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How CVE-2021-41773 Works
This vulnerability affects Apache HTTP Server 2.4.49. A flaw in path normalization allows attackers to bypass security restrictions using directory traversal sequences (../). If `mod_cgi` is enabled, malicious actors can execute arbitrary shell commands by crafting a specially encoded URL. The issue arises due to insufficient input validation in the `ap_normalize_path()` function, permitting unauthorized access to files outside the designated document root. Exploitation leads to remote code execution (RCE) under the web server’s privileges, compromising sensitive data or system integrity.
DailyCVE Form
Platform: Apache HTTP Server
Version: 2.4.49
Vulnerability: Path Traversal → RCE
Severity: Critical
Date: 2021-10-05
What Undercode Say:
Exploit:
curl -v "http://target/cgi-bin/.%2e/%2e%2e/%2e%2e/bin/sh" -d "echo;id"
Detection:
grep -r "mod_alias|mod_cgi" /etc/apache2/
Mitigation:
1. Patch: Upgrade to Apache 2.4.50+.
2. Disable `mod_cgi`:
LoadModule cgi_module modules/mod_cgi.so Comment this line
3. Input Validation:
<Directory "/var/www/html"> AllowOverride None Require all denied </Directory>
4. Log Monitoring:
tail -f /var/log/apache2/access.log | grep -E "../|%2e%2e"
5. WAF Rules:
location ~ .(cgi|sh)$ { deny all; }
Forensics:
strings /proc/<apache_pid>/environ | grep -i "DOCUMENT_ROOT"
Exploit Analysis:
- Payload leverages URL-encoded `../` (
%2e%2e/) to bypass checks.
– `mod_cgi` executes payloads if executable scripts reside in misconfigured directories.
References:
- Apache 2.4.50 Changelog
- CVE-2021-41773 NVD
Note: Replace `target` with the vulnerable server’s IP/hostname. Test exploits only in authorized environments.
Sources:
Reported By: www.cve.org
Extra Source Hub:
Undercode
