Arbitrary File Deletion Bug: Analysis And Fix Guide

Aug 14, 2025 by Felix Dubois 52 views

BUG Arbitrary File Deletion: A Comprehensive Analysis and Remediation Guide

Introduction

Hey guys! Today, we're diving deep into a serious security vulnerability: Arbitrary File Deletion. This bug can be a real headache, allowing attackers to delete any file on your system if not handled correctly. We'll break down what it is, how it works, and, most importantly, how to fix it. So, let's get started and make our systems more secure!

What is Arbitrary File Deletion?

Arbitrary File Deletion vulnerabilities are a critical security flaw. Imagine giving someone the keys to your entire file cabinet and letting them throw away anything they want – that's essentially what this vulnerability does. It occurs when a web application or system allows users to control the file paths used in deletion operations without proper validation. This means an attacker can manipulate these paths to delete files they shouldn't have access to, potentially causing significant damage.

The core issue stems from a lack of security checks on user-supplied file paths. When a function designed to delete files doesn't verify that the user has the right permissions or that the file is within an expected directory, it opens the door to abuse. Attackers can exploit this by crafting malicious requests with file paths that point to critical system files, configuration files, or even other users' data. Think of it as a flaw in the application's design where trust is placed in user input without verification, a classic recipe for disaster in the cybersecurity world. The implications can range from data loss and system instability to complete system compromise, making it a high-priority vulnerability to address.

How Does It Work?

To really understand the danger, let's look at how this vulnerability works under the hood. The root cause is simple: the application doesn't properly check the file path provided by the user before deleting a file. This lack of validation opens the door for sneaky attacks.

Here’s a breakdown of the typical process:

User Input: The application receives a file path from the user, often through a web form or API request. For example, a user might request to delete a file named “report.pdf.”
Path Manipulation: An attacker can manipulate this file path by using techniques like “../” (path traversal) to navigate outside the intended directory. Instead of “report.pdf,” they might input “../../../../../etc/passwd” to target a critical system file.
Unvalidated Deletion: The application, without proper checks, passes this manipulated path to the file deletion function.
File Deletion: The system blindly follows the path and deletes the specified file, regardless of whether it should or not.

The code snippet provided earlier clearly illustrates this vulnerability. The handleLocalFileDelete function constructs a full file path by simply joining the UPLOAD_DIR with the filename extracted from user input. If the filename contains path traversal sequences, it can escape the intended directory. The existsSync check only verifies if the file exists, not whether the path is safe. Similarly, in cloud storage scenarios, the extractCloudKey function attempts to extract the key from the file path but includes a fallback that assumes the incoming string is a raw key, effectively bypassing any path validation if the path starts with “/api/files/serve/”.

This flawed process highlights the importance of rigorous input validation and sanitization. Without these measures, attackers can easily bypass security mechanisms and wreak havoc on the system. Proper validation ensures that the application only operates on files within the intended scope, preventing unauthorized access and deletion.

Code Analysis

Let's break down the vulnerable code snippet to see exactly where things go wrong. We'll focus on the critical sections that lead to the Arbitrary File Deletion vulnerability.

Here’s the code snippet we’re analyzing:

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()
}

Vulnerable Functions

handleLocalFileDelete(filePath: string): This function is the primary culprit for local file deletion vulnerabilities. It constructs the full file path by joining UPLOAD_DIR with the filename extracted from the user-provided filePath. The extractFilename function and path.join do not sanitize the input, allowing directory traversal. Here’s the snippet:

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;
  }
}

The existsSync(fullPath) check only verifies if the file exists but doesn't validate the path's integrity or whether it's within the intended directory. This means an attacker can use path traversal sequences like ../../.. to navigate to and delete files outside the UPLOAD_DIR.

extractCloudKey(filePath: string): This function is vulnerable in cloud storage scenarios. It attempts to extract the cloud storage key from the filePath. However, it includes a fallback mechanism that directly uses the incoming string as a raw key if the path starts with /api/files/serve/. This bypasses any intended path validation. Here’s the relevant code:
```
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;
}
```
The “backwards-compatibility” clause allows an attacker to provide a crafted path that bypasses the extraction logic, potentially leading to arbitrary key usage in cloud storage deletion operations.

Key Vulnerabilities

Path Traversal: The handleLocalFileDelete function's use of path.join without proper sanitization allows attackers to use ../ sequences to navigate the file system and delete arbitrary files.
Raw Key Usage: The extractCloudKey function's fallback mechanism treats the input as a raw key, bypassing validation for cloud storage deletions if the path starts with /api/files/serve/.
Lack of Input Validation: There’s a general lack of validation on the filePath provided by the user, which is the root cause of the vulnerability. The application trusts the user input without ensuring it conforms to expected formats or constraints.

By understanding these vulnerabilities, we can better appreciate the risks and implement effective remediation measures. Next, we’ll dive into a Proof of Concept (POC) to see this vulnerability in action.

Proof of Concept (POC)

Alright, let's get our hands dirty and see this Arbitrary File Deletion vulnerability in action with a Proof of Concept (POC). This will help you understand how an attacker can exploit this flaw.

Scenario

We'll simulate an attack where we try to delete a file outside the intended upload directory. For this example, let's say we want to delete /tmp/cat.txt, a file that should be off-limits.

Steps

Craft the Payload: We'll create a malicious payload containing a file path designed to escape the intended directory. This involves using path traversal sequences like ../ to navigate up the directory tree.

