Automating Authentication Flows in Go: Lessons from a Security Researcher’s No-Docs Approach

Source: Dev.to

Overview

In security research, efficiency and precision are crucial, especially when automating complex authentication flows with little or no documentation. This guide shares key insights and practical Go code snippets for automating OAuth2 authorization flows, focusing on handling redirects, cookies, CSRF tokens, and dynamic parameters securely and reliably.

HTTP Client Setup

A custom http.Client with a cookie jar is essential for tracking sessions and managing redirects.

import (
	"net/http"
	"net/http/cookiejar"
	"log"
)

func newClient() *http.Client {
	jar, err := cookiejar.New(nil)
	if err != nil {
		log.Fatal(err)
	}
	return &http.Client{
		Jar: jar,
		CheckRedirect: func(req *http.Request, via []*http.Request) error {
			// Handle redirects manually if necessary
			return nil
		},
	}
}

Starting the Authorization Flow

Construct the authorization URL with the required query parameters and issue a GET request.

import (
	"net/url"
)

func startAuthFlow(client *http.Client, authEndpoint string, params map[string]string) (*http.Response, error) {
	query := url.Values{}
	for k, v := range params {
		query.Add(k, v)
	}
	authURL := authEndpoint + "?" + query.Encode()
	return client.Get(authURL)
}

// Example parameters
params := map[string]string{
	"client_id":      "your-client-id",
	"redirect_uri":   "https://yourapp.com/callback",
	"response_type":  "code",
	"scope":          "openid profile",
	"state":          "randomStateString",
}

resp, err := startAuthFlow(client, "https://auth.example.com/authorize", params)

Observe the response, especially any redirect URLs that contain the authorization code.

Extracting the Authorization Code

Parse the code parameter from the final redirect URL.

import "net/url"

func extractCodeFromRedirect(resp *http.Response) (string, error) {
	redirectURL := resp.Request.URL
	code := redirectURL.Query().Get("code")
	return code, nil
}

Exchanging the Code for a Token

Send a POST request to the token endpoint with the required form data.

import (
	"net/url"
	"strings"
)

func exchangeCodeForToken(client *http.Client, tokenEndpoint, code string) (*http.Response, error) {
	data := url.Values{}
	data.Set("grant_type", "authorization_code")
	data.Set("code", code)
	data.Set("redirect_uri", "https://yourapp.com/callback")
	data.Set("client_id", "your-client-id")
	data.Set("client_secret", "your-client-secret")

	req, err := http.NewRequest("POST", tokenEndpoint, strings.NewReader(data.Encode()))
	if err != nil {
		return nil, err
	}
	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")

	return client.Do(req)
}

Handle the response to store tokens securely for subsequent API calls.

Best Practices

• Observe real flow interactions before automating to understand required parameters and hidden fields.
• Use robust cookie management to maintain session state across redirects.
• Parse URLs and response bodies carefully to extract tokens and other dynamic data.
• Never hard‑code secrets; load them from secure storage or environment variables.
• Test across environments to catch edge cases and variations in server behavior.

References

• OAuth 2.0 Authorization Framework (RFC 6749)
• OAuth 2.0 Threat Model and Security Considerations
• Go net/http package documentation

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