Broken Access Control in Nestjs with Dynamodb

Broken Access Control occurs when authorization checks are missing, incomplete, or bypassed, allowing a user to access or modify resources they should not. Using NestJS with DynamoDB can unintentionally expose this vulnerability through several common patterns:

• Incomplete attribute-level checks: Developers often validate that a user owns a record by checking the record’s userId attribute, but they may fail to enforce the same check on related or derived operations (e.g., updating nested fields or secondary indexes).
• Overly permissive IAM policies: The AWS credentials used by NestJS (for example via the AWS SDK or an ODBC/DynamoDB connector) may grant broad read/write access to a table, rather than scoping actions to items where the user owns the partition key or has explicit ownership.
• Direct use of user input as a key: If a route parameter such as :id is used directly as a DynamoDB key without verifying that the authenticated subject has permission for that specific key, a BOLA/IDOR arises.
• Missing ownership verification on queries: Queries that filter by an index (e.g., a GSI) may return multiple items; if the service layer does not assert that each returned item belongs to the requesting user, data from other users can be leaked or modified.
• Caching or batch operations without ownership checks: Batch reads or cache-derived results may omit per-item ownership validation, leading to inadvertent data exposure or unauthorized updates across unrelated records.

These issues map to the OWASP API Top 10 Broken Object Level Authorization (BOLA) and are especially important when the data store is DynamoDB, where access patterns are defined by keys and secondary indexes rather than a relational schema.

Remediation combines strict ownership checks in NestJS service logic and carefully scoped DynamoDB access patterns. Below are concrete, working examples.

1. Enforce ownership on reads and updates

Always include the authenticated user identifier in the key expression and verify item ownership before returning or modifying data.

import { Injectable } from '@nestjs/common';
import { DynamoDBDocumentClient, GetCommand, UpdateCommand } from '@aws-sdk/lib-dynamodb';
import { ddbDocClient } from './ddb-client'; // configured DynamoDBDocumentClient

@Injectable()
export class ItemsService {
  constructor(private readonly ddb: DynamoDBDocumentClient) {}

  async getItemByOwner(userId: string, itemId: string) {
    const cmd = new GetCommand({
      TableName: process.env.ITEMS_TABLE,
      Key: { pk: `USER#${userId}`, sk: `ITEM#${itemId}` },
    });
    const { Item } = await this.ddb.send(cmd);
    if (!Item) {
      throw new NotFoundException('Item not found');
    }
    // Double-check ownership (defense in depth)
    if (Item.userId !== userId) {
      throw new ForbiddenException('Access denied');
    }
    return Item;
  }

  async updateItemByOwner(userId: string, itemId: string, updates: Partial) {
    const cmd = new UpdateCommand({
      TableName: process.env.ITEMS_TABLE,
      Key: { pk: `USER#${userId}`, sk: `ITEM#${itemId}` },
      UpdateExpression: 'set #nm = :nm, #st = :st',
      ExpressionAttributeNames: { '#nm': 'name', '#st': 'status' },
      ExpressionAttributeValues: { ':nm': updates.name, ':st': updates.status },
      ConditionExpression: 'attribute_exists(pk) AND attribute_exists(sk)',
    });
    await this.ddb.send(cmd);
  }
}

2. Scope DynamoDB requests to the user’s partition key

Design your access patterns so queries always include the user identifier in the partition key. Avoid queries that scan or filter across partitions without a user filter.

import { DynamoDBDocumentClient, QueryCommand } from '@aws-sdk/lib-dynamodb';

async function listUserItems(userId: string) {
  const cmd = new QueryCommand({
    TableName: process.env.ITEMS_TABLE,
    IndexName: 'UserIdIndex', // GSI with partition key userId
    KeyConditionExpression: 'userId = :uid',
    ExpressionAttributeValues: { ':uid': userId },
  });
  const { Items } = await ddb.send(cmd);
  return Items || [];
}

3. Apply attribute-level authorization on sensitive fields

Even when a user can access an item, restrict fields such as email or internal flags based on role or consent.

function sanitizeForUser(item: any, requestingUserId: string) {
  if (item.userId !== requestingUserId) {
    // Minimal projection: do not return sensitive fields to other users
    return { id: item.id, name: item.name };
  }
  // Owners see full payload
  return item;
}

4. Use condition expressions for optimistic concurrency and ownership

Prevent unauthorized updates by adding a version or ownership condition to UpdateCommand.

const safeUpdate = new UpdateCommand({
  TableName: process.env.ITEMS_TABLE,
  Key: { pk: `USER#${userId}`, sk: `ITEM#${itemId}` },
  UpdateExpression: 'set #v = :v',
  ExpressionAttributeNames: { '#v': 'version' },
  ExpressionAttributeValues: { ':v': nextVersion },
  ConditionExpression: 'attribute_exists(pk) AND version = :expected',
  ExpressionAttributeValues: { ':expected': currentVersion },
});

5. Validate and normalize identifiers

Ensure IDs used in requests are normalized to avoid confusion between formats (e.g., UUID vs. slug) and to prevent IDOR via ambiguous mapping.

import { plainToClass } from 'class-transformer';
import { validateOrReject } from 'class-validator';

class ItemIdDto {
  id: string;
}

async function resolveItemId(dto: ItemIdDto, userId: string) {
  await validateOrReject(dto);
  // Normalize to internal key format
  return `ITEM#${dto.id}`;
}

By combining these patterns, NestJS services can safely interact with DynamoDB while minimizing BOLA/IDOR risks and ensuring that authorization is enforced at the data-access layer.