attack-tree.md

Attack Tree Analysis for pallets/jinja

Objective: Execute Arbitrary Code on Server via Jinja2 Exploit

Attack Tree Visualization

                                      +-------------------------------------------------+
                                      |  Attacker Goal: Execute Arbitrary Code on Server |
                                      |                 via Jinja2 Exploit               |
                                      +-------------------------------------------------+
                                                        |
                                      +-------------------------+
                                      |  Template Injection (SSTI) | [CRITICAL]
                                      +-------------------------+
                                                        |
                                      +------------------------------------------------+
                                      | Untrusted Input to Template | [CRITICAL]
                                      +------------------------------------------------+
                                                        |
                                      +---------+
                                      |   User  |
                                      |  Input  |
                                      +---------+
                                                        |
                                      +------------------------------------------------+
                                      | Bypass Sandboxing/Escaping |
                                      +------------------------------------------------+
                                                        |
                                      +---------+
                                      |   __    |
                                      |  Classes|                                      
                                      +---------+
                                                        |
                                      +------------------------------------------------+
                                      |   Achieve Code Execution (RCE)   | [CRITICAL]
                                      +------------------------------------------------+

Attack Tree Path: Template Injection (SSTI) [CRITICAL]

• Description: This is the core vulnerability. The attacker injects malicious Jinja2 template code into the application. This occurs when the application renders a template using data that the attacker can, at least partially, control.
• How it works: Jinja2, like other templating engines, allows dynamic content generation. If an attacker can inject their own template syntax (e.g., {{ ... }} or {% ... %}), they can potentially execute arbitrary code or access sensitive data.
• Mitigation:
  • Primary: Strict input validation and sanitization. Never directly render user-supplied input in a template without proper escaping and validation.
  • Secondary: Use Jinja2's SandboxedEnvironment and enable autoescaping.

Attack Tree Path: Untrusted Input to Template [CRITICAL]

• Description: This is the root cause of SSTI. The application must be using data that the attacker can influence in the template rendering process.
• Specific Source (High-Risk): User Input
  • Description: The most common and direct source of untrusted input. This includes data submitted through forms, URL parameters, HTTP headers, cookies, etc.
  • How it works: The attacker directly provides the malicious input through a web interface.
  • Mitigation:
    • Validate all user input against a strict whitelist of allowed characters or patterns.
    • Escape output appropriately using Jinja2's escape filter or autoescaping. Note: Escaping alone is not sufficient to prevent SSTI; validation is crucial.
    • Use a Content Security Policy (CSP) to mitigate the impact of XSS, which can sometimes be used in conjunction with SSTI.

Attack Tree Path: Bypass Sandboxing/Escaping

• Description: Jinja2 has built-in security features (sandboxing) to limit what template code can do. Attackers try to circumvent these restrictions.
• Specific Technique (High-Risk): __class__ and related attributes
  • Description: This is a classic SSTI technique. By accessing the __class__ attribute of objects within the template context, attackers can traverse the object hierarchy and potentially reach dangerous classes (like those that allow system command execution).
  • How it works: Python's object model allows access to an object's class through __class__. From there, attackers can access other attributes like __bases__ (to get parent classes), __subclasses__ (to get child classes), and __mro__ (method resolution order). This allows them to "walk" the object tree and find objects with methods they can abuse.
  • Mitigation:
    • Use SandboxedEnvironment to restrict access to potentially dangerous attributes.
    • Consider disabling access to __class__ and related attributes if they are not absolutely necessary. This can be done by customizing the SandboxedEnvironment.
    • Keep Jinja2 updated to the latest version, as vulnerabilities related to sandboxing are often patched.

Attack Tree Path: Achieve Code Execution (RCE) [CRITICAL]

• Description: Once the attacker has bypassed the sandbox, they can execute arbitrary code on the server. This is the ultimate goal, leading to complete system compromise.
• How it works: The attacker uses the access gained through the SSTI vulnerability to execute system commands, read/write files, or interact with the server in other malicious ways. This often involves using Python's built-in functions (if accessible) or leveraging other vulnerabilities.
• Mitigation:
  • Preventative: All the mitigations listed above for SSTI and sandboxing bypasses are crucial to prevent RCE.
  • Detective: Implement robust logging and monitoring to detect suspicious activity, such as unusual system calls, file access patterns, or network connections.
  • Limiting Damage: Run the application with the principle of least privilege. The application should only have the minimum necessary permissions to function, limiting the damage an attacker can do even if they achieve RCE.