Issue
I have been working on a web interface using Flask and having some issues with back button in browser as after logging out hitting it takes user back inside. I have found similar questions and tried their answers but the issue is not resolved. I am attaching a simple example kindly have a look at it.
Main
from flask import Flask, request,session, redirect, url_for, render_template
from os import urandom
app = Flask(__name__)
app.config['SEND_FILE_MAX_AGE_DEFAULT'] = 0
app.secret_key = urandom(24)
@app.route('/', methods=['POST', 'GET'])
def index():
if request.method == 'POST':
session['Email id'] = request.form.get('Email Id')
Pass = request.form.get('Password')
try:
if session['Email id'] == '[email protected]' and Pass == 'KKK':
return render_template('Logged_in.html')
except:
return render_template('login.html')
return render_template('login.html')
@app.route('/sign_out')
def sign_out():
session.pop('Email id')
return redirect(url_for('index'))
@app.after_request
def add_header(r):
r.headers["Cache-Control"] = "no-cache, no-store, must-revalidate"
r.headers["Pragma"] = "no-cache"
r.headers["Expires"] = "0"
r.headers['Cache-Control'] = 'public, max-age=0'
return r
if __name__ == '__main__':
app.run(host="0.0.0.0", debug=True, threaded=True)
login.html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<form action="{{ url_for('index') }}" method="POST" id="login" class="input-group">
<input type="text" class="input-field" placeholder="Email Id" required name="Email Id">
<input type="text" class="input-field" placeholder="Password" required name="Password">
<button type="submit" class="submit-btn" style="color: white;">Log in</button>
</form>
</body>
</html>
Logged_in.html
<h2>You are Logged in</h2>
<a href="/sign_out"><i class="fas fa-sign-out-alt"></i>Log out</a>
Solution
Your problem is that when users push the back button their browser will re-do the POST request. You need to use the POST/redirect/GET pattern to prevent this. For this you need four endpoints in totalt:
GET /: Check in the session that the user is logged in and renderLogged_in.html, otherwise redirect to/login.htmlGET /login.html: Renderlogin.htmlPOST /sign_in: Check username and password. If successful, update the session and redirect to/POST /sign_out: Log out the user session and redirect to/login.html
Do not render templates in your POST endpoints, just make them manipulate the session and then redirect.
Answered By - Emil Vikström
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