Injection Flaws in Loopback with Bearer Tokens

Injection flaws in Loopback applications that use Bearer Tokens occur when untrusted input is concatenated into commands or queries, and the request’s authorization context (the Bearer Token) is not validated for scope or integrity before the injection-prone operation executes. An attacker can supply malicious input such as a crafted query fragment or command-line snippet, while also providing a Bearer Token that may be low-privilege or even missing, but the server processes the request with elevated permissions due to misconfigured authorization checks.

In a typical scenario, an endpoint accepts an id parameter to fetch a resource and also expects a Bearer Token in the Authorization header. If the server-side code builds a database query by directly interpolating the id without validation or parameterization, and then applies authorization using token claims that are not verified against the data ownership, the attacker can manipulate the id to perform Broken Level of Authorization (BOLA) or Insecure Direct Object Reference (IDOR). The Bearer Token may appear to authorize access, but if the token’s scopes or roles are not checked against the targeted resource, the injection flaw effectively bypasses intended access controls.

Another pattern involves logging or diagnostic endpoints that accept a search string and include the Bearer Token’s subject in a shell or command invocation. If the subject is attacker-controlled and not sanitized, command injection can occur. Even when tokens are validated for format, if the server trusts token metadata without re-verifying permissions per request, injection can lead to privilege escalation via BFLA (Broken Function Level Authorization). For example, an attacker may supply a token with a “user” role but exploit an injection path to invoke administrative functions that the token should not permit.

Input Validation checks in middleBrick identify such risks by testing endpoints that consume Bearer Tokens and user-supplied parameters, looking for signs that input is concatenated into commands or queries. Data Exposure findings may surface when injection reveals tokens or sensitive data in error messages or logs. LLM/AI Security probes further test whether token handling logic can be tricked into leaking system prompts or performing unauthorized actions via injected instructions, highlighting how weak authorization coupling with injection surfaces multiplies risk.

Real-world attack patterns mirror these mechanisms: an API endpoint like /api/orders/{orderId} expects a Bearer Token; if the token’s sub is not used to scope the query and the orderId is not parameterized, an attacker can set orderId=1 OR 1=1 to access others’ orders. The Bearer Token may appear valid, but the lack of binding between token identity and data access enables injection-assisted horizontal privilege abuse.

Remediation focuses on strict input validation, parameterized queries, and scoping all data access to the identity or scopes encoded in the Bearer Token. Do not rely on token presence alone; verify ownership and constraints before using any user-controlled input in commands or queries.

Example 1: Parameterized query with token ownership scoping

Instead of building SQL or NoSQL filters by concatenation, use parameterized queries and bind the token’s subject to the query filter. Below is a Loopback connector-style example using a parameterized where clause:

const { securityBindings } = require('loopback-context');

app.models.order.find({
  where: {
    id: req.body.orderId,
    // scope to the user identified by the Bearer Token
    userId: securityBindings.get('userId')
  }
}).then(orders => { /* ... */ });

Example 2: Validate and sanitize input before use in commands

If constructing a shell command for diagnostics, avoid direct interpolation. Use allowlists and explicit escaping, and ensure the Bearer Token’s subject is not directly concatenated:

const allowedCommands = ['summary', 'health'];
if (!allowedCommands.includes(req.query.action)) {
  throw new Error('Invalid action');
}
// Use safe exec with arguments array, never string interpolation
const { execFile } = require('child_process');
execFile('diagnostic', ['--action', req.query.action], (err, stdout) => { /* ... */ });

Example 3: Enforce token scopes and reject high-risk inputs

Check token scopes before performing operations that could be abused via injection. Reject inputs that contain suspicious patterns when combined with privileged scopes:

const tokenScopes = req.accessToken.scopes || [];
const hasAdminScope = tokenScopes.includes('admin');
const userInput = req.query.search;

// Reject inputs with injection indicators when admin scope is present
if (hasAdminScope && /[;|&`$()]/ .test(userInput)) {
  throw new Error('Invalid input for scope');
}

Example 4: Use Loopback’s built-in ACLs and binding

Define strong ACLs in your model JSON or JavaScript config that reference the token’s identity, and avoid dynamic privilege assignment derived from raw input:

{
  "name": "order",
  "base": "PersistedModel",
  "acls": [
    {
      "principalType": "ROLE",
      "principalId": "$everyone",
      "permission": "DENY"
    },
    {
      "principalType": "ROLE",
      "principalId": "authenticated",
      "permission": "ALLOW",
      "property": "find",
      "accessScope": {
        "userId": { "bind": "userId" }
      }
    }
  ]
}

Example 5: Middleware to bind token identity to request context

Use middleware to attach the token’s user ID to the request context and ensure all downstream data access respects this binding:

app.use((req, res, next) => {
  const token = req.accessToken;
  if (token) {
    req.apiContext = { userId: token.userId };
  }
  next();
});

// Later in a remote method
Order.find({
  where: {
    id: req.body.orderId,
    userId: req.apiContext.userId
  }
});