Critical Code Injection In Bottle Framework's Load() Function

by Alex Johnson 62 views

Introduction

In this article, we delve into a critical security vulnerability discovered within the Bottle Framework. Specifically, we'll dissect a code injection flaw residing in the load() function. This vulnerability allows for potential Remote Code Execution (RCE) if an attacker gains control over the input. Understanding the intricacies of this flaw is crucial for developers using Bottle Framework to ensure the security and integrity of their applications. Let's explore the details, impact, and recommended mitigation strategies.

Vulnerability Details

Location and Severity

The vulnerability is located in bottle.py at line 3775, within the load() function. It is classified as a CRITICAL vulnerability with a CVSS score of 9.1, emphasizing its severity and potential impact. The Common Weakness Enumeration (CWE) identifier for this vulnerability is CWE-94, which signifies Code Injection.

Affected Code Snippet

The vulnerable code segment is as follows:

def load(target, **namespace):
 module, target = target.split(":", 1) if ':' in target else (target, None)
 if module not in sys.modules: __import__(module)
 if not target: return sys.modules[module]
 if target.isalnum(): return getattr(sys.modules[module], target)
 package_name = module.split('.')[0]
 namespace[package_name] = sys.modules[package_name]
 return eval('%s.%s' % (module, target), namespace) # VULNERABLE

The load() function utilizes Python's eval() function to execute arbitrary Python expressions. This means that if an attacker can manipulate the input passed to load(), they can inject malicious code that will be executed by the server. The use of eval() is generally discouraged in security-sensitive contexts due to its inherent risks, and this case highlights why careful input validation and alternative approaches are essential.

Attack Vector

The attack unfolds through the following steps:

  1. An attacker gains control over the input to the load() function.
  2. This input can originate from various sources, including:
    • Command-line interface (CLI) parameters (e.g., --plugin "os:system('id')")
    • Configuration file loading mechanisms
    • Direct, programmatic usage of the load() function
  3. The eval() function then executes the attacker-supplied malicious code, leading to code injection.

The key to exploiting this vulnerability is the attacker's ability to inject arbitrary code into the target variable, which is then used in the eval() function. This can be achieved by crafting malicious input strings that are not properly sanitized or validated.

Impact

The consequences of a successful attack are severe and can include:

  • Remote Code Execution (RCE): The attacker can execute arbitrary commands on the server, potentially gaining complete control.
  • File system access: The attacker can read, write, and delete files on the server's file system.
  • Data exfiltration: Sensitive data can be stolen from the server.
  • System compromise: The entire system can be compromised, leading to significant damage and disruption.

Proof of Concept (PoC)

To demonstrate the vulnerability, the following Proof of Concept (PoC) showcases code execution using the whoami command:

from bottle import load
import subprocess

# Execute whoami command
result = load("subprocess:check_output(['whoami'], text=True)")
print(f"WHOAMI: {result.strip()}")
# Output: WHOAMI: desktop-3gh78j5\\trrsl

This code snippet uses the load() function to execute the check_output function from the subprocess module, which in turn runs the whoami command. The output reveals the user account under which the code is executed, demonstrating the ability to execute system commands via code injection.

Additional Attack Examples

Further examples of potential attacks include:

# Execute system commands
load("os:system('id')")

# File system access
load("os:listdir('/')")

# Data exfiltration
load("open('/etc/passwd').read()")

These examples illustrate how an attacker could use the vulnerability to execute arbitrary system commands, access the file system, and even exfiltrate sensitive data like the contents of the /etc/passwd file. The potential for damage is extensive, making it essential to address this vulnerability promptly.

Affected Versions

The vulnerability affects the following versions of Bottle Framework:

  • Bottle 0.14-dev
  • All prior versions that use the load() function

This broad range of affected versions underscores the importance of applying the recommended mitigations to ensure the security of applications built on Bottle Framework.

Recommendations

To mitigate the risks associated with this vulnerability, the following recommendations should be implemented:

  1. Replace eval() with getattr() and callable(): The most effective solution is to replace the eval() function with a safer alternative that avoids executing arbitrary code. Using getattr() to access attributes and callable() to verify if an attribute is callable can provide a more controlled way to achieve the desired functionality.
  2. Implement a strict whitelist of allowed modules and functions: Create a whitelist of modules and functions that are explicitly allowed to be accessed through the load() function. This will prevent attackers from accessing potentially dangerous functions.
  3. Add input validation to restrict allowed expressions: Implement robust input validation to ensure that the input passed to the load() function conforms to a predefined format. This can help prevent attackers from injecting malicious code.
  4. Use AST parsing instead of eval() for safer expression evaluation: Abstract Syntax Tree (AST) parsing can be used to analyze and evaluate expressions in a safer manner than eval(). AST parsing allows you to examine the structure of the expression and ensure that it does not contain any malicious code before executing it.

By implementing these recommendations, developers can significantly reduce the risk of code injection attacks and protect their applications from compromise.

References

The following references provide additional context and information about the vulnerability:

  • Line 3775 in bottle.py: Direct use of eval()
  • _main() function (lines 4565-4572): Usage of load() for CLI
  • load_app() function (line 3786): Another instance of load() usage

Disclosure

This vulnerability has been responsibly disclosed to the Bottle Framework maintainers. Please coordinate with them before public disclosure to ensure that a patch is available and that users are adequately informed.

Conclusion

The code injection vulnerability in Bottle Framework's load() function poses a significant security risk. By understanding the vulnerability details, attack vector, and potential impact, developers can take proactive steps to mitigate the risk and protect their applications. Implementing the recommended mitigations, such as replacing eval() with safer alternatives, whitelisting allowed modules and functions, and adding input validation, is crucial for ensuring the security and integrity of Bottle Framework applications.

For more information on secure coding practices, consider exploring resources like the OWASP Foundation.