CWE-93: Improper Neutralization of CRLF Sequences ('CRLF Injection')

What is Improper Neutralization of CRLF Sequences ('CRLF Injection')?

• Overview: This vulnerability occurs when an application improperly handles carriage return and line feed (CRLF) sequences. Attackers can exploit this flaw to inject malicious CRLF sequences into data streams, potentially manipulating web server responses or log files.

• Exploitation Methods:

Attackers can inject CRLF sequences in HTTP headers to perform HTTP response splitting, leading to cross-site scripting (XSS) or cache poisoning.
Common attack patterns include adding CRLF sequences to user input fields that are not adequately sanitized, causing unintended splitting of HTTP responses.

• Security Impact:

Direct consequences of successful exploitation include unauthorized content injection, which can lead to information disclosure or the execution of malicious code.
Potential cascading effects might involve further exploitation of injected content, such as phishing attacks or session hijacking.
Business impact can range from damaged reputation to financial losses due to security breaches and non-compliance with data protection regulations.

• Prevention Guidelines:

Specific code-level fixes include validating and sanitizing all user inputs by removing or encoding CRLF characters.
Security best practices involve using standardized libraries for input handling and staying updated on security advisories for your software stack.
Recommended tools and frameworks include input validation libraries and web application firewalls (WAFs) that can detect and block CRLF injection attempts.

Corgea can automatically detect and fix Improper Neutralization of CRLF Sequences ('CRLF Injection') in your codebase. [Try Corgea free today](https://corgea.app).

Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Vulnerable Code Example

from flask import Flask, request, Response

app = Flask(__name__)

@app.route('/header')
def add_header():
    # Vulnerable to CRLF Injection: user input is directly included in the HTTP header
    # An attacker can inject newline characters to manipulate HTTP headers or perform HTTP response splitting.
    user_agent = request.args.get('user_agent', '')
    response = Response("Hello, World!")
    response.headers['User-Agent'] = user_agent
    return response

if __name__ == '__main__':
    app.run()

How to fix Improper Neutralization of CRLF Sequences ('CRLF Injection')?

To fix this vulnerability, it is important to sanitize user inputs by removing or encoding CRLF sequences. This ensures that any input that contains newline characters is neutralized and cannot be used to manipulate HTTP headers. In Python, you can use a regular expression to remove any CRLF sequences from the input. Additionally, using a library function that automatically handles header values safely is recommended, whenever available.

Fix Approach:

Sanitize Input: Strip or encode CRLF sequences from user inputs to prevent them from being included in HTTP headers.
Use Safe Libraries: If possible, use libraries or frameworks that automatically handle dangerous characters in HTTP headers.

Fixed Code Example

from flask import Flask, request, Response
import re

app = Flask(__name__)

@app.route('/header')
def add_header():
    # Fix: Sanitize the input by removing CRLF sequences
    user_agent = request.args.get('user_agent', '')
    # Remove CRLF sequences from the input to prevent injection
    sanitized_user_agent = re.sub(r'[\r\n]', '', user_agent)
    response = Response("Hello, World!")
    response.headers['User-Agent'] = sanitized_user_agent
    return response

if __name__ == '__main__':
    app.run()

In this fixed code, the re.sub(r'[\r\n]', '', user_agent) function call removes any carriage return or line feed characters from the user-provided user_agent, effectively neutralizing potential CRLF injection attempts. This ensures that the HTTP headers cannot be manipulated by malicious input.