Tags: wappalyzer redos xss

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# Watchers (2 solves) — writeup by justCatTheFish

Last weekend, our team took part in a cool CTF — Pwn2Win. We've got a pretty nice score and managed to solve a few hard challenges. In this writeup, we will present a solution to the *Watchers* challenge that has been solved only by two teams. The challenge involved exploitation of not so common Regular Expression Denial of Service (ReDoS) technique combined with Cross-Site Scripting (XSS).

*The [author](https://twitter.com/vrechson) of the challenge released the [source code](https://github.com/pwn2winctf/challenges-2020/tree/master/web-Watchers) of the challenge so you can try it out locally.*

## Challenge

> Welcome to Static Web Host! During all these years of static website development, Rebellious Fingers found out that the biggest hole in our security is the use of insecure third-party applications. To help our members mitigate this problem, and not be compromised by our enemies, we decided to implement a third-party app to help them identify which technologies their pages are running so they can check whether there is anything vulnerable before deployment.
>
> Our service is still in beta, so please report any problems you find so our developers can analyze them.

![](./screenshots/website.png)

### Quick overview

There are three functionalities in the web application provided by authors:
- *bug report* that allows to send a URL of a generated page to the admin

Without exploiting any vulnerabilities, URLs of generated pages will not be displayed to the user, and because of that, *check service* and *bug reports* can't be fully utilised. This leads us to the first objective of the mission — **leak the URL of a generated page.**.

### Backend
Although in the challenge we weren't provided with a complete source code of the backend server, there was a link to the source of weaponlyzer function [./weaponlyzer.php](./weaponlyzer.php). In that code, we could see that the engine responsible for fetching metadata about the pages was a tool called [wappalyzer](https://github.com/AliasIO/wappalyzer) that was run with the code below, where $targets[$i] was the URL to one of the generated page: home, about or contact.

php
shell_exec('timeout -k 3s 20s wappalyzer -w 8 ' . escapeshellarg(escapeshellcmd($targets[$i])));


The most and only interesting part of the code is:

php
} elseif ($output == '') { return 'Couldn\'t analyze your file, please ask the admin to try weaponlyze in your page - URL: ' . substr(parse_url($targets[$i])["path"], 1); } else {  If calling shell_exec returns an empty string, we will see the URL of the page that triggered the error. In addition to that, the page and the following ones will not be removed from the disk which would happen otherwise. This additional observation will come handy later on. To achieve the goal of getting a URL of a generated page, we need to **make wappalyzer return nothing**. This can happen, for example, if the timeout command kills the process after exceeded time of 20 seconds. We confirmed that by lowering the timeout from original 20s to 1s and firing the wappalyzer on a constantly loading page. ### Content Security Policy One important addition that can be spotted in the provided screenshot is the code appended to each page. It contains a very restrictive *Content Security Policy* in the <meta> tag set to default-src: none. This prevents us from executing arbitrary JavaScript code and is most likely not bypassable. ### Wappalyzer Wappalyzer is generally known as a browser [extension](https://chrome.google.com/webstore/detail/wappalyzer/gppongmhjkpfnbhagpmjfkannfbllamg?hl=en) that identifies technologies used on websites. It has over 1M users in the store so one could say it's kind of popular. There is a variation of it in the form of a standalone [npm package](https://www.npmjs.com/package/wappalyzer) which was also used in the challenge. The recent versions use a headless browser for dynamic analysis of the page, which respects the CSP for code execution. The tool heavily relies on regular expressions, available [here](https://github.com/AliasIO/wappalyzer/blob/dfe686e524345145bd6286de2148b87d83b575d4/src/apps.json), which were leveraged by the challenge author to create a cool task. ### Defensive strategies We could potentially abuse the page loading time by crafting a very long payload. To prevent that, each page could maximally consist of 1500 characters. Another implemented protection was blocking *meta redirect* that was supposed to block redirects to other pages. We crafted an unintentional bypass to that with <meta http-equiv="refresh" content="1;https://example.org"> but we didn't find it useful in the final solution. The only advantage it gave us was discovering HTTP headers: Referer: http://localhost/page/xxx and User-Agent, indicating Headless Chrome 80 browser from which the admin visited. The third protection was a painful reCAPTCHA where we quickly ran out of available attempts. This was meant to prevent the application from being DDoS'ed through submissions. ### Front end The front-end was [Vue.js](https://vuejs.org/) application that mostly relies on the client-side code. Although there was nothing specific in the source code it guided us to which direction we should take. For instance, the snippet below hinted us that upon sending a link to the admin, it runs checks on it before displaying the page. That is because of two different types of a token in the parameters, probably for bypassing the reCAPTCHA verification. js // we assume it was for admin request.post('/check.php', { url: this.$route.query.url,
token: this.$route.query.token }) //and this was for us request.post('/check.php', { url: this.initial, 'g-recaptcha-response': this.token })  Thanks to that, if we managed to **trigger an XSS on the generated page**, we could execute JavaScript code on the admin's behalf, if only our assumptions were correct. Another hint that we discovered in the front-end code was the Vue's attribute v-html that injects raw HTML from the version to the page, without escaping it. html  This meant that if there was an XSS, it was most likely here. ## Solution ### Idea With the prior research it was clear that we have to perform two tasks: - attack the wappalyzer with [ReDos](https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS) attack - trigger XSS on the *check* endpoint and send it to the admin ### Vulnerabilities We manually researched [./apps.json](apps.json) for interesting regular expressionss from over 1300 different ones. Of course not one by one, but with some more or less clever assumptions. #### ReDoS ![](./screenshots/URL.png) Finding a regular expression that would leverage the ReDoS was the most challenging part. With some hard work and a little bit of luck, we found an interesting regular expression that could potentially lead to denial of the service. It was in the VideoJS component. json "VideoJS": { "cats": [ 14 ], "html": "<div[^>]+class=\"video-js+\">", "icon": "VideoJS.svg", "js": { "videojs.VERSION": "^(.+)$\\;version:\\1",
"videojs": "",
"VideoJS": ""
},
"script": [
"zencdn\\.net/c/video\\.js",
"^(?:https?:)?\\/\\/(?:[^\\/]+\\.)*cdnjs\\.cloudflare\\.com\\/ajax\\/libs\\/video\\.js\\/([\\d\\.]+)\\/\\;version:\\1"
],
"website": "http://videojs.com"
}


