What is Improper Restriction of Names for Files (CWE-641)?

TL;DR: Improper Restriction of Names for Files and Other Resources (CWE-641) is a vulnerability where unvalidated user input constructs file or resource names, leading to security risks. Mitigation involves strict validation of such inputs.

What is Improper Restriction of Names for Files and Other Resources?

Improper Restriction of Names for Files and Other Resources (CWE-641) is a type of vulnerability where the product constructs the name of a file or other resource using input from an upstream component, but it does not restrict or incorrectly restricts the resulting name. As defined by the MITRE Corporation under CWE-641, and classified by the OWASP Foundation as not directly mapped.

Quick Summary

Improper Restriction of Names for Files and Other Resources is a security vulnerability that can lead to unauthorized access, code execution, information leakage, and denial-of-service attacks. It occurs when filenames or resource names are constructed without proper validation based on user input. Jump to: Overview · How it Works · Business Impact · Attack Scenario · Detection · Fix · Framework-Specific Fixes · Ask AI · Best Practices · FAQ · Related Vulnerabilities

Jump to: Quick Summary · Improper Restriction of Names for Files and Other Resources Overview · How Improper Restriction of Names for Files and Other Resources Works · Business Impact of Improper Restriction of Names for Files and Other Resources · Improper Restriction of Names for Files and Other Resources Attack Scenario · How to Detect Improper Restriction of Names for Files and Other Resources · How to Fix Improper Restriction of Names for Files and Other Resources · Framework-Specific Fixes for Improper Restriction of Names for Files and Other Resources · How to Ask AI to Check Your Code for Improper Restriction of Names for Files and Other Resources · Improper Restriction of Names for Files and Other Resources Best Practices Checklist · Improper Restriction of Names for Files and Other Resources FAQ · Vulnerabilities Related to Improper Restriction of Names for Files and Other Resources · References · Scan Your Own Site

Improper Restriction of Names for Files and Other Resources Overview

What: Improper Restriction of Names for Files and Other Resources is a vulnerability where filenames or resource names are constructed without proper validation based on user input. Why it matters: This can lead to unauthorized access, code execution, information leakage, and denial-of-service attacks. Where it occurs: In applications that construct file paths using unvalidated user inputs. Who is affected: Applications that allow users to control filenames or resource names through input parameters. Who is NOT affected: Systems already using strict validation mechanisms for filenames.

How Improper Restriction of Names for Files and Other Resources Works

Root Cause

The root cause lies in the lack of proper validation and restriction when constructing file or resource names based on user inputs, leading to potential security risks such as unauthorized access or code execution.

Attack Flow

1. An attacker provides a malicious input that constructs an unsafe filename or resource name.
2. The application uses this input without proper validation, leading to unintended actions like accessing sensitive files or executing arbitrary commands.
3. The attacker exploits the lack of restrictions to gain unauthorized privileges or cause system disruptions.

   Prerequisites to Exploit

   • The application must accept user inputs that influence file/resource names.
   • There should be no validation or restriction mechanisms in place for these inputs.

     Vulnerable Code

     file_path = input("Enter a filename: ")
     open(file_path, 'r')
     

     This code allows an attacker to specify any file path, potentially leading to unauthorized access.

Secure Code

allowed_paths = ['/safe/path', '/another/safe/path']
file_path = input("Enter a filename: ")

if file_path not in allowed_paths:
    raise ValueError('Invalid file path')

open(file_path, 'r')

This code ensures that only predefined safe paths are accepted.

Business Impact of Improper Restriction of Names for Files and Other Resources

Confidentiality: Attackers can access sensitive files by constructing malicious filenames.

• Example: An attacker constructs a filename to read /etc/passwd.
• Financial impact: Loss of customer data can lead to financial penalties.
• Compliance issues: Breaches may result in non-compliance with regulations like GDPR or HIPAA.

Integrity: Attackers can modify critical files by constructing malicious filenames.

• Example: An attacker constructs a filename to overwrite /etc/shadow.
• Business consequences: Data corruption can disrupt operations and require costly recovery efforts.

Availability: Malformed file names can cause crashes, leading to denial-of-service conditions.

• Example: An attacker constructs a malformed path that causes the application to crash.
• Operational impact: System downtime affects user experience and productivity.

Improper Restriction of Names for Files and Other Resources Attack Scenario

1. The attacker sends a request with a malicious filename parameter.
2. The server uses this input to construct a file path without validation.
3. The constructed path leads to unauthorized access or execution of arbitrary code.
4. The application crashes due to malformed paths, causing denial-of-service conditions.

