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Date: Wed, 16 Oct 2019 14:12:17 +0200
From: SBA Research Advisory <advisory@...-research.org>
To: <oss-security@...ts.openwall.com>
Subject: [SBA-ADV-20190913-02] CVE-2019-16521: WordPress Plugin - Broken Link
 Checker <= 1.11.8 - Reflected XSS

# WordPress Plugin - Broken Link Checker - Reflected XSS #

Link: https://github.com/sbaresearch/advisories/tree/public/2019/SBA-ADV-20190913-02_WordPress_Plugin_Broken_Link_Checker

## Vulnerability Overview ##

The broken-link-checker plugin through 1.11.8 for WordPress (aka Broken Link Checker)
is susceptible to Reflected XSS due to improper encoding and insertion of an HTTP GET
parameter into HTML. The filter function on the page listing all detected broken links
can be exploited by providing an XSS payload in the s_filter GET parameter in a
filter_id=search request.
NOTE: this is an end-of-life product.

* **Identifier**            : SBA-ADV-20190913-02
* **Type of Vulnerability** : Cross-site Scripting
* **Software/Product Name** : [Broken Link Checker](https://wordpress.org/plugins/broken-link-checker/)
* **Vendor**                : [ManageWP](https://managewp.com/)
* **Affected Versions**     : <= 1.11.8
* **Fixed in Version**      : -
* **CVE ID**                : CVE-2019-16521
* **CVSSv3 Vector**         : AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
* **CVSSv3 Base Score**     : 8.1 (High)

## Vendor Description ##

> This plugin will monitor your blog looking for broken links and let you know if any are found.

Active Installations: 700,000+

Source: <https://wordpress.org/plugins/broken-link-checker/>

## Impact ##

By exploiting the documented vulnerability, an external attacker without
any privileges can execute JavaScript code in a victim's browser.
This can be misused, e.g for phishing attacks by displaying a fake
login form and sending the victim's credentials to the attacker.
Furthermore malicious actions can be performed in the context of an authenticated
user. The impact depends on the level of access of the attacked user.
In case of an admin this can lead to the execution of PHP code and the compromise
of the server.

## Vulnerability Description ##

The filter function on the page listing all detected broken links can be exploited
by providing HTML and JavaScript code in one of the parameters.
All users with access to the "Broken Link Checker" are in danger.

This is possible because the CSS class of the table contains the value of the
`s_filter` GET parameter if it is not empty. Additionally it is necessary to provide
the special filter id "search" via parameter `filter_id=search` in order to reach this
execution path:

In `includes/link-query.php` (L803-810) the value of `$base_filter` is determined,
with `filter_id=search` the right path will be reached:

```php
$base_filter = '';
if ( array_key_exists($filter_id, $this->native_filters) ) {
    $base_filter = $filter_id;
} else if ( isset($current_filter['params']['s_filter']) && !empty($current_filter['params']['s_filter']) ) {
    $base_filter = $current_filter['params']['s_filter'];
} else if ( isset($_GET['s_filter']) && !empty($_GET['s_filter']) ) {
    $base_filter = $_GET['s_filter'];
}
```

In `admin/table-printer.php` (L71-82) the value of the CSS class of the table is determined and set.
Here the value of the GET parameter is inside `$current_filter['base_filter']`.

```php
$table_classes = array( 'widefat' );
if ( $compact ) {
    $table_classes[] = 'compact';
};
if ( $this->core->conf->options['table_color_code_status'] ) {
    $table_classes[] = 'color-code-link-status';
};
$table_classes[] = 'base-filter-' . $current_filter['base_filter'];
printf(
    '<table class="%s" id="blc-links"><thead><tr>',
    implode( ' ', $table_classes )
);
```

## Proof of Concept ##

This example shows how an attacker can craft a URL to execute a payload in the
browser of the victim. When a user clicks on the link and is logged-in, the payload
(in this case a simple JavaScript alert) will be executed. Otherwise the user will be taken
to the login page and redirected to the exploit after a successful login.
Replace `<YOUR_DOMAIN_HERE>` with the URL to your WordPress installation:

```text
<YOUR_DOMAIN_HERE>/wp-admin/tools.php?page=view-broken-links&filter_id=search&s_filter=%22+id%3D%22blc-links%22%3E%3Cscript%3Ealert%281%29%3C%2Fscript%3E
```

## Recommended Countermeasures ##

We recommend to properly escape the output by using the encoding functions provided by WordPress,
like the `esc_*`- or `wp_kses_*`-[functions][1].

[1]: https://developer.wordpress.org/themes/theme-security/data-sanitization-escaping/#escaping-securing-output

## Timeline ##

* `2019-09-05` Identified the vulnerability
* `2019-09-06` Contacted ManageWP
* `2019-09-09` Contacted plugins@...dpress.org
* `2019-09-10` Response by ManageWP requesting more details
* `2019-09-11` Response by ManageWP that they are not actively maintaining the plugin and don't provide support
* `2019-09-20` CVE assigned
* `2019-10-16` Public disclosure

## References ##

* <https://wordpress.org/plugins/broken-link-checker/>
* <https://wordpress.org/plugins/broken-link-checker/#developers>

## Credits ##

* Tobias Fink ([SBA Research](https://www.sba-research.org/))

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