Out Of Bounds Write in Fiber with Dynamodb

An Out Of Bounds Write occurs when a program writes data outside the intended memory boundaries. In the context of a Fiber application interacting with DynamoDB, this typically arises from unchecked user input used to control item attributes, table names, or key structures. Because DynamoDB is a managed NoSQL service, the risk does not manifest as classic memory corruption but as unintended writes to attributes, incorrect key construction, or writes to unexpected items or tables. This can lead to data corruption, privilege escalation, or sensitive data modification when input validation is insufficient.

Consider a user profile update endpoint in Fiber that accepts a JSON payload containing an user_id and a list of attribute updates. If the server uses this input directly to construct a DynamoDB key without validating that the user_id belongs to the requesting actor, an attacker can modify other users' items. For example, a crafted request can supply an arbitrary user_id, causing the update operation to target a different partition key. Because DynamoDB operations are permission-bound to the provided credentials, the vulnerability depends on overprivileged IAM policies or missing authorization checks in the application layer.

Another scenario involves using user-controlled data to build attribute names or nested map keys. If an endpoint dynamically sets attribute names based on unvalidated input, it may write to unintended paths within an item. This can corrupt the item structure or overwrite critical fields such as permissions flags. In a DynamoDB context, this often combines with type confusion, where a value expected to be a string is interpreted as a map or list, leading to write failures or unexpected mutations.

OpenAPI/Swagger specifications that define permissive request bodies without strict schema enforcement can exacerbate the issue. When the spec uses additionalProperties: true or lacks explicit required fields, the runtime behavior may accept malformed payloads that produce invalid key constructions. Because middleBrick scans unauthenticated attack surfaces and cross-references spec definitions with runtime findings, it can detect mismatches between declared schemas and actual input handling that contribute to out-of-bounds conditions.

Real-world exploitation patterns align with the OWASP API Top 10 category Broken Object Level Authorization and can intersect with BOLA/IDOR findings. Unlike traditional memory exploits, the impact here is logical: unauthorized data modification, injection of malicious attributes, or disruption of data integrity. Instrumentation such as input validation and strict schema enforcement is essential to constrain writes to expected structures and identities.

Remediation focuses on strict input validation, canonical key construction, and least-privilege IAM. Always validate identifiers against the authenticated subject before constructing DynamoDB keys, and avoid using raw user input as attribute names or key components. Use parameterized update expressions and avoid dynamic attribute name assembly where possible.

Example 1: Safe update using authenticated subject and strict validation. The request body should not contain the target identifier; instead, derive it from the authenticated session.

const { DynamoDB } = require("aws-sdk");
const ddb = new DynamoDB.DocumentClient();
const express = require("express");
const app = express();
app.use(express.json());

app.put("/profile", async (req, res) => {
  // In a real app, subject derived from session or token
  const subject = req.session?.userId; // authenticated subject
  if (!subject) {
    return res.status(401).json({ error: "unauthorized" });
  }
  const { updates } = req.body;
  if (!updates || typeof updates !== "object") {
    return res.status(400).json({ error: "invalid payload" });
  }
  // Validate allowed fields to prevent unexpected attribute writes
  const allowed = new Set(["displayName", "email", "locale"]);
  for (const key of Object.keys(updates)) {
    if (!allowed.has(key)) {
      return res.status(400).json({ error: `invalid field: ${key}` });
    }
  }
  const params = {
    TableName: process.env.PROFILES_TABLE,
    Key: { userId: subject },
    UpdateExpression: "set #d = :val",
    ExpressionAttributeNames: { "#d": "data" },
    ExpressionAttributeValues: { ":val": updates },
    ReturnValues: "UPDATED_NEW",
  };
  try {
    const result = await ddb.update(params).promise();
    res.json(result.Attributes);
  } catch (err) {
    res.status(500).json({ error: "update failed" });
  }
});

Example 2: Conditional write with expected schema checks. Use DynamoDB ConditionExpression to enforce constraints on attributes, preventing overwrites of critical fields like permissions.

const params = {
  TableName: process.env.USERS_TABLE,
  Key: { userId: subject },
  UpdateExpression: "set #st = :status, #lv = :level",
  ConditionExpression: "attribute_exists(#st) AND #lv = :current",
  ExpressionAttributeNames: { "#st": "status", "#lv": "level" },
  ExpressionAttributeValues: {
    ":status": "active",
    ":level": 2,
    ":current": 1,
  },
};
try {
  await ddb.update(params).promise();
} catch (err) {
  if (err.code === "ConditionalCheckFailedException") {
    // Handle race condition or tampering
  }
}

Example 3: Using a whitelist for attribute names when dynamic fields are required. Never directly interpolate user input into expression names; map through a controlled dictionary.

const fieldMap = {
  bio: "profileBio",
  avatar: "avatarUrl",
};
const updateFields = {};
const expressionAttrs = {};
let updateParts = [];
for (const [key, value] of Object.entries(body)) {
  const dbKey = fieldMap[key];
  if (!dbKey) continue; // skip invalid fields
  updateParts.push(`#${dbKey} = :v${dbKey}`);
  expressionAttrs[`#${dbKey}`] = dbKey;
  expressionAttrs[`:v${dbKey}`] = value;
}
if (updateParts.length === 0) {
  return res.status(400).json({ error: "no valid fields" });
}
const params = {
  TableName: process.env.USERS_TABLE,
  Key: { userId: subject },
  UpdateExpression: `set ${updateParts.join(", ")}`,
  ExpressionAttributeNames: expressionAttrs,
  ExpressionAttributeValues: expressionAttrs,
};