Intro to Open-source Bug Bounty Published by Arjun Shibu

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Hello mates, this is my first write-up.

Let’s keep it simple, most of the people who are reading this might have struggled a lot or is still fighting to find his/her first bug. Well I’ve a good news for you, it’s all about how you can earn some extra bucks with open-source bug bounty. Yeah, you heard that right - bounty for vulnerabilities in open-source projects.

Introducing, a bug bounty board for securing open-source code, it helps the open-source community to disclose and fix security issues and get paid to do it. I’ve been using it for few months and the experience is pretty amazing. I was able to disclose over 40 and fixed over 90 security issues (including npm packages that has 250k average downloads per week). 2 CVE IDs were assigned to me for my findings in packages nested-object-assign and apexcharts. Check them out here!

If you like reading code or want to secure the open-source code (but for some $$$😅), join huntr now!

Enough with that, let’s jump into one of my recent disclosures in huntr which was a Directory Traversal vulnerability in the repo of the well known newsletter platform Mailtrain.

I usually choose JavaScript/Node.js apps and packages as my targets (because I love JS😄, it’s best to try to hunt in languages/frameworks that you love). Github of course, and npmjs are my go-to resources for finding targets. For example if you want web apps built with express.js, search require('express') in github. Use filters to get recently active projects.

I was reading code of all the Mailtrain’s routes to identify available features. One of the route handlers is routes/reports.js source

78    router.get('/create', passport.csrfProtection, (req, res) => {
79      const reqData = req.query;
80      // ...
102       if (!reportTemplateId) {
103           res.render('reports/create-select-template', reqData);
104       }
105     // ...
115   });

If you are little bit familiar with express.js, you would say that the line 103 simply renders a template named create-select-template. And what about reqData? Yeah that’s right, it’s an object that has all the query string parameters passed to this route. So basically it is whatever a user gives as GET parameters.

But this simple line of code doesn’t seems to be vulnerable to anything right?

When I saw this, one of the write-ups I read few months ago came into my mind. If you are interested in knowing about the root cause of the issue, read it here (you’ll be amazed to see how a simple feature can be turned into a critical vulnerability if misused).

TL;DR: If an express server is using hbs as view engine for server-side rendering and it allowes user-constructed query string parameters without validation to get passed to express’s render() function, an attacker can use parameter called layout to read arbitrary files in the web server.

If you read the source above, you can see that it uses hbs. At this moment, I was sure it is vulnerable. To test it, I setup Mailtrain locally. Since access to this route requires authentication, the default creds from the repo can be used. With a little curiosity, I gave the layout parameter and got what I expected.

PoC 01

PoC 01

Yeah! Directory traversal🔥. With that, reading local files was easy as…

PoC 02

I quickly disclosed this issue and also opened a PR with fix. The fix is simple. Instead of allowing to pass all query string parameters, allow only what is necessary.

Change diff

Within few days, the maintainer of Mailtrain reviewed and accepted it. And I got the💰.

cyber secure

I’ve also requested a CVE for this bug and waiting to hear back from the CNA.

That’s all for now. Stay safe❤️.

See Arjun's other disclosures and fixes:


Connect with Arjun:

0xsegf#8291 on huntr Discord

Disclose on huntr