Password Spraying in Gin with Bearer Tokens

Password spraying is an authentication attack where one or a few common passwords are tried against many accounts. When an API built with the Gin web framework relies solely on Bearer token authentication without additional protections, password spraying can be enabled by two design characteristics: predictable token issuance endpoints and missing account-specific rate limiting.

In Gin, a common pattern exposes a /login route that accepts a username and password and returns a Bearer token. If this route does not enforce per-account or per-IP rate limits, an attacker can iterate over a list of known usernames and send a small password guess for each, avoiding lockouts that target a single account. Bearer tokens are typically issued as opaque strings or JWTs; if token generation does not require proof of origin beyond the provided credentials, an attacker who discovers a valid username can mount a low-and-slow password spray against that account with minimal risk of detection.

Another contributing factor is the lack of binding between the token and a strong session or risk context. For example, if the token is issued after a single successful credential check and remains valid until explicitly revoked, earlier password guesses that happen to succeed will produce long-lived tokens usable for BOLA/IDOR or data exposure. The Gin application might also leak account existence through timing differences or distinct response codes, which attackers use to enumerate valid usernames before spraying.

Real-world attack patterns reference known weaknesses such as weak password policies and missing multi-factor authentication, which align with the OWASP API Security Top 10 and can map to findings in automated scans. For example, an attacker may use credentials from prior breaches and run a spray against endpoints like /auth/token, checking for 200 responses that indicate success. If the service lacks robust monitoring for distributed attempts across many accounts, the activity remains under typical thresholds, enabling prolonged compromise.

middleBrick detects such risks by scanning the unauthenticated attack surface and correlating authentication behavior with token issuance flows. The scanner checks whether authentication endpoints vary responses by username, whether rate limiting applies at the account level, and whether tokens are issued without sufficient evidence of legitimacy. These checks help surface weaknesses in the interaction between password policies and Bearer token workflows.

Secure Bearer token handling in Gin requires tying issuance to strong verification, rate control, and observable logging. Below are concrete code examples that demonstrate recommended patterns.

1. Rate limiting per username and per IP

Apply a rate limiter that tracks attempts by username and IP to slow down password spraying. Use a sliding window or token bucket algorithm and enforce strict limits.

import (
    "github.com/gin-gonic/gin"
    "github.com/ulule/limiter/v3"
    "github.com/ulule/limiter/v3/drivers/store/memstore"
    "net/http"
)

func SetupRateLimiter() *limiter.Limiter {
    store := memstore.NewStore()
    rate, _ := limiter.NewRateFromFormatted("10-M") // 10 attempts per minute
    return limiter.New(store, rate)
}

func LoginWithRateLimiter(l *limiter.Limiter) gin.HandlerFunc {
    return func(c *gin.Context) {
        // key by username+ip to prevent spraying across accounts
        username := c.PostForm("username")
        ip := c.ClientIP()
        key := username + "|" + ip
        _, err := l.Get(c, key)
        if err != nil {
            c.JSON(http.StatusTooManyRequests, gin.H{"error": "rate limit exceeded"})
            c.Abort()
            return
        }
        // proceed to authenticate
        c.Next()
    }
}

2. Constant-time credential verification

Avoid timing leaks by using constant-time comparison for password checks and by standardizing response times and shapes, regardless of whether the username exists.

import (
    "golang.org/x/crypto/bcrypt"
    "net/http"
    "time"
)

func HashPassword(password string) (string, error) {
    hashed, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
    return string(hashed), err
}

func VerifyPassword(hashed, password string) bool {
    // Use a dummy hash to keep timing consistent when user is not found
    dummyHash, _ := bcrypt.GenerateFromPassword([]byte("dummy"), bcrypt.DefaultCost)
    // Always run a comparison to obscure timing
    if bcrypt.CompareHashAndPassword(dummyHash, []byte(password)) == nil {
        // Only proceed with real hash if username lookup is safe
        return bcrypt.CompareHashAndPassword([]byte(hashed), []byte(password)) == nil
    }
    return false
}

3. Secure Bearer token issuance and validation

Issue tokens only after successful multi-factor flows where applicable, bind tokens to context, and validate them on each request using middleware.

import (
    "github.com/gin-gonic/gin"
    "net/http"
    "strings"
)

func AuthMiddleware() gin.HandlerFunc {
    return func(c *gin.Context) {
        auth := c.GetHeader("Authorization")
        if auth == "" || !strings.HasPrefix(auth, "Bearer ") {
            c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "missing or invalid token"})
            return
        }
        token := strings.TrimPrefix(auth, "Bearer ")
        // Validate token signature, scope, and revocation status here
        if !isValidToken(token) {
            c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "invalid token"})
            return
        }
        c.Set("token", token)
        c.Next()
    }
}

func isValidToken(token string) bool {
    // Implement JWT validation or opaque token introspection
    return token == "expected-secure-token-example"
}

4. Response uniformity and account existence protection

Return the same HTTP status and message shape for both existing and non-existing accounts to prevent enumeration. Log suspicious patterns server-side for further analysis.

func Login(c *gin.Context) {
    var req struct {
        Username string `json:"username" binding:"required"`
        Password string `json:"password" binding:"required"`
    }
    if c.ShouldBindJSON(&req) != nil {
        c.JSON(http.StatusBadRequest, gin.H{"error": "invalid request"})
        return
    }
    // Perform constant-time verification
    hashed, _ := getHashedPassword(req.Username) // returns dummy hash if user not found
    if !VerifyPassword(hashed, req.Password) {
        c.JSON(http.StatusUnauthorized, gin.H{"error": "invalid credentials"})
        return
    }
    token := generateSecureToken(req.Username)
    c.JSON(http.StatusOK, gin.H{"token": token})
}

By combining per-account rate limiting, constant-time checks, secure token validation, and uniform responses, Gin services can significantly reduce the risk of password spraying while maintaining compatibility with Bearer token workflows.