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CVE-2020-1747

PyYAML < 5.3.1, arbitrary code execution via python/object/new

  • CVSS 9.8
  • CWE-20 Improper Input Validation
  • Input Validation and Sanitization
  • Local

A vulnerability was discovered in the PyYAML library in versions before 5.3.1, where it is susceptible to arbitrary code execution when it processes untrusted YAML files through the full_load method or with the FullLoader loader. Applications that use the library to process untrusted input may be vulnerable to this flaw. An attacker could use this flaw to execute arbitrary code on the system by abusing the python/object/new constructor.

CVE
CVE-2020-1747
CWE
CWE-20 Improper Input Validation
CVSS base score
9.8
Published
2020-03-24
OWASP
A06 Vulnerable and Outdated Components
Orthogonal defect classification
Function
Code defect classification
Incorrect Functionality
Category
Input Validation and Sanitization
Subcategory
Insecure Parsing or Deserialization
Accessibility scope
Local
Impact
Arbitrary Code Execution
Fixed by upgrading
Yes

Solution

Update PyYAML to version 5.3.1 or higher.

Vulnerable code sample

def calculate_expression(user_input):
    """Calculate mathematical expression - VULNERABLE"""
    try:
        # Dangerous: eval executes arbitrary Python code
        result = eval(user_input)
        return result
    except Exception as e:
        return f"Error: {e}"

def execute_user_code(code_string):
    """Execute user code - EXTREMELY VULNERABLE"""
    # Dangerous: exec allows arbitrary code execution
    exec(code_string)

# Vulnerable usage examples:
# calculate_expression("__import__('os').system('rm -rf /')")
# execute_user_code("import subprocess; subprocess.call(['rm', '-rf', '/'])")

Patched code sample

import ast
import operator
import math
from typing import Union, Dict, Any

class SafeExpressionEvaluator:
    """Safe mathematical expression evaluator"""
    
    # Allowed operators
    OPERATORS = {
        ast.Add: operator.add,
        ast.Sub: operator.sub,
        ast.Mult: operator.mul,
        ast.Div: operator.truediv,
        ast.Pow: operator.pow,
        ast.Mod: operator.mod,
        ast.USub: operator.neg,
        ast.UAdd: operator.pos,
    }
    
    # Allowed functions
    FUNCTIONS = {
        'abs': abs,
        'min': min,
        'max': max,
        'round': round,
        'sin': math.sin,
        'cos': math.cos,
        'tan': math.tan,
        'sqrt': math.sqrt,
        'log': math.log,
        'exp': math.exp,
    }
    
    def evaluate_expression(self, expression: str) -> Union[float, int, None]:
        """Safely evaluate mathematical expression"""
        try:
            # Parse expression into AST
            tree = ast.parse(expression, mode='eval')
            
            # Validate and evaluate
            result = self._eval_node(tree.body)
            
            # Type validation
            if isinstance(result, (int, float)):
                return result
            else:
                raise ValueError("Result must be numeric")
                
        except Exception as e:
            print(f"Invalid expression: {e}")
            return None
    
    def _eval_node(self, node):
        """Recursively evaluate AST node"""
        if isinstance(node, ast.Constant):
            # Python 3.8+ constant values
            if isinstance(node.value, (int, float)):
                return node.value
            else:
                raise ValueError("Only numeric constants allowed")
        
        elif isinstance(node, ast.Num):
            # Legacy numeric literals
            return node.n
        
        elif isinstance(node, ast.BinOp):
            # Binary operations
            left = self._eval_node(node.left)
            right = self._eval_node(node.right)
            op_func = self.OPERATORS.get(type(node.op))
            
            if op_func is None:
                raise ValueError(f"Unsupported operator: {type(node.op)}")
            
            return op_func(left, right)
        
        elif isinstance(node, ast.UnaryOp):
            # Unary operations
            operand = self._eval_node(node.operand)
            op_func = self.OPERATORS.get(type(node.op))
            
            if op_func is None:
                raise ValueError(f"Unsupported unary operator: {type(node.op)}")
            
            return op_func(operand)
        
        elif isinstance(node, ast.Call):
            # Function calls
            if not isinstance(node.func, ast.Name):
                raise ValueError("Only simple function calls allowed")
            
            func_name = node.func.id
            if func_name not in self.FUNCTIONS:
                raise ValueError(f"Function not allowed: {func_name}")
            
            # Evaluate arguments
            args = [self._eval_node(arg) for arg in node.args]
            
            # Call function
            return self.FUNCTIONS[func_name](*args)
        
        else:
            raise ValueError(f"Unsupported AST node: {type(node)}")

# Secure usage
evaluator = SafeExpressionEvaluator()
result = evaluator.evaluate_expression("2 + 3 * sqrt(16)")  # Returns 14.0
invalid = evaluator.evaluate_expression("__import__('os')")  # Returns None

Cite this entry

@misc{vaitp:cve20201747,
  title        = {{PyYAML < 5.3.1, arbitrary code execution via python/object/new}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2020},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2020-1747},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2020-1747/}}
}
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