Format String in Echo Go with Basic Auth

A format string vulnerability in an Echo Go handler that also uses Basic Authentication can occur when user-controlled input is passed directly to formatted output functions such as fmt.Printf or log.Printf without a explicit format specifier. In Go, the format verb determines how a value is rendered; if the user input contains format verbs like %s, %x, or %n, and the verb is omitted in the call, Go will reuse the format string as the verb table, reading or writing additional values from the stack or heap. When this pattern exists in a route protected by HTTP Basic Auth, the risk surface extends to authenticated behavior and metadata leakage, because the endpoint may log credentials, tokens, or internal identifiers during authentication checks.

Consider an Echo route that authenticates via Basic Auth and then logs the username using a format string bug:

package main

import (
    "fmt"
    "net/http"
    "github.com/labstack/echo/v4"
)

func main() {
    e := echo.New()
    e.GET("/profile", func(c echo.Context) error {
        user, pass, ok := c.Request().BasicAuth()
        if !ok {
            return c.String(http.StatusUnauthorized, "auth required")
        }
        // Vulnerable: user input used directly as format string
        fmt.Printf(user + " logged in\n")
        return c.String(http.StatusOK, "ok")
    })
    e.Start(":8080")
}

If an attacker sends a request with user=%s%s%s%s%s and valid Basic Auth credentials, the format string can read stack memory, potentially exposing session tokens cached in variables, memory addresses, or other sensitive runtime data. Because Basic Auth typically carries credentials in the request header, the format string bug can turn an otherwise harmless log line into a data-exposure vector. In a scan that tests unauthenticated attack surfaces and authenticated behaviors, such a route would flag both the authentication mechanism and the format string misuse as findings, since the vulnerability is tightly coupled to the auth flow.

An API security scan that supports OpenAPI/Swagger spec analysis would cross-reference the spec definition for the route with runtime behavior. If the spec does not document that the endpoint uses Basic Auth and the runtime reveals format string behavior through injected payloads, the scanner reports a finding that spans both the Authentication and Input Validation categories. This demonstrates how a format string bug in an authenticated route can amplify impact by exposing credentials or tokens that would otherwise remain protected by the authentication layer.

Remediation centers on two rules: never use untrusted input as a format string, and avoid logging or printing credentials directly. Use explicit format verbs and structured logging, and ensure credentials are handled as opaque values.

1. Use explicit format verbs and avoid concatenation with user input

Replace concatenation with a fixed format string that includes explicit verbs. This prevents Go from reinterpreting user input as a format string.

// Safe: explicit verb, user treated as data
fmt.Printf("%s logged in\n", user)

2. Do not log or print credentials

Credentials should never be written to logs. If you need to correlate requests, use a request-scoped identifier that does not expose usernames or passwords.

// Safe: log an identifier, not credentials
requestID := c.Get("requestId")
fmt.Printf("request %v processed\n", requestID)

3. Validate and sanitize authentication inputs

Although Basic Auth credentials are base64-encoded in the header, treat the decoded username and password as untrusted. Validate length and character sets where possible, and avoid using them in any formatting context.

// Validate length and reject suspicious patterns
if len(user) > 128 || len(pass) > 256 {
    return c.String(http.StatusBadRequest, "invalid credentials")
}

4. Use structured logging with explicit fields

Structured loggers reduce the risk of accidental format string misuse because fields are passed as key-value pairs, not as format strings.

import "github.com/sirupsen/logrus"

func main() {
    e := echo.New()
    e.GET("/profile", func(c echo.Context) error {
        user, _, ok := c.Request().BasicAuth()
        if !ok {
            return c.String(http.StatusUnauthorized, "auth required")
        }
        // Structured logging: fields are not interpreted as format verbs
        logrus.WithFields(logrus.Fields{
            "user": user,
            "route": "/profile",
        }).Info("authentication succeeded")
        return c.String(http.StatusOK, "ok")
    })
    e.Start(":8080")
}

5. Enforce secure authentication patterns in middleware

Centralize authentication logic in middleware to reduce duplication and ensure consistent handling. Keep credentials out of downstream handlers and avoid formatting them at any layer.

func BasicAuthMiddleware(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        user, pass, ok := c.Request().BasicAuth()
        if !ok || !validateUserPass(user, pass) {
            return c.String(http.StatusUnauthorized, "auth required")
        }
        // Store user identity without formatting credentials
        c.Set("user", user)
        return next(c)
    }
}

By following these patterns, the combination of Echo Go and Basic Auth remains secure against format string vulnerabilities while preserving the simplicity of header-based authentication.