mitigations.md

Mitigation Strategies Analysis for nikic/php-parser

Mitigation Strategy: AST Analysis and Validation (Post-Parsing)

Mitigation Strategy: AST Analysis and Validation (Post-Parsing)

• Description:

  1. Define Allowed Node Types: Create a precise whitelist of allowed PhpParser\Node types. This list should be as restrictive as possible, including only the node types absolutely necessary for the intended analysis or manipulation. For example, if you're only analyzing function definitions, the whitelist might include PhpParser\Node\Stmt\Function_, PhpParser\Node\Param, PhpParser\Node\Identifier, etc., but exclude nodes like PhpParser\Node\Expr\Eval_, PhpParser\Node\Expr\ShellExec, PhpParser\Node\Stmt\Class_, etc.
  2. Node Visitor Implementation: Create a custom class that implements PhpParser\NodeVisitor. This visitor will traverse the AST generated by php-parser.
  3. Whitelist Check (in enterNode()): Within the enterNode(PhpParser\Node $node) method of your custom NodeVisitor, check if the type of the current node ($node->getType()) is present in your predefined whitelist of allowed node types.
  4. Rejection (Exception/Error Handling): If the node type is not in the whitelist, immediately reject the input. This can be done by:
     • Throwing a custom exception (e.g., InvalidCodeStructureException).
     • Returning NodeTraverser::DONT_TRAVERSE_CHILDREN to stop further traversal of the current node's children.
     • Returning NodeTraverser::STOP_TRAVERSAL to completely stop the AST traversal.
     • Setting an error flag and returning. Choose the method that best integrates with your error handling strategy. The key is to prevent any further processing of the invalid AST.
  5. Restrict Node Modification (in leaveNode()): If you are modifying the AST (within the leaveNode(PhpParser\Node $node) method), be extremely cautious. Only allow modifications that are strictly necessary and that cannot introduce new vulnerabilities. For example, if you're renaming variables, ensure that the new names are validated and don't conflict with existing variables or keywords. Avoid any modifications that could lead to code execution or other unintended behavior.
  6. Data Flow Analysis (Advanced, within Visitor): If you're performing data flow analysis (tracking the flow of data through the code), implement checks within your NodeVisitor to ensure that tainted data (from user input) doesn't influence the analysis in a way that could lead to vulnerabilities. This is a more advanced technique and requires careful consideration of how data is tracked and used.

• Threats Mitigated:

  • Remote Code Execution (RCE): (Severity: Medium) - Reduces the risk of RCE by preventing the analysis or manipulation of malicious code structures (e.g., eval, system calls) that are represented by specific AST node types.
  • Denial of Service (DoS): (Severity: Medium) - Prevents the analysis of overly complex or deeply nested code structures (e.g., deeply nested arrays or loops) that could lead to resource exhaustion during AST traversal.
  • Application Logic Modification: (Severity: High) - Prevents attackers from subtly altering the application's logic by manipulating the AST in unauthorized ways (e.g., changing conditional statements, modifying function calls).

• Impact:

  • RCE: Moderately reduces risk by adding an extra layer of validation after parsing, specifically targeting the structured representation of the code.
  • DoS: Moderately reduces risk by preventing the processing of overly complex ASTs that could consume excessive resources.
  • Application Logic Modification: Significantly reduces risk by preventing unauthorized and potentially dangerous modifications to the AST.

• Currently Implemented:

  • Example: AstValidatorVisitor.php implements the PhpParser\NodeVisitor interface and performs whitelist checks within enterNode().
  • Example: The AstValidatorVisitor is added to a NodeTraverser in CodeAnalyzer.php and run after the code is parsed using ParserFactory.

• Missing Implementation:

  • Example: Data flow analysis within the NodeVisitor is not yet implemented.
  • Example: The whitelist of allowed node types in AstValidatorVisitor.php is not yet fully comprehensive and needs to be reviewed and expanded.
  • Example: Restrictions on AST modification within leaveNode() are not yet fully implemented and documented.

Mitigation Strategy: Output Handling (Using PrettyPrinter Safely)

Mitigation Strategy: Output Handling (Using PrettyPrinter Safely)

• Description:

  1. PrettyPrinter Usage: Always use PhpParser\PrettyPrinter\Standard (or a custom pretty printer that extends it) to generate PHP code from the AST. Never manually construct PHP code by concatenating strings. The PrettyPrinter ensures that the generated code is syntactically correct and avoids common code injection vulnerabilities.
  2. Custom PrettyPrinter (If Necessary): If you need to customize the code generation (e.g., to add specific formatting or comments), create a custom class that extends PhpParser\PrettyPrinter\Standard. Override the specific methods you need to modify, but be extremely careful not to introduce any vulnerabilities. Thoroughly review and test any custom pretty printing logic.
  3. Avoid eval() with Generated Code: Absolutely never use the generated code with PHP's eval() function. This is inherently dangerous and bypasses many security protections. If you need to execute the generated code, use the sandboxing techniques described previously (but outside the scope of this php-parser-specific list).
  4. Review Code Generation Logic: Carefully examine the code that uses the PrettyPrinter. Ensure that the AST being passed to the PrettyPrinter is itself safe and hasn't been tampered with. This connects back to the AST validation strategy. The flow should be: Input Validation -> Parsing -> AST Validation -> (Optional AST Modification, with extreme caution) -> Pretty Printing.

• Threats Mitigated:

  • Remote Code Execution (RCE): (Severity: Critical) - By avoiding manual string concatenation and eval(), this strategy significantly reduces the risk of RCE vulnerabilities that could arise from improperly generated code.
  • Code Injection: (Severity: High) - The PrettyPrinter ensures that the generated code is syntactically correct, preventing various forms of code injection that could exploit parsing flaws.

• Impact:

  • RCE: Significantly reduces risk by preventing the most common causes of RCE related to code generation.
  • Code Injection: Significantly reduces risk by ensuring syntactic correctness and avoiding manual string manipulation.

• Currently Implemented:

  • Example: CodeGenerator.php uses PhpParser\PrettyPrinter\Standard exclusively to generate PHP code from the AST. The $prettyPrinter->prettyPrintFile($ast) method is used.

• Missing Implementation:

  • Example: A custom PrettyPrinter is planned for adding specific code comments, but it hasn't been implemented and thoroughly reviewed for security implications yet.