Insecure Design in Buffalo with Mongodb

Insecure design in a Buffalo application that uses MongoDB often stems from modeling data without considering authorization boundaries and from trusting the ORM layer to enforce constraints that MongoDB does not enforce automatically. When route parameters or query inputs are mapped directly to MongoDB selectors without validation or scoping, this can enable IDOR and BOLA patterns that are difficult to detect without runtime analysis.

Buffalo does not enforce a default scope on MongoDB queries; developers must explicitly apply tenant or user filters. If a handler builds a filter like bson.M{"_id": id} using an untrusted URL parameter, an attacker can enumerate or access documents across users by iterating IDs. This becomes more dangerous when the handler also uses session values (e.g., a user ID from the session) to construct queries without intersection checks, leading to privilege escalation where a lower-privilege account can read or modify another account’s data.

Additionally, embedding sensitive fields (password hashes, API keys, payment tokens) directly in MongoDB documents without field-level encryption or careful schema design increases data exposure risk. If the application uses polymorphic references (e.g., storing multiple entity types in the same collection without strict type segregation), an attacker may leverage injection or confused deputy patterns to pivot across collections. Insecure default configurations—such as allowing unauthenticated connections to a development MongoDB instance from the Buffalo app—also expand the attack surface during deployment.

When using MongoDB with Buffalo, the lack of built-in row-level security means the framework cannot automatically prevent IDOR; this must be implemented at the handler and query level. For example, a handler that does not intersect the authenticated user ID with the requested resource ID can lead to Insecure Direct Object Reference. Similarly, if the application reflects MongoDB errors to the client, it may leak collection names or field structures that aid reconnaissance.

These design issues are detectable by middleBrick because the scanner runs 12 security checks in parallel, including BOLA/IDOR, Property Authorization, Input Validation, and Authentication. It evaluates the unauthenticated attack surface of your Buffalo endpoints and, when provided with an OpenAPI spec, cross-references definitions with runtime findings to highlight missing authorization scoping and over-privileged data access patterns specific to MongoDB-backed routes.

Remediation centers on explicit scoping, strict input validation, and avoiding exposure of internal identifiers. Always intersect the authenticated user or tenant identifier with any MongoDB selector, and prefer parameterized queries to avoid injection.

Example 1: Secure handler with user-scoped query

import (
    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/packr/v2"
    "github.com/pkg/errors"
    "go.mongodb.org/mongo-driver/bson"
    "go.mongodb.org/mongo-driver/mongo"
)

func showProfile(c buffalo.Context) error {
    userID := c.Session().Get("user_id")
    if userID == nil {
        return c.Render(401, r.JSON(map[string]string{"error": "unauthorized"}))
    }

    // Validate and convert input
    oid, err := primitive.ObjectIDFromHex(c.Param("profile_id"))
    if err != nil {
        return c.Render(400, r.JSON(map[string]string{"error": "invalid id"}))
    }

    var profile Profile
    // Explicitly scope by user_id to prevent IDOR
    filter := bson.M{
        "_id":       oid,
        "user_id":   userID, // ensure the record belongs to the authenticated user
    }
    err = db.Collection("profiles").FindOne(c.Request().Context(), filter).Decode(&profile)
    if err != nil {
        if errors.Is(err, mongo.ErrNoDocuments) {
            return c.Render(404, r.JSON(map[string]string{"error": "not found"}))
        }
        return c.Render(500, r.JSON(map[string]string{"error": "server error"}))
    }
    return c.Render(200, r.JSON{profile})
}

Example 2: Insert with schema enforcement and field-level protection

func createPost(c buffalo.Context) error {
    userID := c.Session().Get("user_id")
    if userID == nil {
        return c.Render(401, r.JSON(map[string]string{"error": "unauthorized"}))
    }

    var input struct {
        Title   string `json:"title" bson:"title" validate:"required,max=200"`
        Content string `json:"content" bson:"content" validate:"required,max=10000"`
    }
    if err := c.Bind(&input); err != nil {
        return c.Render(400, r.JSON(map[string]string{"error": "invalid payload"}))
    }
    if err := c.Validate(&input); err != nil {
        return c.Render(422, r.JSON(map[string]string{"error": err.Error()}))
    }

    doc := bson.M{
        "user_id": userID,
        "title":   input.Title,
        "content": input.Content,
        "status":  "draft",
        // Avoid storing sensitive fields directly; store references or encrypt
    }
    _, err := db.Collection("posts").InsertOne(c.Request().Context(), doc)
    if err != nil {
        return c.Render(500, r.JSON(map[string]string{"error": "server error"}))
    }
    return c.Render(201, r.JSON(bson.M{"_id": doc["_id"]}))
}

Example 3: Aggregation with $match scoping and safe lookup

func listUserResources(c buffalo.Context) error {
    userID := c.Session().Get("user_id")
    if userID == nil {
        return c.Render(401, r.JSON(map[string]string{"error": "unauthorized"}))
    }

    cursor, err := db.Collection("resources").Aggregate(c.Request().Context(), mongo.Pipeline{
        {{"$match", bson.D{{"user_id", userID}}}}, // enforce tenant scope
        {{"$project", bson.D{{"name", 1}, "type", 1}, {"$unset", "internal_token"}}}, // redact sensitive fields
    })
    if err != nil {
        return c.Render(500, r.JSON(map[string]string{"error": "server error"}))
    }
    var results []bson.M
    if err = cursor.All(c.Request().Context(), &results); err != nil {
        return c.Render(500, r.JSON(map[string]string{"error": "server error"}))
    }
    return c.Render(200, r.JSON{results})
}

Operational and schema guidance

• Always validate and sanitize ObjectID and numeric inputs before using them in selectors.
• Apply the principle of least privilege at the database user and connection level; avoid broad read/write roles for the Buffalo app’s MongoDB user.
• Use MongoDB’s field-level encryption or application-layer encryption for sensitive fields rather than relying on network isolation alone.
• Do not expose internal IDs directly in URLs when an alternate, non-guessable reference can be used; if you must use direct IDs, enforce ownership checks on every query.
• Log rejected attempts without revealing internal schema details to reduce information leakage.

middleBrick’s checks for BOLA/IDOR, Property Authorization, and Input Validation help surface missing scoping and over-privileged queries. Its OpenAPI/Swagger analysis can map declared parameters against required security scopes, highlighting where runtime filters do not align with the spec when an OpenAPI definition is provided.