attack-tree.md

Attack Tree Analysis for pallets/flask

Objective: To gain unauthorized Remote Code Execution (RCE) on the server hosting the Flask application by exploiting vulnerabilities or misconfigurations specific to the Flask framework or its common usage patterns.

Attack Tree Visualization

                                     +-------------------------------------------------+
                                     |  Attacker Goal: RCE or DoS via Flask Exploitation |
                                     +-------------------------------------------------+
                                                      |
         +--------------------------------------------------------------------------------+
         |                                                                                |
+-------------------------+                                                +-------------------------+
|  1. Exploit Template     |                                                |  2. Exploit Request      |
|     Rendering (SSTI)    |                                                |     Handling            |
+-------------------------+                                                +-------------------------+
         |                                                                                |
+--------+--------+                                                +--------+
| *1.1*  | 1.2    |                                                | *2.1*  |
| *Unsafe*| Unsafe |                                                | *Unsafe*|
| *Jinja2*| Flask  |                                                | *Deser.*|
| *Config*| Config |                                                | *of*    |
|        |        |                                                | *Data*  |
+--------+--------+                                                +--------+
         |                                                                                |
+--------+--------+                                                +--------+
|**->1.1.1**|        |                                                |**->2.1.1**|
|**[CRITICAL]**|        |                                                |**[CRITICAL]**|
|*User*   |        |                                                |*Pickle* |
|*Input*  |        |                                                |*Used w/*|
|*to*     |        |                                                |*User*   |
|*Template*|        |                                                |*Input*  |
+--------+--------+                                                +--------+
         |
+--------+
|**->1.2.2**|
|**[CRITICAL]**|
|*`send_`*|
|*file`*  |
|*Used*   |
|*Insec-* |
|*urely*  |
+--------+

Attack Tree Path: High-Risk Path 1: 1. -> 1.1 -> 1.1.1 (User Input to Template)

• Description: This is the classic Server-Side Template Injection (SSTI) vulnerability. An attacker injects malicious Jinja2 code into a template through user-supplied input that is not properly sanitized or escaped.
• Vulnerability: Flask applications that directly embed user input into Jinja2 templates without using auto-escaping or explicit escaping functions (like flask.escape()) are vulnerable.
• Exploitation:
  • The attacker identifies a field or parameter where user input is reflected in the rendered output.
  • The attacker crafts a malicious Jinja2 payload, such as {{ config }} (to leak configuration), {{ self.__class__.__init__.__globals__ }} (to access global variables), or more complex payloads to achieve RCE (e.g., using subprocess.Popen).
  • The attacker submits the payload through the vulnerable input field.
  • The server renders the template, executing the attacker's injected code.
• Impact: Remote Code Execution (RCE) - The attacker can execute arbitrary commands on the server, potentially leading to full system compromise.
• Mitigation:
  • Always use Jinja2's auto-escaping: This is enabled by default in Flask, but ensure it's not accidentally disabled.
  • Explicitly escape user input: If you must handle user input before passing it to the template, use flask.escape() to sanitize it.
  • Use context processors: Provide safe data to templates through context processors instead of directly embedding user input.
  • Content Security Policy (CSP): A CSP can limit the impact of successful injections, even if they occur.
  • Input Validation: Validate all user input to ensure it conforms to expected types and formats.

Attack Tree Path: High-Risk Path 2: 1. -> 1.2 -> 1.2.2 (send_file Used Insecurely)

• Description: This is a Path Traversal vulnerability. An attacker manipulates the filename passed to Flask's send_file function to access files outside the intended directory.
• Vulnerability: Flask applications that construct file paths directly from user input without proper sanitization or validation are vulnerable.
• Exploitation:
  • The attacker identifies a parameter used to specify a filename for download or display.
  • The attacker crafts a malicious filename containing path traversal characters, such as ../../etc/passwd or ../../../../sensitive_config.ini.
  • The attacker submits the malicious filename.
  • The server uses the attacker-controlled filename to access and return the requested file, potentially exposing sensitive information.
• Impact: Arbitrary File Read - The attacker can read any file on the server that the Flask application process has access to. This can lead to exposure of sensitive data (configuration files, source code, etc.), which can then be used for further attacks.
• Mitigation:
  • Never construct file paths directly from user input.
  • Use a whitelist: Maintain a list of allowed filenames and only serve files from that list.
  • Generate unique filenames: Create random, unique filenames for uploaded files and store the mapping to the original filename in a database.
  • Sanitize user input: Remove any path traversal characters (../, etc.) from user-supplied filenames.
  • Use safe_join (with caution): While safe_join is designed to prevent path traversal, always combine it with other mitigation techniques. Keep Flask updated to address any potential bypasses.

Attack Tree Path: High-Risk Path 3: 2. -> 2.1 -> 2.1.1 (Pickle Used with User Input)

• Description: This is an Unsafe Deserialization vulnerability using Python's pickle module. An attacker crafts a malicious pickle payload that executes arbitrary code when deserialized.
• Vulnerability: Flask applications that deserialize data from untrusted sources (e.g., user input) using pickle.loads() without any validation are vulnerable.
• Exploitation:
  • The attacker identifies a route or endpoint that accepts serialized data.
  • The attacker crafts a malicious pickle payload that, when deserialized, executes arbitrary Python code. This often involves defining a class with a __reduce__ method that returns a tuple specifying a callable (like os.system) and its arguments.
  • The attacker sends the malicious payload to the vulnerable endpoint.
  • The server deserializes the payload using pickle.loads(), triggering the execution of the attacker's code.
• Impact: Remote Code Execution (RCE) - The attacker can execute arbitrary commands on the server, potentially leading to full system compromise.
• Mitigation:
  • Never use pickle to deserialize data from untrusted sources.
  • Use safer alternatives: Use JSON (json.loads()) for simple data structures. For more complex objects, consider well-vetted serialization libraries with built-in security features.
  • Input Validation: If you must use a potentially unsafe deserialization method, rigorously validate the structure and content of the deserialized data before using it. This is extremely difficult to do securely with pickle, however.