attack-tree.md

Attack Tree Analysis for ultraq/thymeleaf-layout-dialect

Objective: Execute Arbitrary Code on the Server or Leak Sensitive Information via Exploitation of the Thymeleaf Layout Dialect.

Attack Tree Visualization

                                      +-----------------------------------------------------+
                                      |  Execute Arbitrary Code/Leak Sensitive Data via    |
                                      |        Thymeleaf Layout Dialect Exploitation       |
                                      +-----------------------------------------------------+
                                                       |
                                                       | [HIGH RISK]
                                        +-------------------------+
                                        |  Template Injection via  |
                                        |   Layout Attributes     |
                                        +-------------------------+
                                                       |
                                        +---------+---------+
                                        |  Unsafe Use of   |
                                        |  `layout:replace` |
                                        |  or `layout:insert`|
                                        +---------+---------+
                                                       |
                                        +---------+---------+
                                        |  Dynamic Fragment |
                                        |  Names from      |
                                        |  Untrusted Input |  **CRITICAL NODE** [HIGH RISK]
                                        +---------+---------+

Attack Tree Path: High-Risk Path: Template Injection via Layout Attributes

• Description: This attack path focuses on injecting malicious code or template fragments through the Thymeleaf Layout Dialect's attributes, primarily layout:replace and layout:insert. The attacker aims to control the template or fragment that is being rendered, allowing them to execute arbitrary Thymeleaf expressions or include malicious content.

• Likelihood: High (if untrusted input is used) / Medium (if some validation exists but is flawed)

• Impact: Very High (potential for Remote Code Execution (RCE) or significant data leakage)

• Effort: Low to Medium (depending on the complexity of the injection)

• Skill Level: Intermediate

• Detection Difficulty: Medium to Hard (requires careful log analysis and potentially intrusion detection systems)

• Attack Steps:

  1. Unsafe Use of layout:replace or layout:insert: The application uses these attributes in a way that allows user input to influence the template or fragment being rendered.

  2. Dynamic Fragment Names from Untrusted Input (CRITICAL NODE): The application constructs the name of the template fragment to be included dynamically, using data that originates from an untrusted source (e.g., user input, URL parameters, request headers). This is the core vulnerability.

• Example Scenario:

  • A web application allows users to select a "theme" for their profile page.
  • The theme selection is implemented using a URL parameter: /profile?theme=dark.
  • The application uses the theme parameter directly in a layout:replace attribute:

    <div th:replace="${'themes/' + param.theme + ' :: profile'}"></div>

  • An attacker can craft a malicious URL: /profile?theme=../../../../etc/passwd.
  • If the application doesn't validate the theme parameter, Thymeleaf might attempt to load the /etc/passwd file (or a crafted template designed to execute code).

Attack Tree Path: Critical Node: Dynamic Fragment Names from Untrusted Input

• Description: This is the most critical vulnerability within the high-risk path. It represents the direct point of injection where an attacker can control the template fragment being rendered.

• Likelihood: High (if this pattern is used)

• Impact: Very High (RCE is highly likely)

• Effort: Low (simple string manipulation)

• Skill Level: Intermediate

• Detection Difficulty: Medium (suspicious template paths in logs might be a clue)

• Why it's Critical: This node is the direct cause of the template injection vulnerability. If this node is secured (by preventing untrusted input from influencing fragment names), the entire high-risk path is effectively mitigated.

• Mitigation Strategies (Detailed):

  1. Strict Input Validation (Whitelist):

     • Implement a whitelist of allowed fragment names. This is the most secure approach.
     • Use an enum (if the set of fragments is known at compile time) or a predefined Map (if the set is known but might change at runtime) to store the allowed fragment names.
     • Validate user input against this whitelist before using it in any template-related operation.
     • Reject any input that doesn't match an entry in the whitelist.

  2. Parameterization (if Dynamic Selection is Necessary):

     • If dynamic fragment selection is absolutely required, avoid direct string concatenation.
     • Instead, use a safe mechanism like a lookup table.
     • For example, the user might select a theme ID (e.g., 1, 2, 3).
     • The application would then use this ID as a key in a Map to retrieve the actual fragment name.
     • The Map acts as a safe intermediary, preventing direct injection.
     • Example (Conceptual):

       Map<Integer, String> themeFragments = new HashMap<>();
       themeFragments.put(1, "themes/dark :: profile");
       themeFragments.put(2, "themes/light :: profile");
       themeFragments.put(3, "themes/custom :: profile");
       
       Integer themeId = // Get validated theme ID from user input
       String fragmentName = themeFragments.get(themeId); // Safe lookup
       
       // Use fragmentName in th:replace (it's now safe)

  3. Avoid Dynamic Fragments Where Possible:

     • Favor static fragment inclusion using literal strings in the layout:replace or layout:insert attributes.
     • If the set of possible fragments is known, use static inclusion. This eliminates the injection risk entirely.

  4. Sanitization (Least Preferred - Use as a Last Resort):

     • If whitelisting and parameterization are not feasible, carefully sanitize user input.
     • This is the least preferred approach because it's prone to errors and bypasses.
     • Sanitization involves removing or escaping potentially dangerous characters from the input.
     • However, it's difficult to create a sanitizer that is guaranteed to be secure against all possible injection attacks.
     • Never rely solely on sanitization.