How to Detect Improper Restriction of Names for Files and Other Resources

Manual Testing

• Check if filenames are validated against a strict allowlist.
• Verify that user inputs influencing file/resource names are properly sanitized.

  Automated Scanners (SAST/DAST)

  Static analysis can identify code paths where file or resource names are constructed without validation. Dynamic testing is required to confirm the absence of protective measures in runtime scenarios.

PenScan Detection

PenScan’s automated scanners such as ZAP and Nuclei detect improper restrictions by identifying unvalidated input used to construct filenames.

False Positive Guidance

False positives can occur if benign paths or resource names are flagged due to lack of context. Ensure that only truly unsafe patterns trigger alerts.

How to Fix Improper Restriction of Names for Files and Other Resources

• Do not allow users to control names of resources used on the server side.
• Perform allowlist input validation at entry points before consuming the resources.
• Make sure technologies consuming the resources are not vulnerable in a way that would allow code execution if the name is malformed.

Framework-Specific Fixes for Improper Restriction of Names for Files and Other Resources

Python/Django

from django.core.validators import FileExtensionValidator

def handle_uploaded_file(file):
    allowed_extensions = ['txt', 'csv']
    validator = FileExtensionValidator(allowed_extensions)
    try:
        validator(file.name)
    except ValidationError:
        raise ValueError('Invalid file extension')

Java Spring Boot

@PostMapping("/upload")
public ResponseEntity<String> uploadFile(@RequestParam("file") MultipartFile file) {
    List<String> allowedExtensions = Arrays.asList("txt", "csv");
    if (!allowedExtensions.contains(file.getOriginalFilename().substring(file.getOriginalFilename().lastIndexOf('.') + 1))) {
        return new ResponseEntity<>("Invalid file extension", HttpStatus.BAD_REQUEST);
    }
    // Proceed with safe operations
}

PHP

$allowed_extensions = array('txt', 'csv');
$file_extension = pathinfo($_FILES['file']['name'], PATHINFO_EXTENSION);

if (!in_array($file_extension, $allowed_extensions)) {
    die("Invalid file extension");
}

// Proceed with safe operations

How to Ask AI to Check Your Code for Improper Restriction of Names for Files and Other Resources

Review the following Python code block for potential CWE-641 Improper Restriction of Names for Files and Other Resources vulnerabilities and rewrite it using allowlist validation:

Improper Restriction of Names for Files and Other Resources Best Practices Checklist

✅ Do not allow users to control names of resources used on the server side. ✅ Perform strict input validation before constructing file or resource paths. ✅ Ensure technologies consuming the resources are not vulnerable to malformed inputs. ✅ Use a whitelist approach for validating filenames and resource names. ✅ Regularly audit code for improper restrictions in filename construction.

Improper Restriction of Names for Files and Other Resources FAQ

How does improper restriction of names lead to security vulnerabilities?

Improperly restricted file names can allow attackers to access or manipulate files they shouldn’t be able to, leading to unauthorized actions such as code execution or data theft.

Can you provide an example of vulnerable code for CWE-641?

Vulnerable code constructs a file path using user input without proper validation, allowing paths like ‘../etc/passwd’.

How can I detect improper restriction of names in my application?

Manual testing involves checking how your application handles file paths and resource names. Automated scanners can also identify potential vulnerabilities.

What is the best way to prevent CWE-641 from occurring?

Use allowlist validation for all input that influences file or resource names, ensuring only safe patterns are accepted.

How does improper restriction of names affect application availability?

Improperly restricted names can cause denial-of-service attacks by crashing the application with malformed paths.

What is an example of secure code to prevent CWE-641?

Secure code validates and restricts file path inputs before using them, ensuring only safe patterns are accepted.

How does OWASP recommend mitigating improper restriction of names for files?

OWASP recommends validating all input that influences resource names against a strict allowlist to prevent unauthorized access.

Vulnerabilities Related to Improper Restriction of Names for Files and Other Resources

| CWE | Name | Relationship | |—|—|—| | CWE-99 | Resource Injection (ChildOf) |

References

• MITRE
• NVD

Scan Your Own Site

Manual code review catches what you know to look for. An automated scan catches what you didn’t. Scan your own website using PenScan to find Improper Restriction of Names for Files and Other Resources and other risks before an attacker does.