Insecure Direct Object Reference in Buffalo with Mongodb

Insecure Direct Object Reference (IDOR) occurs when an API exposes a reference to an object—such as a record ID—and uses that value directly to retrieve data without verifying that the requesting user is authorized to access it. In Buffalo, this commonly arises when route parameters or query strings are passed directly to MongoDB queries without an authorization check. Because Buffalo does not enforce permissions automatically, developers must explicitly scope data access to the requesting actor, and MongoDB’s flexible document model can inadvertently facilitate access to other users’ records if object references are not validated.

Consider a Buffalo application that handles user profiles with a route like /users/:userID. If the handler retrieves a document by ID using only the parameter value, an attacker can change :userID to another valid ObjectId and access or enumerate profiles they should not see. MongoDB’s ObjectId type does not prevent this; it only ensures the value is a valid identifier. Without ownership or role checks, the database returns the requested document, resulting in an IDOR. This pattern is especially risky when combined with predictable ObjectIds or non-sequential identifiers that can be easily guessed or enumerated.

In a typical Buffalo handler, a developer might write code like the following insecure example, which directly uses a URL parameter to fetch a document:

// Insecure: uses raw ID from URL without authorization checks
func UserShow(c buffalo.Context) error {
    userID := c.Param("userID")
    var user models.User
    // Directly querying with userID from the route
    if err := models.DB.Collection("users").FindOne(c.Request().Context(), bson.M{"_id": userID}).Decode(&user); err != nil {
        return c.Error(404, errors.New("not found"))
    }
    return c.Render(200, r.JSON(user))
}

Here, the ObjectId string from the URL is used verbatim in a MongoDB filter. If the ID exists in the collection, the record is returned regardless of whether the authenticated user owns it or has permission to view it. This pattern ignores context such as tenant isolation, team membership, or role-based access control. Attackers can automate enumeration by iterating through plausible ObjectIds, potentially accessing other users’ sensitive data. Even when authentication is enforced elsewhere, the lack of server-side authorization in the data access layer makes the endpoint vulnerable.

Additional risk arises when related data is referenced indirectly. For example, an endpoint that accepts a project ID and then fetches associated tasks without confirming the user’s access to that project can expose cross-boundary references. Because MongoDB supports rich document structures and flexible references, developers must explicitly enforce scoping rules at the query level. Failing to do so allows attackers to traverse relationships using known IDs and infer the existence of resources through timing differences or error messages.

Remediation focuses on ensuring every MongoDB query includes authorization constraints that align with the authenticated subject. In Buffalo, this means explicitly adding ownership or access rules to the filter before executing a database operation. Developers should avoid using raw IDs directly and instead construct queries that incorporate the user’s identifier or permissions.

For the user profile example, the fix involves scoping the query to the current user. Assuming authentication provides a user identity and an ID, the handler should combine the requested ID with the user’s own ID (or a list of accessible IDs) in the MongoDB filter:

// Secure: scope the query to the authenticated user
func UserShow(c buffalo.Context) error {
    userID := c.Param("userID")
    currentUser := c.Value("current_user").(*models.User)
    var user models.User
    // Combine requested ID with user ownership check
    filter := bson.M{
        "_id": userID,
        "owner_id": currentUser.ID,
    }
    if err := models.DB.Collection("users").FindOne(c.Request().Context(), filter).Decode(&user); err != nil {
        return c.Error(404, errors.New("not found"))
    }
    return c.Render(200, r.JSON(user))
}

This pattern ensures that even if an attacker supplies a valid but other user’s ID, the query will not return a document unless the owner_id matches the authenticated user. The additional field in the filter acts as a mandatory authorization guard. If your data model uses roles or team-based access, extend the filter to include allowed team identifiers or policy expressions.

When relationships are involved—for example, fetching tasks for a project—apply the same principle by embedding access conditions in the query:

// Secure: scope tasks by project and user membership
func TasksList(c buffalo.Context) error {
    projectID := c.Param("projectID")
    currentUser := c.Value("current_user").(*models.User)
    var tasks []models.Task
    // Ensure user has access to the project before fetching tasks
    filter := bson.M{
        "project_id": projectID,
        "$or": []bson.M{
            {"members.user_id": currentUser.ID},
            {"visibility": "public"},
        },
    }
    cursor, err := models.DB.Collection("tasks").Find(c.Request().Context(), filter)
    if err != nil {
        return c.Error(500, err)
    }
    defer cursor.Close(c.Request().Context())
    if err = cursor.All(c.Request().Context(), &tasks); err != nil {
        return c.Error(500, err)
    }
    return c.Render(200, r.JSON(tasks))
}

In this example, the query explicitly checks that the current user is listed as a member or that the project is public. This prevents ID-based traversal across project boundaries. Using $or with structured subdocuments allows flexible access policies while keeping the filter efficient. For complex scenarios, consider precomputed access collections or application-level joins that enforce permissions before issuing database requests.

Additionally, validate and normalize incoming identifiers. While MongoDB ObjectIds are 12-byte values, accepting them as strings and passing them directly can still lead to injection-like behavior if not handled consistently. Use bson.ObjectIdHex with error handling to ensure malformed IDs are rejected early:

oid, err := bson.ObjectIdHex(userID)
if err != nil {
    return c.Error(400, errors.New("invalid id"))
}
filter := bson.M{"_id": oid, "owner_id": currentUser.ID}
// proceed with FindOne using filter

By combining strict ID parsing with scoped queries, you reduce both accidental exposure and injection risks. These patterns integrate naturally into Buffalo handlers and align with how middleBrick scans may surface IDOR findings, emphasizing the need for explicit authorization in data access logic.