Session Fixation in Buffalo with Bearer Tokens

Session fixation occurs when an application assigns a user a session identifier before authentication and does not rotate or validate that identifier after login. In Buffalo, this risk is amplified when Bearer Tokens are used for authentication because a token issued before authentication may be accepted as proof of identity after authentication without being regenerated or bound to the authenticated user.

Buffalo applications that rely on token-based mechanisms may expose a session fixation surface when a pre-authentication token (for example, a session cookie or an unverified Bearer token presented via an Authorization header) continues to be used post-login. If the server accepts the same token value after the user logs in, an attacker who can set or predict a token value can hijack the authenticated session by persuading a victim to use a known token.

Consider a Buffalo API that accepts Bearer Tokens in the Authorization header and uses it to identify a user session without additional validation. If the token is issued during an unauthenticated visit (for example, generated as part of a middleware or init process) and the server does not require re-authentication or token rebinding on login, an attacker can fix the token and later use it to access protected endpoints as the victim. This violates the principle that authentication should establish a fresh, authoritative identity binding.

Real-world attack patterns mirror issues cataloged in OWASP API Top 10 and can be detected by security scans that cross-reference spec definitions with runtime behavior. For instance, an unauthenticated endpoint that returns the same token used post-login indicates a broken session-binding flow. The presence of Bearer Tokens does not inherently cause fixation, but insecure handling—such as accepting pre-auth tokens as post-auth identity—creates the condition for session hijacking.

Because Buffalo does not enforce automatic token rotation on authentication, developers must explicitly ensure that any session or token state is re-evaluated after login. This includes invalidating pre-authentication tokens, issuing new tokens bound to the authenticated subject, and validating token scope and audience on each request. Without these controls, an API that appears to authenticate via Bearer Tokens may still allow an attacker to retain access using a fixed token value.

Remediation focuses on ensuring that authentication establishes a fresh token or session binding and that pre-authentication tokens cannot be accepted post-login. Below are concrete code examples for a Buffalo API that uses Bearer Tokens, demonstrating secure token issuance and validation.

First, define a helper to generate a cryptographically secure token and bind it to a user identity. Store only a token hash server-side, and ensure the token is transmitted only over HTTPS.

// app/models/auth_token.go
package models

import (
    "crypto/rand"
    "encoding/hex"
    "time"
)

type AuthToken struct {
    ID        int
    UserID    int
    TokenHash string
    ExpiresAt time.Time
    CreatedAt time.Time
}

func NewAuthToken(userID int) (*AuthToken, error) {
    token, err := generateSecureToken(32)
    if err != nil {
        return nil, err
    }
    // In practice, store only a hashed version of the token
    tokenHash, err := hashToken(token)
    if err != nil {
        return nil, err
    }
    return &AuthToken{
        UserID:    userID,
        TokenHash: tokenHash,
        ExpiresAt: time.Now().Add(24 * time.Hour),
        CreatedAt: time.Now(),
    }, nil
}

func generateSecureToken(length int) (string, error) {
    bytes := make([]byte, length)
    _, err := rand.Read(bytes)
    if err != nil {
        return "", err
    }
    return hex.EncodeToString(bytes), nil
}

Second, in your authentication handler, invalidate any pre-authentication token and issue a new one after successful credentials verification.

// app/controllers/sessions_controller.go
package controllers

import (
    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/pop/v6"
    "yourapp/app/models"
)

func Login(c buffalo.Context) error {
    tx := c.Value("tx").(*pop.Connection)
    var creds struct {
        Username string `json:"username"`
        Password string `json:"password"`
    }
    if err := c.Bind(&creds); err != nil {
        return c.Render(400, r.JSON(map[string]string{"error": "invalid_request"}))
    }
    // Validate credentials against your user store
    user, err := models.FindUserByUsername(tx, creds.Username)
    if err != nil || !user.CheckPassword(creds.Password) {
        return c.Render(401, r.JSON(map[string]string{"error": "invalid_grant"}))
    }
    // Revoke any existing tokens for this user and create a new one
    if err := models.RevokeUserTokens(tx, user.ID); err != nil {
        return c.Render(500, r.JSON(map[string]string{"error": "internal_error"}))
    }
    token, err := models.NewAuthToken(user.ID)
    if err != nil {
        return c.Render(500, r.JSON(map[string]string{"error": "internal_error"}))
    }
    // Persist token record (store hash only)
    if err := tx.Create(token); err != nil {
        return c.Render(500, r.JSON(map[string]string{"error": "internal_error"}))
    }
    // Return the plain token only once over HTTPS
    return c.Render(200, r.JSON(map[string]string{"access_token": token.TokenHash, "token_type": "Bearer"}))
}

Third, validate Bearer Tokens on each request and ensure the token is bound to the authenticated identity by checking against the stored hash. Do not accept tokens that were issued before authentication.

// app/middleware/auth_token.go
package middleware

import (
    "net/http"
    "strings"
    "yourapp/app/models"
)

func AuthTokenMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        auth := r.Header.Get("Authorization")
        if auth == "" || !strings.HasPrefix(auth, "Bearer ") {
            http.Error(w, `{"error": "unauthorized"}`, http.StatusUnauthorized)
            return
        }
        tokenValue := strings.TrimPrefix(auth, "Bearer ")
        // Lookup token hash by value (constant-time comparison)
        token, err := models.FindTokenByHash(tokenValue)
        if err != nil || token.Expired() {
            http.Error(w, `{"error": "invalid_token"}`, http.StatusUnauthorized)
            return
        }
        // Ensure token is tied to a valid user and scope
        if !token.IsValidFor(r.Context()) {
            http.Error(w, `{"error": "insufficient_scope"}`, http.StatusForbidden)
            return
        }
        // Set user identity in context for downstream handlers
        c := context.WithValue(r.Context(), "user_id", token.UserID)
        next.ServeHTTP(w, r.WithContext(c))
    })
}

Additionally, enforce HTTPS to protect token transmission and implement short token lifetimes with refresh rotation to reduce the impact of token leakage. Regularly rotate signing keys if you use signed tokens, and avoid storing raw tokens in logs or client-side storage.