Cross-site Scripting (XSS) - Stored in elgg/elgg

Successful exploitation of CVE-2021-4072 allows an attacker to execute arbitrary JavaScript code within the context of a victim's browser. This can be leveraged to steal session cookies, redirect users to malicious websites, deface the elgg instance, or even gain complete control over user accounts. The impact is particularly severe because elgg is often used in environments where sensitive user data is stored and processed, such as educational institutions and community organizations. The attacker could potentially gain access to private messages, personal information, and other sensitive data stored within the elgg system. This vulnerability shares similarities with other XSS vulnerabilities, where improper input sanitization leads to the execution of attacker-controlled code.

Organizations and individuals using elgg versions 3.3.24 and earlier are at risk. This includes educational institutions, community organizations, and any other entity relying on elgg for social networking or collaboration purposes. Shared hosting environments running elgg are particularly vulnerable, as a compromise of one site can potentially impact others on the same server.

• php / web:

curl -I https://your-elgg-site.com/ | grep -i content-type

• php / web: Examine elgg plugin files for instances of htmlspecialchars or similar encoding functions. Look for areas where user input is directly inserted into HTML without proper sanitization. • generic web: Monitor access logs for unusual requests containing JavaScript code or suspicious URL parameters. • generic web: Use a WAF to detect and block requests containing common XSS payloads.

1. 2021-12-24Disclosure

   disclosure

1. Réservé2021-12-06
2. Publiée2021-12-24
3. Modifiée2024-08-03
4. EPSS mis à jour2026-04-02

The primary mitigation for CVE-2021-4072 is to immediately upgrade elgg to version 3.3.24 or later. If upgrading is not immediately feasible, consider implementing input validation and output encoding on all user-supplied data to prevent the injection of malicious scripts. Web Application Firewalls (WAFs) can be configured to filter out potentially malicious requests containing XSS payloads. Regularly review and update elgg's configuration to ensure that all security settings are properly configured. After upgrading, confirm the fix by attempting to inject a simple XSS payload (e.g., <script>alert('XSS')</script>) into a form field and verifying that the script is not executed.