CWE-691: Insufficient Control Flow Management

What is Insufficient Control Flow Management?

• Overview: Insufficient Control Flow Management occurs when code does not adequately manage the sequence and conditions of its execution, potentially allowing the control flow to be altered in unanticipated and possibly harmful ways.

• Exploitation Methods:

• Attackers can exploit this vulnerability by manipulating the execution flow to bypass security checks or execute unintended code paths.
• Common attack patterns include altering program execution via input tampering, exploiting assumptions in logic flow, and using control flow hijacking techniques.

• Security Impact:

• Direct consequences of successful exploitation include unauthorized access, privilege escalation, and execution of arbitrary code.
• Potential cascading effects involve exposure of sensitive data, system instability, and further vulnerability exploitation.
• Business impact may include data breaches, financial loss, reputation damage, and legal liabilities.

• Prevention Guidelines:

• Specific code-level fixes involve implementing strict control flow checks and validating assumptions about execution paths.
• Security best practices include using defensive programming techniques, performing thorough code reviews, and employing automated testing to detect control flow anomalies.
• Recommended tools and frameworks include static analysis tools for code path validation, control flow integrity tools, and languages or frameworks that emphasize safe control flow management.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not Technology-Specific

Vulnerable Code Example

Python Example

def process_user_input(user_input):
    # Vulnerable: Using eval() on user input without validation
    # This can lead to arbitrary code execution if the input is malicious
    result = eval(user_input)
    return result

user_input = input("Enter a calculation: ")  # e.g., "2 + 2" or "os.system('rm -rf /')"
output = process_user_input(user_input)
print(f"Output: {output}")

In this vulnerable code example, the use of eval() on user input without any validation poses a significant security risk. The eval() function can execute arbitrary code, which makes the control flow susceptible to manipulation by malicious inputs.

How to fix Insufficient Control Flow Management?

The vulnerability in the code arises from using eval() on user input, which can execute arbitrary code. This is a classic example of insufficient control flow management, as the control flow can be altered by any input. To fix this, avoid using eval() with untrusted input. Instead, use safer alternatives like parsing and evaluating expressions using a library that restricts the execution context.

Specific Fixes:

1. Avoid eval() with Untrusted Input: Never use eval() on input that can be influenced by a user or attacker.
2. Use Safer Libraries: Use libraries designed for safely evaluating mathematical expressions, such as ast.literal_eval for basic literals or sympy for mathematical calculations.
3. Validate and Sanitize Input: Always validate and sanitize inputs to ensure they conform to expected formats.

Fixed Code Example

import ast

def process_user_input(user_input):
    # Fixed: Using ast.literal_eval to safely evaluate basic expressions
    try:
        # Safely parse the user input using ast.literal_eval
        result = ast.literal_eval(user_input)
    except (ValueError, SyntaxError):
        # Handle cases where the input is not a valid expression
        return "Invalid input"
    return result

user_input = input("Enter a calculation: ")
output = process_user_input(user_input)
print(f"Output: {output}")

In this fixed version, we replace eval() with ast.literal_eval() to safely evaluate the user input as a literal expression. This approach prevents the execution of arbitrary code, ensuring that the control flow can't be exploited through malicious inputs. The use of ast.literal_eval() limits the execution to expressions that are considered safe, such as numbers, strings, tuples, lists, dicts, booleans, and None.