Prototype Pollution in Chi with Dynamodb

Prototype pollution in JavaScript frameworks can affect server-side behavior, and when those frameworks interact with Amazon DynamoDB clients in Chi, the risk becomes implementation-specific rather than theoretical. In Chi applications, developers often construct DynamoDB parameters dynamically based on user input, such as query conditions or update expressions. If input validation is weak, an attacker can supply keys like __proto__, constructor, or prototype that modify shared JavaScript object prototypes before data is passed to the DynamoDB layer.

For example, a Chi handler that merges incoming JSON into a DynamoDB update expression without sanitization can inadvertently cause property additions or overwrites on shared objects. This may not directly corrupt DynamoDB data, but it can alter how the application interprets responses or builds subsequent requests. A common pattern looks like this:

let updateExpression = 'SET ';
for (const key in input) {
  updateExpression += `${key} = :${key},`;
}
const params = {
  TableName: 'users',
  Key: { id: userId },
  UpdateExpression: updateExpression.slice(0, -1),
  ExpressionAttributeValues: input,
};

If input contains a polluted property such as constructor, the resulting expression may behave unexpectedly in the application logic layer, even though DynamoDB itself treats attribute names literally. The vulnerability is therefore about how Chi routes and handlers transform untrusted data into DynamoDB operations, not about DynamoDB being prototype-pollutable in its native form. Attack scenarios include privilege escalation by modifying role-checking logic or data exposure by altering how responses are serialized after DynamoDB returns results.

Another vector involves DynamoDB Document Client usage in Chi services. The Document Client marshals JavaScript types into DynamoDB’s attribute values, but if the input object has modified prototypes, default type coercion rules might produce unexpected mappings. For instance, a polluted toJSON method could cause sensitive fields to be omitted or injected during marshaling, leading to incomplete or malicious payloads being sent to DynamoDB. This is a runtime logic issue in Chi, not a DynamoDB storage issue.

Because middleBrick tests unauthenticated attack surfaces across 12 security checks including Input Validation and Unsafe Consumption, it can flag cases where user-controlled data reaches DynamoDB parameter construction without normalization. The scanner does not fix these flaws but provides prioritized findings with remediation guidance to help developers review how Chi routes and request handlers prepare data for DynamoDB operations.

Remediation focuses on ensuring that data flowing from Chi routes and handlers into DynamoDB parameter objects is normalized and validated. The safest approach is to avoid dynamic key insertion entirely or to strictly whitelist allowed keys before constructing expressions.

Instead of iterating over raw input, define an allowlist of safe attribute names and map them explicitly:

const safeKeys = new Set(['email', 'status', 'score']);
let updateExpression = 'SET ';
const expressionValues = {};
let index = 1;
for (const key of safeKeys) {
  if (key in input) {
    const placeholder = `:val${index++}`;
    updateExpression += `${key} = ${placeholder},`;
    expressionValues[placeholder] = input[key];
  }
}
if (updateExpression.endsWith(',')) {
  updateExpression = updateExpression.slice(0, -1);
}
const params = {
  TableName: 'users',
  Key: { id: userId },
  UpdateExpression: updateExpression,
  ExpressionAttributeValues: expressionValues,
};

This pattern eliminates prototype keys by design and ensures only expected fields are included. For query conditions, prefer parameterized filters with explicit attribute names rather than concatenating user input into key paths.

When using DynamoDB Document Client, validate and sanitize values before passing them to marshaling functions. You can strip or reject properties that match prototype pollution patterns:

function sanitizeForDynamo(obj) {
  if (obj && typeof obj === 'object' && !Array.isArray(obj)) {
    return Object.fromEntries(
      Object.entries(obj).filter(([key]) =>
        !['__proto__', 'constructor', 'prototype'].includes(key)
      ).map(([key, value]) => [key, typeof value === 'object' ? sanitizeForDynamo(value) : value])
    );
  }
  return obj;
}

const cleanInput = sanitizeForDynamo(userData);
const params = {
  TableName: 'profiles',
  Item: cleanInput,
};

Additionally, use middleware in Chi to enforce schema validation (for example with a lightweight runtime checker) before requests reach DynamoDB parameter construction. This complements middleBrick’s detection by reducing the likelihood that untrusted data influences DynamoDB operation structures. The scanner’s findings can guide which input fields require stricter validation and where allowlists should be applied.