Prototype Pollution in Buffalo

Prototype pollution occurs when an attacker manipulates JavaScript object prototypes via unsafe property assignment, potentially leading to remote code execution or privilege escalation. In Buffalo applications, this commonly arises in JSON payload handling where user input is merged into objects without proper validation.

Buffalo’s default JSON unmarshalling uses Go’s encoding/json package. When combined with libraries like github.com/gobuffalo/pop/v6 for database operations or custom middleware that merges request bodies into structs, prototype pollution can emerge if developers use patterns like map[string]interface{} and recursively merge nested fields.

Example vulnerable code in a Buffalo handler:

func UpdateUser(c buffalo.Context) error {
    u := &models.User{}
    if err := c.Bind(u); err != nil {
        return c.Error(400, err)
    }
    // Vulnerable: direct bind without field filtering
    if err := models.DB.Update(u); err != nil {
        return c.Error(500, err)
    }
    return c.Render(200, r.JSON(u))
}

If the User struct has embedded types or if the application uses generic update functions that copy all fields from a map, an attacker could send a payload like:

{
  "__proto__": {
    "isAdmin": true
  },
  "name": "attacker"
}

Although Go does not have JavaScript-style prototypes, prototype pollution in Buffalo typically manifests through:

• Misuse of map[string]interface{} in JSON handlers where keys like __proto__, constructor, or prototype are incorrectly processed
• Template rendering where polluted objects affect HTML output (e.g., via github.com/gobuffalo/plush)
• Object merging in utility functions (e.g., deep copy helpers) that don’t sanitize key names

Real-world parallels include CVE-2019-11358 (lodash) and CVE-2020-28477 (hoek), where unsafe recursive merges allowed prototype tampering. In Buffalo, similar risks exist when developers implement generic PATCH handlers or use third-party merging utilities without key validation.

Detecting prototype pollution in Buffalo applications requires observing how user input flows into object construction and whether dangerous keys are sanitized. middleBrick identifies these risks during its unauthenticated black-box scan by sending payloads containing prototype pollution vectors and analyzing responses for behavioral changes.

During the Input Validation and Property Authorization checks, middleBrick tests for:

• Reflection of __proto__, constructor, or prototype keys in JSON responses
• Changes in object behavior after submitting polluted payloads (e.g., unexpected admin flags appearing)
• Error messages that reveal unsafe object merging (e.g., "cannot set property 'isAdmin' of undefined")

Example detection via middleBrick CLI:

middlebrick scan https://api.example.com/users

If prototype pollution is suspected, the report will include a finding under "Property Authorization" or "Input Validation" with details like:

middleBrick does not require source code or configuration—it detects these issues by interacting with the live API and observing deviations in response structure, status codes, or data integrity. This makes it effective for spotting issues in staging or production environments where internal code may not be accessible for review.

Fixing prototype pollution in Buffalo applications involves preventing dangerous keys from being processed during JSON binding and object merging. Since Buffalo uses Go’s standard encoding/json package, the primary defense is struct-based binding with explicit field control and input sanitization.

Recommended remediation strategies:

• Use struct tags to limit bindable fields
• Sanitize keys in generic maps before processing
• Avoid recursive merge functions that don’t filter dangerous keys

Example of safe binding in a Buffalo handler:

type UserUpdate struct {
    Name  string `json:"name"`
    Email string `json:"email"`
    // Explicitly omit dangerous fields
}

func UpdateUser(c buffalo.Context) error {
    u := &UserUpdate{}
    if err := c.Bind(u); err != nil {
        return c.Error(400, err)
    }
    // Only update allowed fields
    user := &models.User{}
    if err := models.DB.Find(user, c.Param("user_id")); err != nil {
        return c.Error(404, err)
    }
    user.Name = u.Name
    user.Email = u.Email
    if err := models.DB.Update(user); err != nil {
        return c.Error(500, err)
    }
    return c.Render(200, r.JSON(user))
}

If generic map handling is unavoidable (e.g., for PATCH endpoints), sanitize keys before use:

func sanitizeMap(data map[string]interface{}) map[string]interface{} {
    result := make(map[string]interface{})
    for k, v := range data {
        switch k {
        case "__proto__", "constructor", "prototype":
            continue // skip dangerous keys
        default:
            if mv, ok := v.(map[string]interface{}); ok {
                result[k] = sanitizeMap(mv)
            } else {
                result[k] = v
            }
        }
    }
    return result
}

func PatchUser(c buffalo.Context) error {
    raw := map[string]interface{}{}
    if err := c.Bind(&raw); err != nil {
        return c.Error(400, err)
    }
    clean := sanitizeMap(raw)
    // Now safely apply clean map to user model
    // ...
}

Additionally, avoid using reflect-based deep copy or merge libraries unless they explicitly exclude dangerous keys. Buffalo’s built-in validators (via github.com/go-ozzo/ozzo-validation) can also enforce field-level constraints to prevent unintended property assignment.

By combining strict struct binding, key sanitization, and avoiding generic merge patterns, Buffalo applications can effectively mitigate prototype pollution risks without relying on external blocking mechanisms—aligning with middleBrick’s detection-only, guidance-driven approach.