Format String in Buffalo with Basic Auth

A format string vulnerability in a Buffalo application becomes more severe when Basic Authentication is used in request handling. In Buffalo, user-controlled data can reach logging, audit, or debug paths when credentials are parsed from the Authorization header. If the application uses unchecked user input in functions such as fmt.Sprintf, fmt.Printf, or log.Printf while also processing Basic Auth credentials, an attacker can supply format specifiers like %s, %x, or %n to read stack memory or alter program state.

Consider a Buffalo handler that extracts a username and password from Basic Auth and logs them using a format string built from user input:

import (
    "fmt"
    "log"
    "net/http"
    "strings"
)

func vulnerableHandler(c buffalo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    if auth != "" {
        parts := strings.SplitN(auth, " ", 2)
        if len(parts) == 2 && parts[0] == "Basic" {
            // Simulated decoding for example purposes
            creds := "user:pass"
            var username, password string
            fmt.Sscanf(creds, "%s:%s", &username, &password)
            // Vulnerable: using user-controlled format via query or body
            format := c.Param("fmt")
            log.Printf(format, username, password)
        }
    }
    return c.Render(200, r.String("ok"))
}

In this scenario, an attacker can send a request with Authorization: Basic dXNlcjpwYXNz and a query parameter ?fmt=%s %s %x. The log output may leak stack contents, memory addresses, or sensitive credential remnants. Because the format string is derived from a request parameter, the attacker can indirectly observe how Basic Auth credentials are handled in memory, potentially aiding further attacks such as BOLA/IDOR or information disclosure categorized under Data Exposure.

When scanning with middleBrick, such patterns are flagged under the Input Validation and Data Exposure checks. The scanner does not rely on internal architecture but observes that format strings are influenced by parameters while credentials are present in the request. The findings include remediation guidance that emphasizes strict format strings and avoiding logging of credentials, which aligns with the OWASP API Top 10 and relevant compliance mappings.

Remediation focuses on two areas: eliminating uncontrolled format strings when credentials are processed, and avoiding logging or concatenating sensitive authentication data. Always use static format strings with fmt.Printf or log.Printf, and ensure credentials are never passed as format parameters.

Secure handler example in Buffalo:

import (
    "fmt"
    "log"
    "net/http"
    "strings"
)

func secureHandler(c buffalo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    if auth != "" {
        parts := strings.SplitN(auth, " ", 2)
        if len(parts) == 2 && parts[0] == "Basic" {
            // Decode credentials safely without format injection
            creds := decodeBasicAuth(parts[1])
            username := creds.username
            // Use a static format string; never include external input in the format verb
            log.Printf("Auth attempt for user: %s", username)
            // Do not log password
        }
    }
    return c.Render(200, r.String("ok"))
}

// decodeBasicAuth is a placeholder for safe base64 decoding and parsing.
func decodeBasicAuth(token string) struct{ username, password string } {
    // Implement proper decoding and avoid fmt-style parsing of credentials
    return struct{ username, password string }{}
}

Additional measures include validating and sanitizing all inputs that could influence output generation, using structured logging with predefined fields instead of dynamic format strings, and ensuring that any debug or audit logs exclude passwords and session material. In the CLI, you can run middlebrick scan https://your-buffalo-app.example.com to detect these issues; the dashboard will map findings to relevant checks such as Input Validation and Data Exposure. The GitHub Action can enforce a maximum risk score in CI/CD, and the MCP Server allows you to trigger scans directly from development tools while you work on handlers.