Here’s the payload we’ll use:
```
{
  "filePath": "/api/files/serve/../../../../../../../../../tmp/cat.txt"
}
```
This payload uses /api/files/serve/ to trigger the vulnerable fallback in extractCloudKey and then uses ../ sequences to navigate to the root directory and target /tmp/cat.txt.

Send the Request: We'll send a POST request to the /api/files/delete endpoint with our crafted payload. This simulates an attacker sending a malicious request to the server.

Here’s the HTTP request:

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"
}

Observe the Outcome: If the application is vulnerable, the file /tmp/cat.txt will be deleted. This demonstrates the Arbitrary File Deletion vulnerability.

Visual Evidence

The provided images further illustrate the POC:

The first image shows the crafted payload being sent to the server.
The second image likely shows the successful deletion of the targeted file.

This POC clearly shows how an attacker can exploit the vulnerability by manipulating file paths. By sending a crafted request, they can bypass security checks and delete files outside the intended directory. This emphasizes the importance of proper input validation and sanitization.

Next up, we'll discuss how to remediate this vulnerability and protect your system from such attacks. Let's get to it!

Remediation

Okay, guys, now for the most important part: how to fix this mess! We've seen how dangerous Arbitrary File Deletion can be, so let's talk about the steps you can take to secure your application.

The key to remediation is implementing robust input validation and sanitization. Here’s a breakdown of the best practices:

1. Input Validation and Sanitization

Input validation is the first line of defense. It involves verifying that the input provided by the user meets the expected format and constraints. Sanitization is the process of cleaning the input to remove any potentially harmful characters or sequences.

Whitelist Allowed Characters: Instead of trying to blacklist dangerous characters, create a whitelist of characters that are allowed in file names and paths. This prevents attackers from using unexpected characters or sequences.
Path Canonicalization: Use path canonicalization to resolve symbolic links and remove relative path components (. and ..). This ensures that the file path points to the intended location and prevents path traversal attacks.
Validate Against a Known Base Directory: Ensure that the file path stays within the expected base directory. If a user tries to access a file outside this directory, reject the request.

2. Secure File Handling

Use UUIDs for File Names: Instead of using user-provided file names, generate UUIDs (Universally Unique Identifiers) for file names. This prevents attackers from predicting file names and manipulating paths.
Restrict Access with Least Privilege: Grant the application only the necessary permissions to access files. Avoid running the application with administrative privileges, as this can escalate the impact of a vulnerability.
Centralized File Management: Implement a centralized file management system that enforces access controls and performs security checks. This makes it easier to manage file access and prevent unauthorized operations.

3. Code-Level Fixes

Let's look at how we can fix the vulnerable code snippets we discussed earlier.

handleLocalFileDelete Fix: Instead of directly joining the user-provided filename with UPLOAD_DIR, we need to validate and sanitize the path.

Here’s a safer version:

import path from 'path';

async function handleLocalFileDelete(filePath: string) {
  const filename = extractFilename(filePath);
  const fullPath = path.resolve(UPLOAD_DIR, filename); // Use path.resolve for security

  // Check if the resolved path is within the allowed directory
  if (!fullPath.startsWith(UPLOAD_DIR)) {
    logger.warn(`Attempted path traversal: ${fullPath}`);
    return {
      success: false as const,
      message: 'Invalid file path',
    };
  }

  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;
  }
}

Key changes:

path.resolve: Using path.resolve helps canonicalize the path, resolving .. and . sequences.
startsWith Check: We check if the resolved path starts with UPLOAD_DIR. If it doesn’t, we reject the request, preventing path traversal.

extractCloudKey Fix: Remove the vulnerable fallback that treats the input as a raw key.

Here’s the improved version:

function extractCloudKey(filePath: string): string {
  if (isS3Path(filePath)) {
    return extractS3Key(filePath);
  }

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

  // Remove the vulnerable fallback
  // 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;

  // Throw an error if the path is not recognized
  throw new Error('Invalid cloud file path');
}

By removing the fallback, we force the function to only accept valid S3 or Blob paths, preventing attackers from using arbitrary keys.

4. Logging and Monitoring

Log File Access: Keep detailed logs of all file access and deletion attempts. This helps in identifying suspicious activity and investigating security incidents.
Monitor for Anomalies: Set up monitoring systems to detect unusual file access patterns or deletion attempts. Alert administrators when suspicious activity is detected.

5. Regular Security Audits and Testing

Security Audits: Conduct regular security audits to identify vulnerabilities in your application. Use automated tools and manual code reviews to ensure comprehensive coverage.
Penetration Testing: Perform penetration testing to simulate real-world attacks and identify weaknesses in your security measures. This helps validate your remediation efforts and uncover new vulnerabilities.

By implementing these measures, you can significantly reduce the risk of Arbitrary File Deletion vulnerabilities in your application. Remember, security is an ongoing process, so stay vigilant and keep your systems updated.

Conclusion

So, there you have it, guys! We've taken a comprehensive look at Arbitrary File Deletion vulnerabilities. We've defined what they are, explored how they work, analyzed vulnerable code, demonstrated a Proof of Concept, and, most importantly, discussed effective remediation strategies.

This vulnerability can be a serious threat, but with the right knowledge and practices, you can protect your applications. Remember to prioritize input validation, sanitize file paths, and implement secure file handling practices. Regular security audits and testing are also crucial for maintaining a secure system.

Stay vigilant, keep learning, and let's make the web a safer place together! If you have any questions or want to share your experiences, feel free to drop a comment below. Happy coding, and stay secure!