CWE-1286: Improper Validation of Syntactic Correctness of Input

What is Improper Validation of Syntactic Correctness of Input?

• Overview: Improper Validation of Syntactic Correctness of Input occurs when software receives input that is expected to follow a specific syntax, but fails to validate or improperly validates that the input adheres to this syntax, allowing malformed data to be processed.

• Exploitation Methods:

• Attackers can exploit this vulnerability by sending inputs that do not conform to the expected syntax, leading to parsing errors or unexpected behaviors.
• Common attack patterns include injecting malformed data to disrupt processing logic or exploit hidden vulnerabilities.

• Security Impact:

• Direct consequences include application crashes, denial of service, or execution of unintended code paths.
• Potential cascading effects involve exposing other vulnerabilities that rely on syntactically correct inputs.
• Business impact may include data breaches, loss of customer trust, and financial damage due to service downtime.

• Prevention Guidelines:

• Specific code-level fixes involve implementing robust input validation routines that strictly enforce expected syntax rules.
• Security best practices include employing whitelisting techniques and rejecting inputs that do not match the expected format.
• Recommended tools and frameworks involve using libraries and parsers designed to handle specific data formats securely and performing regular security audits.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Vulnerable Code Example

import re

def is_valid_email(email):
    # Vulnerable code: Improper validation of email syntax
    # This regex is too permissive and allows invalid email formats
    return re.match(r".+@.+\..+", email) is not None
    
# Example usage
print(is_valid_email("not-an-email"))  # Returns True, which is incorrect

Explanation

In the vulnerable code example, the regular expression used for email validation is overly simplistic and permissive. It matches any string containing an '@' followed by a period, which is not sufficient to ensure a valid email format. This can lead to security issues by allowing malformed email addresses to be processed, potentially leading to injection attacks or logical errors.

How to fix Improper Validation of Syntactic Correctness of Input?

To address this vulnerability, the email validation logic should be made more stringent by using a regular expression that closely adheres to the standard email format as defined by RFC 5322. For critical applications, consider using specialized libraries for email validation, as they handle edge cases and ensure compliance with email standards.

Fixed Code Example

import re

def is_valid_email(email):
    # Fixed code: Proper validation of email syntax using a stricter regex pattern
    # This regex pattern is more aligned with the standard email format
    pattern = r"^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z]{2,}\$"
    return re.match(pattern, email) is not None

# Example usage
print(is_valid_email("not-an-email"))  # Returns False, which is correct
print(is_valid_email("example@example.com"))  # Returns True, which is correct

Explanation

In the fixed code example, the regular expression used for email validation is significantly more precise. It checks for a sequence of alphanumeric characters, underscores, dots, plus signs, or hyphens before the '@' symbol, followed by a valid domain name and a top-level domain of at least two characters. This pattern better reflects the standard email syntax, preventing improperly formatted emails from being accepted. By ensuring that the input adheres to the expected syntax, the risk of processing invalid data and potential security vulnerabilities is reduced.