Out Of Bounds Read in Fiber with Dynamodb

An Out Of Bounds Read occurs when a program reads memory outside the intended allocation. In a Fiber application that uses Amazon DynamoDB, this typically arises from unchecked index or key values derived from user input that affect how records are retrieved or sliced. Because DynamoDB itself does not have a traditional memory buffer, the risk manifests at the application layer: unsafe use of pagination tokens, numeric identifiers, or slice parameters can cause the service client to request items beyond the intended dataset or process results in an unsafe way.

Consider a route that fetches a paginated list of items and allows a page number query parameter. If the parameter is parsed as an integer and used to compute an offset without validating bounds, a request for a very large page can cause the application to skip over valid results or attempt to read indices that do not exist in the retrieved slice. Even though DynamoDB returns only existing items, the application code may assume a fixed page size and iterate over a result array using the unchecked offset, leading to an out-of-bounds access when the code tries to read an index that is beyond the returned slice.

Another common pattern involves using a user-supplied index to select from a list obtained from a DynamoDB query. For example, an endpoint might accept an index parameter to choose a specific item from a sorted list. If the index is not validated against the actual number of items returned, an attacker can supply an out-of-range value. When the application later accesses the list at that index, it performs an out-of-bounds read. This can expose unintended data from the process memory or cause a crash, depending on how the runtime handles the invalid access. The DynamoDB request itself is valid, but the unsafe handling of the response in Fiber makes the application vulnerable.

Because middleBrick tests unauthenticated attack surfaces, such input handling flaws are detectable. The scanner exercises endpoints that use DynamoDB queries with manipulated parameters, including large numeric offsets or indexes, to observe whether the application exhibits instability or returns unexpected data patterns. Findings from these checks highlight cases where pagination or indexing logic does not properly constrain user input, which is an important signal for developers to review bounds enforcement.

Remediation focuses on strict validation of any value that influences pagination or indexing before it is used to access collections. Ensure that page numbers, offsets, and indexes are non-negative, within feasible ranges, and consistent with the size of the result set. Use safe iteration practices, such as iterating over actual items rather than assuming fixed sizes, and avoid raw pointer arithmetic or unchecked indexing. These measures reduce the likelihood of out-of-bounds reads when working with DynamoDB-backed Fiber services.

To prevent out-of-bounds reads, validate and sanitize all inputs that affect how data is retrieved from DynamoDB in a Fiber application. Below are concrete examples that show safe patterns for pagination and indexed access.

Safe Pagination with Limit/ExclusiveStartKey

Use DynamoDB's native pagination tokens instead of computing offsets from user input. If you must support page numbers, validate them rigorously and compute the token from the previous page's last key.

const { DynamoDBClient, QueryCommand } = require("@aws-sdk/client-dynamodb");
const client = new DynamoDBClient({ region: "us-east-1" });

app.get("/items", async (req, res) => {
  const page = parseInt(req.query.page, 10) || 1;
  const limit = parseInt(req.query.limit, 10) || 10;

  if (page < 1 || limit < 1 || limit > 100) {
    return res.status(400).json({ error: "Invalid pagination parameters" });
  }

  const params = {
    TableName: "ItemsTable",
    Limit: limit,
  };

  try {
    const result = await client.send(new QueryCommand(params));
    res.json({ items: result.Items, lastEvaluatedKey: result.LastEvaluatedKey });
  } catch (err) {
    res.status(500).json({ error: "Failed to fetch items" });
  }
});

Validated Index Access

When selecting an item by index from a list retrieved from DynamoDB, ensure the index is within bounds of the actual returned items.

app.get("/items/:index", async (req, res) => {
  const index = parseInt(req.params.index, 10);
  if (Number.isNaN(index) || index < 0) {
    return res.status(400).json({ error: "Invalid index" });
  }

  const result = await client.send(new ScanCommand({ TableName: "ItemsTable" }));
  const items = result.Items;

  if (index >= items.length) {
    return res.status(404).json({ error: "Item not found" });
  }

  res.json(items[index]);
});

Using Middleware to Enforce Bounds

In Fiber, you can add middleware to validate query and parameter values before handlers run, centralizing bounds checks.

app.use((req, res, next) => {
  const page = parseInt(req.query.page, 10);
  const limit = parseInt(req.query.limit, 10);
  if ((page !== null && (isNaN(page) || page < 1)) || (limit !== null && (isNaN(limit) || limit < 1 || limit > 100))) {
    return res.status(400).json({ error: "Invalid pagination values" });
  }
  next();
});

These patterns demonstrate how to align DynamoDB usage with safe indexing practices in Fiber, reducing the chance of out-of-bounds reads while preserving the ability to inspect API behavior during scans.