Anti tamper: avoid submitting any suffix

app/config.py

@@ -123,6 +123,7 @@ DB_URI = os.environ["DB_URI"]
 # Flask secret
 FLASK_SECRET = os.environ["FLASK_SECRET"]
 MAILBOX_SECRET = FLASK_SECRET + "mailbox"
+CUSTOM_ALIAS_SECRET = FLASK_SECRET + "custom_alias"
 
 # AWS
 AWS_REGION = "eu-west-3"

app/dashboard/templates/dashboard/custom_alias.html

@@ -42,7 +42,7 @@
         <div class="col-sm-6 p-1">
           <select class="form-control" name="suffix">
             {% for suffix in suffixes %}
-              <option value="{{ suffix[1] }}">
+              <option value="{{ suffix[2] }}">
                 {% if suffix[0] %}
                   {{ suffix[1] }} (your domain)
                 {% else %}

app/dashboard/views/custom_alias.py

@@ -1,7 +1,12 @@
 from flask import render_template, redirect, url_for, flash, request
 from flask_login import login_required, current_user
+from itsdangerous import TimestampSigner, SignatureExpired
 
-from app.config import DISABLE_ALIAS_SUFFIX, ALIAS_DOMAINS
+from app.config import (
+    DISABLE_ALIAS_SUFFIX,
+    ALIAS_DOMAINS,
+    CUSTOM_ALIAS_SECRET,
+)
 from app.dashboard.base import dashboard_bp
 from app.email_utils import email_belongs_to_alias_domains, get_email_domain_part
 from app.extensions import db
@@ -9,6 +14,8 @@ from app.log import LOG
 from app.models import Alias, CustomDomain, DeletedAlias, Mailbox
 from app.utils import convert_to_id, random_word, word_exist
 
+signer = TimestampSigner(CUSTOM_ALIAS_SECRET)
+
 
 @dashboard_bp.route("/custom_alias", methods=["GET", "POST"])
 @login_required
@@ -24,27 +31,24 @@ def custom_alias():
         return redirect(url_for("dashboard.index"))
 
     user_custom_domains = [cd.domain for cd in current_user.verified_custom_domains()]
-    # List of (is_custom_domain, alias-suffix)
+    # List of (is_custom_domain, alias-suffix, time-signed alias-suffix)
     suffixes = []
 
     # put custom domain first
     for alias_domain in user_custom_domains:
-        suffixes.append((True, "@" + alias_domain))
+        suffix = "@" + alias_domain
+        suffixes.append((True, suffix, signer.sign(suffix).decode()))
 
     # then default domain
     for domain in ALIAS_DOMAINS:
-        suffixes.append(
+        suffix = ("" if DISABLE_ALIAS_SUFFIX else "." + random_word()) + "@" + domain
-            (
+        suffixes.append((False, suffix, signer.sign(suffix).decode()))
-                False,
-                ("" if DISABLE_ALIAS_SUFFIX else "." + random_word()) + "@" + domain,
-            )
-        )
 
     mailboxes = [mb.email for mb in current_user.mailboxes()]
 
     if request.method == "POST":
         alias_prefix = request.form.get("prefix")
-        alias_suffix = request.form.get("suffix")
+        signed_suffix = request.form.get("suffix")
         mailbox_email = request.form.get("mailbox")
         alias_note = request.form.get("note")
 
@@ -55,6 +59,18 @@ def custom_alias():
                 flash("Something went wrong, please retry", "warning")
                 return redirect(url_for("dashboard.custom_alias"))
 
+        # hypothesis: user will click on the button in the 300 secs
+        try:
+            alias_suffix = signer.unsign(signed_suffix, max_age=300).decode()
+        except SignatureExpired:
+            LOG.error("Alias creation time expired")
+            flash("Alias creation time is expired, please retry", "warning")
+            return redirect(url_for("dashboard.custom_alias"))
+        except Exception:
+            LOG.error("Alias suffix is tampered, user %s", current_user)
+            flash("Unknown error, refresh the page", "error")
+            return redirect(url_for("dashboard.custom_alias"))
+
         if verify_prefix_suffix(
             current_user, alias_prefix, alias_suffix, user_custom_domains
         ):

tests/dashboard/test_custom_alias.py

@@ -1,6 +1,7 @@
 from flask import url_for
 
 from app.config import EMAIL_DOMAIN
+from app.dashboard.views.custom_alias import signer
 from app.extensions import db
 from app.models import Mailbox
 from app.utils import random_word
@@ -12,14 +13,12 @@ def test_add_alias_success(flask_client):
     db.session.commit()
 
     word = random_word()
+    suffix = f".{word}@{EMAIL_DOMAIN}"
+    suffix = signer.sign(suffix).decode()
 
     r = flask_client.post(
         url_for("dashboard.custom_alias"),
-        data={
+        data={"prefix": "prefix", "suffix": suffix, "mailbox": user.email,},
-            "prefix": "prefix",
-            "suffix": f".{word}@{EMAIL_DOMAIN}",
-            "mailbox": user.email,
-        },
         follow_redirects=True,
     )