More specifically

re
/^(?:https?:)?\/\/(?:[^\/]+\.)*cdnjs\.cloudflare\.com\/ajax\/libs\/video\.js\/([\d\.]+)\/\;version:\1/


*Please note that the \;version:\1 part is only appended to retrieve version from the regular expression when parsing the code and therefore is not part of the executed expression.*

The regular expression can be visualized as:

![](./screenshots/redos.png)

We can see that there is a double loop where a dot character matches both None of: "/" and .. This is exactly what we need to perform backtracking based [ReDoS](https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS) attack. With the payload including enough dots, we can achieve our goal with the below snippet.

html
<script
</script>


We also found another vulnerable RegExp in Carbon Ads package which is almost identical to the above one, shown below.

json
"cats": [
36
],
"js": {
},
}


#### Pass the check

![](./screenshots/error.png)

We managed to retrieve the URL for our single note, but when we submit it to the *check* endpoint, we get a message *Couldn't analyze your file :(*.

With enough attempts, we could probably create a note that is executing near the allowed time and therefore returns two different results with the same note. But would the admin spend over 20 seconds to wait for the page to load? We doubted it and went into researching other possibilities despite the fact that this solution would be even more beautiful in our opinion :)

Now, the fact that other pages are not being removed comes really handy. We noticed that if we change the -home.html suffix in the URL of the pages/b699e61f9d5ce40a7caa8d7b3e66be98979a5b9f838073a833739c8b3f54d726/fd8f9ba6ede0-home.html page to -about.html we could see our second page. Viola, that's exactly what we need.

#### Getting the XSS

We've got all we needed but where was the XSS? We've analyzed a lot of expressions in the search for something that would allow us to inject <> characters. There are dozens of rules in form of (.+), for example the two below.

json
{
"js": {
"AFRAME.version": "^(.+)$\\;version:\\1" } }  or json { "meta": { "ajs-version-number": "^(.+)$\\;version:\\1",
}
}


Looks like simple injection right? We first tried with the code below but had no luck.

html
<script>
var AFRAME = {version: "123"}
</script>


Although it worked locally, we quickly discovered that because of the CSP, the code will not be evaluated on the challenge. This is because the driver [evaluates scripts](https://github.com/AliasIO/wappalyzer/blob/dfe686e524345145bd6286de2148b87d83b575d4/src/drivers/npm/driver.js#L386) from the page and then looks if the variable was defined.

js
const js = processJs(
await page.evaluate(getJs),
this.wappalyzer.jsPatterns
)


But the attached CSP blocks script evaluation and because of that, it will fail.

What about <meta> tag? Simple code below should get us an XSS, right?

html
<meta
name="ajs-version-number"
content="<u>123</u>"
>


Nope, it didn't work. After investigation we learned that meta tags are parsed with a simple [regex](https://github.com/AliasIO/wappalyzer/blob/dfe686e524345145bd6286de2148b87d83b575d4/src/wappalyzer.js#L185) that basically doesn't allow > character inside the <meta> tag.

js
const regex = /<meta[^>]+>/gi


With these two observations we stopped looking at the js and meta properties and focused on other resources. After some time we discovered the *AppDynamics* package with a very interesting RegExp.

json
"AppDynamics": {
"cats": [
10
],
"icon": "AppDynamics.png",
"website": "https://appdynamics.com"
},


The expression aboce is visualized on the image below.

![](./screenshots/xss_regex.png)

After adrum.1 prefix, we can add any payload as long as it ends with .js suffix. We tried a simple payload shown below but it returns a urlencoded string, unfortunately.

html
<script
</script>



Although it looks like a dead at the first glance, we knew a bypass to this already and which is a custom protocol that does not enforce percent-encoding. One of them is cid:, so by sending the below payload the alert pops out.

html
<script
</script>


### Summary

To sum up, we performed the following steps to retrieve the flag.

1. We submitted two pages to the server:
html
<script
</script>


and

html
<script
</script>



2. We got the path from the error message: pages/b699e61f9d5ce40a7caa8d7b3e66be98979a5b9f838073a833739c8b3f54d726/77f5aaead21d-home.html and sent the replaced one pages/b699e61f9d5ce40a7caa8d7b3e66be98979a5b9f838073a833739c8b3f54d726/77f5aaead21d-about.html to the admin.

3. We enjoyed the flag that arrived at our server directly from the admin's cookies ;)

*We recorded these steps in the form of a video: https://youtu.be/bXBSTmWqVys, enjoy!*

**CTF-BR{stackoverflow.com/revisions/1732454/11}**

Thanks to the authors for such an enjoyable puzzle!

Original writeup (https://github.com/justcatthefish/ctf-writeups/tree/master/2020-05-31-Pwn2win/web-watcher#watchers-2-solves--writeup-by-justcatthefish).