This blog is written about a bug (I believe), that was found on my last weekend. It located on a website from a private program X on Hackerone.
That website has an API, allows users input their URL and email. In the backend, there is a simulated browser, which tries to open that URL and send the screenshot to user’s email. Actually, I don’t have any idea about that simulated browser, not sure if it is a real one or not. First thing, I inject a requestb.in URL, and the received User-Agent is
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.101 Safari/537.36
Personally, I am interested in SSRF bug, and this case I have the response, so, I thought that I might have some funs.
After the requestb.in URL, I tried my luck with:
The server accepts these URL, open it and send the result to my inbox. But the received responses are
Err: timeout for almost times.
The hard thing is the limited number of URLs to be inputted. So, I couldn’t do the brute-force to test Boolean-based.
OK. Now is the time for the local file:
These above payloads had been used to injected as URLs. However, this time, the server stopped me and said that they are invalid URLs.
I paused when there are not any ideas in my head.
That night, after a cup of Asahi beer, I decided to brainstorm again.
Is there anything else that I forgot?
http ? Yes.
file ? Yes.
chrome ? Yes…. How about other services?
ftp://example.com has been inserted. It worked. Then, I was confused about which keyword had been filtered on the server. I replaced
file , and it worked also. How so?
Yes. The problem is
/// , we only need a couple of
/ , if there are more than, the server would think it is an invalid URL. Then, I tried with
Index of page was in the response. Awesome!
Then, how about
/etc/passwd ? Hmm. The URL would be:
And that is my PoC.
I submitted the report to that program; unfortunately, they fixed and said that they aware of that issue and closed as Informative.
Actually, I still don’t really understand that decision, however, I don’t want to argue with triggers, then, it should be fine.
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