HIGH out of bounds writefeathersjsdynamodb

Out Of Bounds Write in Feathersjs with Dynamodb

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Out Of Bounds Write in Feathersjs with Dynamodb — how this specific combination creates or exposes the vulnerability

An Out Of Bounds Write occurs when an application writes data past the intended allocation boundaries, which in web APIs often manifests as unchecked array indices or unsafe numeric offsets used to address storage. With FeathersJS, a typical service definition can accept payloads that include numeric fields used as array indices or as keys in data structures. If these values are not validated, an attacker can supply an index that is negative, excessively large, or otherwise outside expected ranges. When the service logic uses that value to index into an array or to compute a memory-like offset, the runtime may write to an unintended location.

In a FeathersJS service that persists data to Amazon DynamoDB, the vulnerability is shaped by how the application translates HTTP payloads into DynamoDB operations. DynamoDB itself is a managed NoSQL store with strict schema enforcement for primary keys and defined attribute types, but the application layer can still create unsafe patterns. For example, a service might accept an array identifier and an index from the client, then construct an UpdateExpression that targets a specific position in a list attribute stored as a DynamoDB list. If the index is not validated, a large or negative value can cause the SDK to generate an expression that targets an unexpected part of the item’s attribute space, or the client-side SDK may produce an invalid request structure that bypasses intended constraints.

Consider a FeathersJS service that manages a user’s ordered tags stored as a DynamoDB list attribute. A route like PATCH /tags/:id{ "index": 99999, "value": "malicious" } might be interpreted by the service as an instruction to update the 99999th element of the list. The DynamoDB SET update expression could become SET tags[99999] = :val. While DynamoDB may extend the list to accommodate the new high index, this behavior can lead to sparse lists and unexpected memory consumption on the server side, and it may expose internal data handling logic that does not account for such gaps. More critically, if the index is derived from unchecked user input and combined with other unsafe operations—such as iterating over list elements using client-provided offsets—the service may inadvertently overwrite adjacent attributes or metadata stored within the same item, leading to data corruption or privilege escalation.

The risk is compounded when the FeathersJS application uses dynamic expression building without proper sanitization. An attacker could supply negative indices or non-integer values that cause type coercion errors, leading to malformed UpdateExpression strings that write to unintended attribute names. For instance, if the application concatenates the index directly into the expression string without validation, a payload with a special character or a string like "0; DELETE FROM users" could result in malformed expressions that the SDK interprets in unexpected ways. Although DynamoDB does not execute arbitrary code, the resulting malformed updates can violate data integrity constraints, overwrite critical fields such as permissions flags, or bypass application-level checks that rely on consistent list ordering.

Because middleBrick scans the unauthenticated attack surface of a FeathersJS endpoint, it can detect indicators of such unsafe patterns. The LLM/AI Security checks may flag endpoints that accept array indices without strong type and range constraints, while the Input Validation and Property Authorization checks can surface missing safeguards on numeric parameters. The scanner does not fix the issue but provides findings with severity ratings and remediation guidance, helping developers enforce strict index validation and adopt safe update patterns when working with DynamoDB list attributes in FeathersJS services.

Dynamodb-Specific Remediation in Feathersjs — concrete code fixes

To remediate Out Of Bounds Write risks in a FeathersJS service that uses DynamoDB, you must enforce strict validation on any user-supplied numeric indices before they are used in UpdateExpression strings. The following example demonstrates a safe approach using Joi validation and guarded update logic.

const { Service } = require('feathersjs');
const Joi = require('joi');

const tagSchema = Joi.object({
  index: Joi.number().integer().min(0).max(1000).required(),
  value: Joi.string().max(256).required()
});

class TagsService {
  constructor(options) {
    this.options = options || {};
  }

  async create(data) {
    const validated = tagSchema.validate(data);
    if (validated.error) {
      throw new Error(`Validation error: ${validated.error.message}`);
    }
    const { index, value } = validated.data;
    const params = {
      TableName: process.env.TAGS_TABLE,
      Key: { id: 'tags' },
      UpdateExpression: 'SET #list.#idx = :val',
      ExpressionAttributeNames: {
        '#list': 'tags',
        '#idx': index.toString()
      },
      ExpressionAttributeValues: {
        ':val': value
      },
      ReturnValues: 'UPDATED_NEW'
    };
    const dynamoDb = new AWS.DynamoDB.DocumentClient();
    await dynamoDb.update(params).promise();
    return { index, value };
  }
}

module.exports = function () {
  const app = this;
  app.use('/tags', new TagsService());
};

This snippet ensures that the index is an integer within a safe range before it is interpolated into the UpdateExpression. Using ExpressionAttributeNames prevents injection into attribute names, while limiting the maximum index avoids creating excessively sparse lists. For list updates that involve multiple positions, consider using SET list[0] = :a, list[1] = :b with explicit bounds rather than dynamic index calculation.

In addition to validation, prefer transactional patterns where feasible. The DynamoDB TransactWriteItems API can group multiple updates atomically, reducing the impact of an out-of-bounds operation on related attributes. If your FeathersJS service supports batch operations, validate each index individually and reject the entire batch if any value is out of bounds.

For projects on the Pro plan, middleBrick enables continuous monitoring of your FeathersJS endpoints and flags missing index constraints in the Input Validation checks. The dashboard and GitHub Action integrations can fail CI/CD builds when unsafe patterns are detected, helping you catch these issues before deployment. These integrations do not auto-fix the code but provide prioritized findings and remediation guidance to steer developers toward safer implementation practices.

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Frequently Asked Questions

Can DynamoDB prevent Out Of Bounds Write by itself?
No. DynamoDB enforces schema rules for keys and data types but does not block unsafe numeric indices supplied by the client. Application-level validation in FeathersJS is required to prevent out-of-bounds writes.
Does middleBrick fix the vulnerability automatically?
No. middleBoard detects and reports the issue with severity and remediation guidance. Developers must apply the suggested code fixes and validation logic to address the vulnerability.

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