CVE-2021-42013: Apache HTTP Server Path Traversal & RCE Walkthrough
⚠️ EDUCATIONAL PURPOSE ONLY: This video is for security research, system administration, and authorized testing purposes only. The goal is to demonstrate the mechanics of a vulnerability to help organizations identify, verify, and patch critical risks.
Overview
CVE-2021-42013 is a critical vulnerability in Apache HTTP Server (versions 2.4.49 and 2.4.50) that allows for unauthenticated Path Traversal and Remote Code Execution (RCE). This flaw is an incomplete fix for the earlier CVE-2021-41773; while the first patch blocked standard URL encoding, it failed to account for double-URL encoding. Due to its high impact and active use by threat actors, it is listed in the CISA Known Exploited Vulnerabilities (KEV) Catalog.
Technical Breakdown
The vulnerability stems from improper input validation in the URL path normalization routine.
Incomplete Patch Bypass: The initial fix for CVE-2021-41773 only decoded and validated paths at a single level. Attackers found that by using double-URL encoding (e.g., %%32%65 instead of %2e for a dot), they could bypass the first decoding pass. The server would then decode the sequence a second time after the security validation had already occurred.
Path Traversal: By crafting a URL with these double-encoded sequences (e.g., .%%32%65/.%%32%65/), an attacker can escape the web root and access arbitrary files on the filesystem, such as /etc/passwd.
RCE Impact: If the Apache module mod_cgi (or mod_cgid) is enabled and the traversed path leads to an executable binary (like /bin/sh), attackers can send a POST request to execute arbitrary system commands. This allows for full system compromise.
Remediation
Organizations should immediately upgrade to Apache HTTP Server 2.4.51 or later, which correctly handles recursive URL decoding during path normalization.
Additional Best Practices:
Ensure the Require all denied directive is applied to all directories outside the intended web root.
Disable mod_cgi and mod_cgid if CGI functionality is not strictly required.
🔗 Links & Resources:
https://thehackernews.com/2021/10/new...
https://httpd.apache.org/security/vul...
https://github.com/projectdiscovery/n...
https://github.com/vulhub/vulhub/tree...
https://www.exploit-db.com/exploits/5...
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Tags:
#CVE202142013 #CyberSecurity #ApacheHTTPD #RCE #PathTraversal #Infosec #CISA #Pentesting #BugBounty #PatchBypass