Symlink Attack in Fiber with Dynamodb

A symlink attack in a Fiber application that uses DynamoDB typically occurs when the service writes user-controlled or untrusted data into the filesystem and that data influences file paths used to access DynamoDB export files, backups, or local caches. A common pattern is exporting DynamoDB table contents to a local directory for processing or analysis. If a filename or key derived from user input is used to build a filesystem path without canonicalization, an attacker can supply a path that traverses directories and places a symlink at a location the application trusts. When the application later writes a file at that path, it may overwrite a file that is subsequently read by another process or by administrative tooling that expects trusted content, potentially leading to data exfiltration, tampering, or privilege escalation.

Consider a scenario where a Fiber endpoint accepts a table name and a local directory to store a DynamoDB export file. If the code concatenates user-supplied values directly into the filesystem path, an attacker can provide a filename like ../../../etc/some-sensitive-file or a crafted name that results in a symlink being created in a directory the application does not control. Even though DynamoDB itself is a managed service and does not interact directly with the local filesystem, the vulnerability arises from insecure handling of local file paths in the application layer. The DynamoDB data becomes part of the file content written by the attacker-controlled symlink, which may cause the application to read sensitive system files or overwrite configuration artifacts when the export is later processed or imported.

The risk is compounded when the application runs with elevated privileges or when the exported files are later consumed by automated tooling that trusts file location and naming conventions. For example, an import routine that reads files from a known directory and applies them to a DynamoDB table may inadvertently process malicious content introduced via the symlink. This does not exploit DynamoDB itself but leverages the interaction between untrusted input, filesystem path resolution, and the use of DynamoDB as the data store. Therefore, securing the path construction, avoiding direct use of user input in filesystem operations, and ensuring strict validation are essential to mitigate symlink-based attacks in a Fiber application using DynamoDB.

To prevent symlink attacks in a Fiber application that interacts with DynamoDB, ensure that any filesystem operations related to DynamoDB exports, backups, or caches do not use untrusted input in constructing file paths. Use strict allowlists for table names, store exports in a dedicated directory with restrictive permissions, and resolve paths to their canonical form before writing files. Below are concrete code examples for a Fiber handler that safely exports DynamoDB table data to a local file.

First, define a small set of valid table names and sanitize inputs:

const validTables = ['users', 'orders', 'products'];

function isValidTable(name) {
  return validTables.includes(name);
}

Next, implement the export handler using the AWS SDK for JavaScript v3 and secure path handling:

import { DynamoDBClient, ScanCommand } from '@aws-sdk/client-dynamodb';
import { writeFileSync } from 'fs';
import { join, resolve } from 'path';
import { fileURLToPath } from 'url';
import { dirname } from 'path';
import { defineRoutes } from '..path-to-fiber';

const __dirname = dirname(fileURLToPath(import.meta.url));
const exportDir = resolve(__dirname, 'exports');

// Ensure export directory exists and is not writable by unrelated users
// On Unix: mkdir -p exports && chmod 700 exports

exportRoutes.defineRoutes((app) => {
  app.get('/export/:table', async (req, res) => {
    const tableName = req.params.table;

    if (!isValidTable(tableName)) {
      res.status(400).send({ error: 'Invalid table name' });
      return;
    }

    // Canonicalize and restrict export path to exportDir only
    const resolvedExportDir = resolve(exportDir);
    const targetPath = join(resolvedExportDir, `${tableName}.json`);
    const canonicalTarget = resolve(targetPath);

    // Prevent path traversal: ensure the canonical path is within the allowed directory
    if (!canonicalTarget.startsWith(resolvedExportDir + '/')) {
      res.status(400).send({ error: 'Invalid path' });
      return;
    }

    const client = new DynamoDBClient({ region: 'us-east-1' });
    const command = new ScanCommand({ TableName: tableName });

    try {
      const data = await client.send(command);
      writeFileSync(canonicalTarget, JSON.stringify(data, null, 2));
      res.download(canonicalTarget, `${tableName}.json`);
    } catch (err) {
      console.error(err);
      res.status(500).send({ error: 'Export failed' });
    }
  });
});

Key points in this approach:

• Allowlist validation: Only known table names are accepted, preventing arbitrary table access.
• Path canonicalization: Using resolve and startsWith ensures symlinks and .. sequences cannot escape the export directory.
• Dedicated export directory: The export directory is created with minimal permissions and kept separate from application code and sensitive system files.

For dynamic table names where an allowlist is impractical, enforce strict naming conventions (e.g., only alphanumeric characters and underscores) and avoid using user input directly in file names. Always prefer server-side paths and never expose internal file system details in responses.