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Merge pull request #557 from aidantwoods/documentation/safe-mode

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Talk about safe mode in the README
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@ -38,7 +38,32 @@ More examples in [the wiki](https://github.com/erusev/parsedown/wiki/) and in [t
### Security
Parsedown does not sanitize the HTML that it generates. When you deal with untrusted content (ex: user comments) you should also use a HTML sanitizer like [HTML Purifier](http://htmlpurifier.org/).
Parsedown is capable of escaping user-input within the HTML that it generates. Additionally Parsedown will apply sanitisation to additional scripting vectors (such as scripting link destinations) that are introduced by the markdown syntax itself.
To tell Parsedown that it is processing untrusted user-input, use the following:
```php
$parsedown = new Parsedown;
$parsedown->setSafeMode(true);
```
If instead, you wish to allow HTML within untrusted user-input, but still want output to be free from XSS it is recommended that you make use of a HTML sanitiser that allows HTML tags to be whitelisted, like [HTML Purifier](http://htmlpurifier.org/).
In both cases you should strongly consider employing defence-in-depth measures, like [deploying a Content-Secuity-Policy](https://scotthelme.co.uk/content-security-policy-an-introduction/) (making use of browser security feature) so that your page is likely to be safe even if an attacker finds a vulnerability in one of the first lines of defence above.
#### Security of Parsedown Extensions
Safe mode does not necessarily yield safe results when using extensions to Parsedown. Extensions should be evaluated on their own to determine their specific safety against XSS.
### Escaping HTML
> ⚠️  **WARNING:** This method isn't safe from XSS!
If you wish to escape HTML **in trusted input**, you can use the following:
```php
$parsedown = new Parsedown;
$parsedown->setMarkupEscaped(true);
```
Beware that this still allows users to insert unsafe scripting vectors, such as links like `[xss](javascript:alert%281%29)`.
### Questions