Parameter Tampering in Buffalo

Parameter tampering in Go applications built with the Buffalo framework often occurs when developers rely on client-supplied data without proper validation, particularly in handler functions that bind request parameters directly to structs. Buffalo’s c.Bind() method, which uses the binding package to map query strings, form data, or JSON payloads to Go structs, can inadvertently allow attackers to modify unexpected fields if the struct includes unexported or sensitive fields that should not be user-controllable.

For example, consider a Buffalo handler for updating a user profile where the developer binds the request body to a User struct that includes fields like Role or IsAdmin. If these fields are not explicitly ignored via struct tags or protected by middleware, an attacker can tamper with the role parameter in a JSON payload to escalate privileges:

// handlers/users.go
func UpdateUser(c buffalo.Context) error {
    u := &models.User{}
    if err := c.Bind(u); err != nil {
        return c.Error(400, err)
    }
    // ... save u to DB
    return c.Render(200, r.JSON(u))
}

// models/user.go
type User struct {
    ID        int    `db:"id"`
    Email     string `db:"email"`
    Password  string `db:"password"`
    Role      string `db:"role"` // Should not be user-editable
    IsAdmin   bool   `db:"is_admin"`
}

An attacker could send a PATCH request with { "role": "admin" } to gain administrative access. This becomes especially dangerous in Buffalo applications that use resource generators, where CRUD handlers are auto-generated and may bind entire structs without field-level restrictions. Tampering can also occur via query parameters in GET requests (e.g., tampering with ?status=pending to ?status=approved) if the handler uses c.Param() or c.Request().URL.Query() without validation.

This pattern aligns with OWASP API Security Top 10:2023 — API1:2023 Broken Object Property Level Authorization, where attackers exploit excessive data exposure in API responses or insufficient input validation to modify properties they should not control.

Detecting parameter tampering in Buffalo applications requires examining both the handler logic and the data flow from request to model binding. Since Buffalo abstracts much of the HTTP handling, manual code review can miss subtle binding issues, especially in large codebases with auto-generated resource code. middleBrick helps identify these vulnerabilities by scanning the unauthenticated attack surface and analyzing how client-supplied parameters are processed.

When scanning a Buffalo API endpoint, middleBrick performs active tests such as injecting unexpected parameters into JSON bodies, query strings, and form data to see if they are accepted and processed by the backend. For example, if a PATCH /users/{id} endpoint expects only name and email but accepts and stores a role field, middleBrick will flag this as a potential property authorization violation (BOLA/IDOR or BFLA, depending on context).

middleBrick also cross-references OpenAPI/Swagger specs (if available) with runtime behavior. If the spec defines a User schema without a role property in the PATCH request body, but the live endpoint accepts it, this discrepancy is reported as an unsafe consumption finding — indicating the API consumes more data than documented, increasing attack surface.

To detect such issues early, developers can integrate middleBrick into their CI/CD pipeline using the GitHub Action. By adding a step that runs middlebrick scan https://staging.example.com on every pull request, teams can catch parameter tampering risks before deployment. The action can be configured to fail the build if the security score drops below a threshold (e.g., grade C), ensuring that regressions in input validation are blocked.

Example GitHub Action workflow snippet:

# .github/workflows/middlebrick.yml
name: API Security Scan
on: [pull_request]
jobs:
  scan:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - name: Run middleBrick scan
        uses: middlebrick/action@v1
        with:
          url: https://staging.example.com
          fail-below: c  # Fail if score is C or lower

This approach provides continuous feedback without requiring agents, configuration, or credentials — only the public URL of the API under test.

Fixing parameter tampering in Buffalo applications involves enforcing strict input validation at the handler level, leveraging Buffalo’s built-in struct tagging and binding controls to limit which fields can be populated from client data. The most effective strategy is to use explicit struct tags to ignore sensitive fields during binding, ensuring they cannot be overridden by request parameters.

In the earlier User example, the Role and IsAdmin fields should be marked with -" in the JSON binding tag to prevent them from being set via c.Bind():

// models/user.go
type User struct {
    ID        int    `db:"id" json:"-"`
    Email     string `db:"email" json:"email"`
    Password  string `db:"password" json:"- "` // Never expose or accept via API
    Role      string `db:"role" json:"-"` // Ignore during binding
    IsAdmin   bool   `db:"is_admin" json:"-"`
}

With this change, even if an attacker sends { "role": "admin" } in the request body, the Role field will remain unchanged during binding. Note that the -" tag must be placed in the json struct tag (not db) to affect JSON binding, which is what c.Bind() uses by default for JSON payloads.

For query parameters or form data, developers should avoid using c.Param() or c.Request().URL.Query() directly without validation. Instead, define a dedicated input struct for each endpoint and bind only expected fields. For example, a handler that updates a user’s email should use a narrow struct:

// handlers/users.go
type UpdateUserInput struct {
    Email string `json:"email" binding:"required,email"`
}

func UpdateUser(c buffalo.Context) error {
    input := &UpdateUserInput{}
    if err := c.Bind(input); err != nil {
        return c.Error(400, err)
    }

    u := &models.User{ID: c.Param("id")}
    if err := models.Find(u); err != nil {
        return c.Error(404, err)
    }

    u.Email = input.Email
    if err := models.Update(u); err != nil {
        return c.Error(500, err)
    }

    return c.Render(200, r.JSON(u))
}

This approach ensures that only the email field is accepted from the request, eliminating the risk of tampering with other properties. Additionally, using validation tags like binding:"required,email" enforces format checks at the binding stage.

For applications using Buffalo’s resource generators, developers should customize the generated handlers to use specific input structs rather than binding directly to the model. This can be done by overriding the default Update or Create methods in the resource file and applying the same input struct pattern.

Finally, to prevent tampering via URL parameters (e.g., ?status=approved), always validate and sanitize query values against an allowlist of expected values, especially when they influence business logic or data access.