Prototype Pollution in Fiber with Basic Auth

Prototype pollution in Fiber applications that use HTTP Basic Authentication can arise when user-controlled input reaches object-merge operations before authentication is fully validated or when parsed request bodies are merged into prototype objects. In JavaScript, objects inherit properties from their prototype chain; if an attacker can inject a property such as __proto__, constructor.prototype, or prototype into a target object, they can alter behavior across the application, including escalating privileges or tampering with security defaults.

When Basic Auth is used, credentials are typically extracted from the Authorization header and validated before business logic executes. However, if the application parses and merges request bodies (e.g., JSON or URL-encoded payloads) and then performs authentication checks, an attacker can send a polluted object that affects authentication checks or middleware behavior. For example, an attacker might add a property that modifies how credentials are compared or how middleware interprets request state. Because Fiber routes requests without inherent server-side state, pollution can persist within the request lifecycle if objects are reused or cached inadvertently.

Consider a scenario where route handlers merge incoming JSON into a configuration or context object. An exploit payload like { "__proto__": { "isAdmin": true } } could, if merged unsafely, cause the resulting object to inherit isAdmin: true. If the application later evaluates permissions based on that object, the attacker may bypass intended access controls. MiddleBrick’s checks for BOLA/IDOR and Input Validation help surface these risky merge patterns by correlating spec definitions with runtime behavior, revealing endpoints where authentication boundaries intersect with mutable object construction.

SSRF and unsafe consumption checks are also relevant because an attacker may leverage external endpoints or crafted content to indirectly poison object prototypes through deserialization or dependency chains. LLM/AI Security probes do not apply here, as prototype pollution is a classic input validation and object handling issue rather than a prompt injection vector. Nonetheless, understanding how pollution can interact with authentication flows helps prioritize remediation based on severity and compliance mappings to OWASP API Top 10 and other frameworks.

To mitigate prototype pollution in Fiber when using Basic Authentication, ensure that request-body parsing and object merging occur after successful authentication and that inputs are validated and sanitized before being merged into any object, including prototypes. Use strict parsing options and avoid merging user input into global or shared prototypes. Below are concrete code examples demonstrating secure patterns.

Example 1: Validate credentials before parsing mutable bodies

Authenticate first, then handle the body. This prevents polluted objects from affecting authentication logic.

const { app } = require("@Fiber/core");
const base64 = require("base-64");

app.use(async (req, res, next) => {
  const authHeader = req.headers["authorization"];
  if (!authHeader || !authHeader.startsWith("Basic ")) {
    return res.status(401).send("Unauthorized");
  }
  const token = authHeader.split(" ")[1];
  const decoded = base64.decode(token);
  const [user, pass] = decoded.split(":");
  const isValid = (user === "admin" && pass === "secret"); // replace with secure check
  if (!isValid) {
    return res.status(403).send("Forbidden");
  }
  next();
});

app.post("/data", (req, res) => {
  const safeData = sanitizeObject(req.body); // custom validation/whitelist
  res.json({ received: safeData });
});

function sanitizeObject(obj) {
  if (obj === null || typeof obj !== "object") return obj;
  const clean = Array.isArray(obj) ? [] : {};
  for (const key of Object.keys(obj)) {
    if (key === "__proto__" || key === "constructor" || key === "prototype") continue;
    clean[key] = obj[key];
  }
  return clean;
}

Example 2: Use a body parser with explicit type checks and no prototype pollution

Configure the JSON parser to reject prototypes and use a library or custom logic to strip dangerous keys.

const { app } = require("@Fiber/core");
const bodyParser = require("body-parser");

// Reject prototype pollution keys in parsed JSON
app.use(bodyParser.json({
  strict: true,
  verify: (req, res, buf) => {
    try {
      const parsed = JSON.parse(buf.toString());
      if (parsed && typeof parsed === "object") {
        if (parsed.__proto__ !== undefined || parsed.constructor !== undefined) {
          throw new Error("Polluted object rejected");
        }
      }
    } catch (err) {
      throw new Error("Invalid JSON");
    }
  }
}));

app.post("/submit", (req, res) => {
  // req.body is now safer; still sanitize if merging or reflecting
  res.json({ ok: true });
});

General best practices

• Validate and whitelist allowed fields before merging.
• Avoid using objects as prototypes for user-controlled data.
• Ensure authentication checks are independent of mutable request bodies.

These steps reduce the risk that polluted properties affect authentication or authorization logic. MiddleBrick scans can help identify endpoints where input validation and authentication boundaries are not properly isolated, providing prioritized findings with remediation guidance.