Broken Access Control in Gin with Mongodb

Broken Access Control occurs when API endpoints fail to enforce proper authorization checks, allowing one user to act on another user's resources. Using the Gin framework with MongoDB can unintentionally expose this vulnerability when route handlers validate identities but omit per-request ownership checks or rely on client-supplied identifiers without verification.

In Gin, a handler might extract a user ID from a JWT and use it to query MongoDB. If the developer constructs queries by directly concatenating user input (e.g., an ID from the URL) into the database filter, an attacker can tamper with that input to access or modify other users' documents. For example, an endpoint like /users/:id/profile that trusts the :id path parameter can allow horizontal privilege escalation when the parameter is changed to another valid user ID and the backend does not re-confirm that the requesting user owns that ID.

MongoDB query structure plays a critical role. Consider a filter built as bson.M{"_id": userID} where userID is taken from the URL without confirming it matches the authenticated subject. If the JWT payload contains sub but the handler uses a different ID sourced from the request, the access control boundary is effectively bypassed. Additionally, MongoDB's flexible schema can inadvertently expose sensitive fields if projections are not explicitly restricted, returning data that should remain invisible to the caller.

Common implementation patterns in Gin that contribute to this issue include omitting ownership validation in list or get endpoints, using raw user input in aggregation pipelines, and neglecting to scope queries by tenant or organization in multi-tenant setups. Without explicit checks that the authenticated subject has the necessary relationship to the target resource, the API surface remains wide open to authenticated attackers who can iterate through known IDs.

These risks map to the OWASP API Top 10 category Broken Access Control and can lead to unauthorized data access, modification, or deletion. Proper scoping of queries, server-side ownership verification, and strict use of identifiers from the authentication layer are essential to prevent insecure direct object reference (IDOR) in this technology stack.

To secure Gin handlers that use MongoDB, always tie database queries to the authenticated subject and avoid relying on client-provided identifiers for access decisions. Use the identity from the token to scope every query, and explicitly define projections to limit returned fields.

Below is a secure pattern for retrieving a user profile. The authenticated subject's ID from the JWT is used as the sole filter, and the path parameter is ignored for authorization purposes.

// Assume authentication middleware sets c.MustGet("userID") from the token
userID, _ := c.Get("userID")
var profile UserProfile
err := collection.FindOne(ctx, bson.M{"_id": userID}).Decode(&profile)
if err != nil {
    c.JSON(http.StatusNotFound, gin.H{"error": "not_found"})
    return
}
c.JSON(http.StatusOK, profile)

For endpoints that accept an ID in the path but still require strict ownership checks, re-assert the relationship server-side instead of trusting the URL. If the endpoint is meant to allow users to fetch their own data, do not use the URL ID for access control; use it only for logging or informational purposes.

When querying for lists or related resources, embed the subject ID in the filter and avoid returning references that the user should not see.

cursor, err := collection.Find(ctx, bson.M{"owner_id": userID, "status": "active"}, options.Find().SetProjection(bson.M{"name": 1, "email": 1, "_id": 1}))
if err != nil {
    c.JSON(http.StatusInternalServerError, gin.H{"error": "server_error"})
    return
}
var results []PublicProfile
if err = cursor.All(ctx, &results); err != nil {
    c.JSON(http.StatusInternalServerError, gin.H{"error": "server_error"})
    return
}
c.JSON(http.StatusOK, results)

In multi-tenant scenarios, include a tenant or organization identifier in every filter and ensure the subject belongs to that tenant before executing the query.

err := collection.FindOne(ctx, bson.M{
    "_id": userID,
    "tenant_id": currentTenant,
}).Decode(&resource)

Always validate and sanitize inputs even when they are not used for access control, and prefer parameterized queries to avoid injection risks. By consistently scoping queries to the authenticated identity and explicitly defining which fields are returned, you mitigate the likelihood of broken access control in the Gin and MongoDB stack.