRP, which could be used to discover and query internal Google DNS servers to extract all kinds of corporate information like used internal IP addresses across the company as well as A and NS records exposing all kinds of hosts like Google’s Active Directory structures and also a fancy Minecraft server! :-) So here’s a quick write-up about the vulnerability.

As you might already know, the G-Suite Toolbox can be used to perform all kinds of trouble-shootings. Among all the available tools, there is one called “Dig” which - on Linux - can be used to query a DNS server for its records of a given domain, just like A- or MX records. In this case Google implemented a nice web interface for that tool to visually lookup DNS information. While it looks like a useful tool to query DNS servers from a Google perspective…

…the “Name server” field probably raises the attention of every bug hunter ;-) While playing around and trying to query 127.0.0.1 for DNS records, the web application simply responds with a “Server did not respond message”:

So it really looks like the tool is trying to connect to 127.0.0.1:53 to fetch the DNS information for my domain - this really smells like a Server Side Request Forgery, doesn’t it?

Ok Google, give me a responding internal DNS server!

Thanks to BurpSuite’s intruder, brute-forcing for responding IP addresses can be quickly automated by instrumenting the corresponding HTTP POST “nameserver” parameter:

POST /apps/dig/lookup HTTP/1.1 Host: toolbox.googleapps.com User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:51.0) Gecko/20100101 Firefox/51.0 Accept: application/json, text/javascript, */*; q=0.01 Accept-Language: en-US,en;q=0.5 Content-Type: application/x-www-form-urlencoded; charset=UTF-8 X-Requested-With: XMLHttpRequest Referer: https://toolbox.googleapps.com/apps/dig/ Content-Length: 107 Cookie: csrftoken=NE5nKGrbPNRoEwm0mahDzop9iJfsxU4H; _ga=GA1.2.2102640869.1486420030; _gat=1 Connection: close csrfmiddlewaretoken=NE5nKGrbPNRoEwm0mahDzop9iJfsxU4H & domain=www.rcesecurity.com & nameserver=§127.0.0.1§ & typ=a

Some minutes later I’ve found one promising internal IP, which responded to the request - however just with an empty A record for my domain:

Since I do know about my own domain very well, it’s even more interesting to find out whether it’s possible to extract some internal information from Google, which are not publicly available.

Ok Google, give me your internal domain name!

Et voila. Found something here. It seems like Google is using “corp.google.com” as their corporate domain - at least some of their tools including one called “MoMa - Inside Google” is hosted under that domain. Now you could either brute force all subdomains of “corp.google.com” using the very same POST request to discover more subdomains (that’s what I actually did), or just google for them, which will reveal an interesting A record called “ad.corp.google.com”.

Ok Google, just give me all your A records for “ad.corp.google.com”!

This looks even more interesting in comparison to what is available on public DNS records:

dig A ad.corp.google.com @8.8.8.8 ; << >> DiG 9.8.3-P1 << >> A ad.corp.google.com @8.8.8.8 ;; global options: +cmd ;; Got answer: ;; ->>HEADER <<- opcode : QUERY, status: NXDOMAIN, id: 5981 ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0 ;; QUESTION SECTION: ;ad.corp.google.com. IN A ;; AUTHORITY SECTION: corp.google.com. 59 IN SOA ns3.google.com. dns-admin.google.com. 147615698 900 900 1800 60 ;; Query time: 28 msec ;; SERVER: 8.8.8.8#53(8.8.8.8) ;; WHEN: Wed Feb 15 23:56:05 2017 ;; MSG SIZE rcvd: 86

This is now very internal!

Ok Google, give me the NS records (and their internal IPs) associated with that domain!

Ok Google, let’s get more specific. Give me information about the “gc._msdcs”!

Ok Google, anything else you want me to see?

After reporting this vulnerability to Google via their VRP, they quickly fixed this vulnerability. Thanks Google for the nice bounty!