Incomplete Patch: More Joomla! Core XSS Vulnerabilities Are Found

July 12, 2017 admin

Credit to Author: Zhouyuan Yang| Date: Wed, 12 Jul 2017 15:50:00 +0000

Joomla! is one of the world's most popular content management systems (CMS). It enables users to build Web sites and powerful online applications. More than 3 percent of Web sites are running Joomla!, and it accounts for more than 9 percent of CMS market share.

As of July 2017, Joomla! has been downloaded over 82 million times. Over 7,800 free and commercial extensions are available from the official Joomla! Extension Directory, and more are available from other sources.

In my last blog, I discovered 2 Cross-Site Scripting (XSS) vulnerabilities in Joomla!. They are identified as CVE-2017-7985 and CVE-2017-7986. After analyzing the patches for these issues, I discovered 3 more XSS vulnerabilities. Joomla! still identifies these 3 new vulnerabilities as CVE-2017-7985, and has posted a separate security announcement.

As detailed in my last blog, these vulnerabilities exist because Joomla! fails to sanitize malicious user input when users post or edit an article. Remote attackers could exploit these vulnerabilities to run malicious code on a victim’s browser. This could allow the remote attacker to gain control of the victim’s Joomla! account. However, if the victim has higher permission, like system administrator, the remote attacker could actually gain full control of the web server.

These vulnerabilities affect Joomla! CMS versions 1.5.0 through 3.7.2. In this blog, I will share the details of these vulnerabilities.

Background

In the patches for CVE-2017-7985 and CVE-2017-7986, Joomla! filtered special characters, like the right double quotation mark, and dangerous HTML codes like “formaction.” But the filtering process is similar to a blacklist sanitizer, which means it simply matches the bad codes and drops them.

Analysis

In this analysis I use the same test account ‘yzy1’ as in my last post. This account only has publisher permission, which means it’s not allowed to use full HTML elements.

In the CVE-2017-7985 and CVE-2017-7986 patches, Joomla! sanitized my PoC

as shown in Figures 1 and 2.

Figure 1. Fix for CVE-2017-7985

Figure 2. Fix for CVE-2017-7986

Because it’s a blacklist XSS filter, I found that by adding the HTML “%0d%0a” mark, an attacker is able to bypass the filter and insert arbitrary HTML codes.

For example, the PoC in CVE-2017-7985 can be changed to . I added the %0d%0a behind the right double quotation, which allowed the XSS code to be triggered in both the front page and the backend, as shown in figures 3, 4, and 5.

The fix for CVE-2017-7986 can be bypassed in the same way.

Figure 3. Inserting new PoC for CVE-2017-7985

Figure 4. New PoC for CVE-2017-7985 triggered in the front page

Figure 5. New PoC for CVE-2017-7985 triggered in administrator page

Moreover, in HTML, “%0d%0a” serves as a line break, and the HTML codes can work by breaking with “%0d%0a”. For example, in figure 6, the codes in the two SVG tags have the same function.

Figure 6. HTML codes break using “%0d%0a”

Based on this, I found an easier way to trigger the XSS attack by simply adding the “%0d%0a”string to arbitrary HTML tags, like a, svg, img, and so on. Here is an example with an SVG tag.

Figure 7. Inserting PoC in SVG tag

The codes will be triggered in both the front page and the administrator page, as shown in figures 8 and 9.

Figure 8. SVG tag XSS in the front page

Figure 9. SVG tag XSS in the administrator page

There is no length limitation, so the attacker can insert the codes as many times as he wants and the XSS codes in the SVG tag will be triggered automatically. The victim only needs to view the article to trigger the attack.

Exploit

As I did in my last post, I will now show how an attacker using a low permission account is able to exploit this vulnerability to create a Super User account and then upload a web shell.

To accomplish this, I will first build a small JavaScript for adding a Super User. This script leverages the site administrator’s permission to get the CSRF token from the user edit page “index.php?option=com_users&view=user&layout=edit”, and then posts the add Super User request to the server using the CSRF token. In this example, the new Super User is Fortinet Yzy and the password is fortinet.

An attacker can add this code to Joomla! by exploiting the XSS vulnerability, as shown in figure 10.