Format String in Feathersjs with Dynamodb

A format string vulnerability occurs when user-controlled input is directly used in functions that interpret format specifiers, such as printf-style operations. In a Feathersjs application using Dynamodb, this typically arises when constructing request parameters or logging data where untrusted input is inserted into format strings. For example, if a service handler uses util.format or string interpolation to build a condition expression for Dynamodb queries without proper validation, an attacker can supply format specifiers like %s, %d, or %n that cause unintended data leakage or application behavior changes.

Feathersjs services often interact with Dynamodb through an ORM-like layer or raw SDK calls. Consider a scenario where a before hook builds a filter expression using user input:

const util = require('util');
app.service('items').hooks({
  before: {
    create: [context => {
      const userInput = context.data.search;
      context.params.dynamodb = {
        TableName: 'Items',
        FilterExpression: util.format('contains(#name, %s)', userInput)
      };
      return context;
    }]
  }
});

If userInput contains format specifiers, util.format will interpret them, potentially reading stack memory or causing errors that expose internal state. In a Dynamodb context, this can lead to malformed condition expressions or information disclosure through error messages. An attacker might submit a payload like \'%s\' %s to probe how the application handles format arguments, or use %n to write arbitrary memory locations in native bindings, though the primary risk in server-side JavaScript is information disclosure via crafted error responses.

Moreover, logging mechanisms in Feathersjs that include user data without sanitization can amplify the impact. If the application logs formatted strings that include Dynamodb request parameters, an attacker can inject format specifiers into logs, leading to log injection or sensitive data exposure through log retrieval mechanisms. The combination of Feathersjs's flexible hook system and Dynamodb's expression-based query syntax increases the surface for improper input handling if validation is not strictly enforced.

Another vector involves error messages generated by the Dynamodb SDK when malformed expressions are submitted. If user input is embedded in these expressions via format strings, the resulting errors might reveal internal details such as table structure or expected attribute types. This aligns with common web vulnerabilities where improper input validation leads to information exposure, emphasizing the need for strict input sanitization before constructing Dynamodb expressions in Feathersjs services.

To mitigate format string risks when using Feathersjs with Dynamodb, always treat user input as data rather than executable parts of expressions. Use parameterized approaches and explicit validation instead of string formatting functions. Below are concrete remediation patterns with valid Dynamodb code examples for Feathersjs services.

1. Avoid util.format for query construction. Instead, build FilterExpression using placeholders and the ExpressionAttributeNames and ExpressionAttributeValues parameters:

app.service('items').hooks({
  before: {
    create: [context => {
      const userSearch = context.data.search;
      // Safe: use ExpressionAttributeNames for attribute placeholders
      context.params.dynamodb = {
        TableName: 'Items',
        FilterExpression: 'contains(#nm, :val)',
        ExpressionAttributeNames: {
          '#nm': 'name'
        },
        ExpressionAttributeValues: {
          ':val': userSearch
        }
      };
      return context;
    }]
  }
});

This approach ensures user input is passed as a value, not interpreted as part of the expression syntax, eliminating format specifier interpretation by the runtime.

2. Validate and sanitize input before use. Implement validation that rejects input containing format specifiers or unexpected characters for the expected data context:

const hasFormatSpecifier = (str) => {
  // Basic check for printf-style specifiers
  return /[%#\0]/.test(str);
};

app.service('items').hooks({
  before: {
    create: [context => {
      const userInput = context.data.search;
      if (hasFormatSpecifier(userInput)) {
        throw new Error('Invalid input: format specifiers not allowed');
      }
      context.params.dynamodb = {
        TableName: 'Items',
        FilterExpression: 'contains(#name, :searchVal)',
        ExpressionAttributeNames: { '#name': 'name' },
        ExpressionAttributeValues: { ':searchVal': userInput }
      };
      return context;
    }]
  }
});

3. Use Feathersjs hooks to enforce schema-based validation. Leverage hooks like feathers-hooks-common to validate input against a defined schema, ensuring only safe values reach Dynamodb expressions:

const { iff, isProvider } = require('feathers-hooks-common');
const { validator } = require('feathers-joi');

app.service('items').hooks({
  before: {
    create: [
      iff(isProvider('external'), validator({
        search: Joi.string().pattern(/^[\w\-\s]+$/).required()
      }))
    ]
  }
});

These patterns ensure that user input never participates in string formatting operations that could interpret format specifiers, thereby securing the interaction between Feathersjs and Dynamodb.