Pii Leakage in Buffalo with Bearer Tokens

Buffalo is a popular Go web framework for building rapid web applications. When developers use Bearer Tokens for authentication in Buffalo without additional safeguards, PII leakage can occur through multiple vectors. Bearer Tokens are typically transmitted in the Authorization header as Authorization: Bearer <token>; if responses inadvertently include sensitive user data or if endpoints do not enforce strict access controls, that data can be exposed to unauthorized parties.

One common scenario involves JSON serialization in Buffalo actions. If a developer renders a user object containing fields such as email, address, or government ID into a JSON response without explicitly filtering attributes, those fields can be exposed alongside the Bearer Token usage. For example, an endpoint like /api/users/me that relies on the token to identify the current user might return the full user struct, inadvertently including PII if the struct is not carefully controlled.

Logging practices in Buffalo applications can also contribute to PII leakage. If request logging includes the Authorization header or user details without redaction, tokens and associated PII may be written to log stores that are accessible to more people than intended. This risk is compounded when Bearer Tokens are used across multiple services, as a single log exposure can lead to cross-service data exposure.

Another vector arises from improper route or parameter handling. Buffalo applications that accept user-supplied identifiers and use them to fetch records may expose PII if those identifiers are not validated or scoped correctly. Even with Bearer Tokens confirming the requester’s identity, an endpoint that returns data for a given ID without verifying ownership can expose one user’s PII to another.

middleBrick detects these risks through its 12 security checks, including Data Exposure, Input Validation, and Authentication. When scanning a Buffalo API that uses Bearer Tokens, the scanner validates whether responses contain PII, whether logs or error messages disclose tokens or user data, and whether access controls properly scope data to the authenticated subject. The scanner also cross-references OpenAPI specifications to identify endpoints that accept Bearer Tokens but lack sufficient data exposure controls.

Real-world attack patterns such as OWASP API Top 10:2023 A1: Broken Object Level Authorization and A3: Excessive Data Exposure align closely with these Buffalo and Bearer Token risks. A token may be valid, but if the API returns more data than necessary, the token effectively amplifies the impact of the leakage. This is why scanning with context-aware tools that understand framework conventions and token usage is essential.

To mitigate PII leakage in Buffalo applications using Bearer Tokens, developers should adopt explicit data control, secure logging, and strict authorization checks. Below are concrete examples that demonstrate secure patterns.

1. Explicitly control JSON serialization

Define a dedicated struct for responses that includes only necessary fields, and use it instead of returning the full model.

import (
    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/packr/v2"
)

type UserResponse struct {
    ID    string `json:"id"`
    Email string `json:"email"`
    Name  string `json:"name"`
}

func ShowUser(c buffalo.Context) error {
    userID := c.Param("user_id")
    var user User
    if err := db.Where("id = ?", userID).First(&user).Error; err != nil {
        return c.Error(404, err)
    }
    resp := UserResponse{
        ID:    user.ID,
        Email: user.Email,
        Name:  user.Name,
    }
    return c.Render(200, r.JSON(resp))
}

2. Avoid logging sensitive headers

Configure logging middleware to redact or exclude the Authorization header.

import (
    "github.com/gobuffalo/middleware"
)

func skipAuthLogging(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Create a copy of the request without the Authorization header for logging
        rq := cloneRequest(r)
        rq.Header.Del("Authorization")
        // Safe logging of rq instead of r
        logger.Info(rq)
        next.ServeHTTP(w, r)
    })
}

func cloneRequest(r *http.Request) *http.Request {
    r2 := new(http.Request)
    *r2 = *r
    r2.Header = make(http.Header)
    for k, s := range r.Header {
        r2.Header[k] = s
    }
    r2.Header.Del("Authorization")
    return r2
}

3. Enforce ownership checks before returning data

Ensure that any lookup by ID verifies that the resource belongs to the authenticated subject.

func UpdateProfile(c buffalo.Context) error {
    tokenString := c.Request().Header.Get("Authorization")
    if len(tokenString) < 7 || tokenString[:7] != "Bearer " {
        return c.Error(401, errors.New("unauthorized"))
    }
    token := tokenString[7:]
    subjectID, err := extractSubjectID(token)
    if err != nil {
        return c.Error(401, err)
    }
    profileID := c.Param("profile_id")
    var profile Profile
    if err := db.Where("id = ? AND user_id = ?", profileID, subjectID).First(&profile).Error; err != nil {
        return c.Error(404, err)
    }
    // process update
    return c.Render(200, r.JSON(profile))
}

4. Use middleware to validate tokens and limit scope

Leverage Buffalo middleware to verify Bearer Tokens and attach user context safely.

func AuthMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        header := r.Header.Get("Authorization")
        if header == "" {
            http.Error(w, "Authorization header required", 401)
            return
        }
        parts := strings.Split(header, " ")
        if len(parts) != 2 || parts[0] != "Bearer" {
            http.Error(w, "Invalid authorization format", 401)
            return
        }
        claims, err := parseToken(parts[1])
        if err != nil {
            http.Error(w, "Invalid token", 401)
            return
        }
        ctx := context.WithValue(r.Context(), "userID", claims.Subject)
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

These patterns reduce the likelihood of PII leakage by ensuring that only intended data is returned, tokens are never logged, and access is strictly scoped. When combined with automated scanning using tools that understand Buffalo conventions, teams can identify subtle exposure issues before they reach production.