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Response snapshot Oct 27, 2026, 16:01 UTC

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VulnerabilityCVE-2026-76216

Vikunja: Link-share principal-type confusion enables cross-account team removal, bot takeover, and roster disclosure

1. **`TeamMember.CanDelete`** (`pkg/models/team_members_permissions.go:31-40`): the self-removal fast path `if u.ID == a.GetID() { return true }` (:36) executes **before** `IsAdmin`. `IsAdmin` (:48-51) is the ONLY place that rejects link shares (`if _, is := a.

At a glance

Exploited
Not known
Severity
7.5 High (CVSS 3.1)
Chance of exploitation
Not estimated
Sources
1 source

What to do

GitHub lists first patched versions: code.vikunja.io/api: 2.5.0. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-32r8-5843-4qw2 (opens a new tab)

What happened

## Summary Vikunja's `web.Auth` interface (`pkg/web/web.go`, single method `GetID() int64`) is satisfied by BOTH `*user.User` and `*models.LinkSharing`. A link-share's `GetID()` returns the **raw positive** `share.ID` (`pkg/models/link_sharing.go:83-85`), which lives in the same positive autoincrement ID space as `users.id`. The safe negated form `getUserID() = share.ID * -1` (`link_sharing.

go:126-128`) exists but is NOT used at three permission sinks. As a result, a link-share principal with id `N` — which should have zero authority over teams or bot users — is treated as the *user* whose `users.id == N` at three permission checks that lack the `a.(*LinkSharing)` guard their sibling methods have.

This is the same principal-type-confusion class as CVE-2026-68581 (GHSA-vvcv-vpph-h844), but at three code paths that advisory/fix never touched.

## Root Cause `web.Auth` is a one-method interface (`GetID() int64`). `*LinkSharing.GetID()` returns the raw positive share id. Three permission methods compare this raw id directly and omit the link-share type guard used elsewhere in the same files:

Continue reading

1. **`TeamMember.CanDelete`** (`pkg/models/team_members_permissions.go:31-40`): the self-removal fast path `if u.ID == a.GetID() { return true }` (:36) executes **before** `IsAdmin`. `IsAdmin` (:48-51) is the ONLY place that rejects link shares (`if _, is := a.(*LinkSharing); is { return false }`, :50) — and it is never reached when the fast path returns true. 2. **`BotUser.

isOwner`** (`pkg/models/bot_users_permissions.go:47-56`): `return u.BotOwnerID == a.GetID()` (:55), used by `CanRead`/`CanUpdate`/`CanDelete` (:36-45). Unlike `CanCreate` (:27-30) which type-asserts `a.(*user.User)`, these three paths have no principal guard. 3. **`Team.CanRead`** (`pkg/models/teams_permissions.go:68-78`): matches membership on `And("user_id = ?", a.

GetID())` (:76) with no link-share guard, unlike sibling `IsAdmin` (:45-49, guard at :47).

Link-share JWTs reach these routes: `SetupTokenMiddleware` validates the signature only, `GetAuthFromClaims` returns `*models.LinkSharing`, and the `/user`/`/teams` route groups add no link-share rejection. Link sharing is enabled by default (`config.go` `ServiceEnableLinkSharing.setDefault(true)`).

## Impact A link-share principal (obtainable from any public share link, or self-registered via a share on the attacker's own project) whose id `N` collides with a victim's `users.

id == N` can, without being that user or any user: - **Integrity (I:H):** remove the victim from any team they belong to (`DELETE /api/v1/teams/{T}/members/{username}`) → revokes all project permissions the victim inherited through that team. - **Availability/Integrity (A:H, I:H):** enumerate the victim's bot users (`GET /api/v1/user/bots` → `bot_owner_id = a.

GetID()`), then disable/rename or **permanently delete** them (`DELETE /api/v1/user/bots/{id}` → `DeleteUser`), destroying data owned solely by those bots. - **Confidentiality (C:H):** read the roster + metadata (name, description, full member list) of any team the colliding user belongs to (`GET /api/v1/teams/{T}`), plus read bot-user records via `isOwner`-gated reads.

## Attack Chain

### Sink 1 — TeamMember.CanDelete (integrity) 1. **Entry:** `POST /api/v1/shares/{hash}/auth` → link-share JWT with `id = N`. **Guard:** JWT middleware — signature only. **Bypass proof:** `GetAuthFromClaims` returns `*models.LinkSharing`; no route-group link-share rejection on the `/teams` group. 2.

**Action:** `DELETE /api/v1/teams/{T}/members/{usernameOfUserN}` with the link-share bearer, targeting a team `T` (≥2 members) that user `N` belongs to. **Guard:** `CanDelete` → `GetUserByUsername(tm.Username)` returns user N, then `u.ID == a.GetID()` (`team_members_permissions.go:36`). **Bypass proof:** `a.GetID()` returns `N` (raw positive `share.ID`, `link_sharing.go:84`) == user N's id → `true`. No `a.

(*LinkSharing)` check on this branch (only `IsAdmin` at :50 has it, never reached). 3. **Sink:** `Delete` (`team_members.go`) removes user N from team T (last-member check passes when team has ≥2 members). **Impact:** victim loses all project access inherited through team T.

### Sink 2 — BotUser.isOwner (bot takeover / destruction) 1. **Entry:** link-share JWT id N (as above). **Guard:** signature-only; `/user` group adds no link-share reject. 2. **Enumerate:** `GET /api/v1/user/bots` → `ReadAll` runs `Where("bot_owner_id = ?", a.GetID())` = bots owned by user N. **Bypass proof:** `a.GetID()` = N; returns victim's bot ids self-contained (removes the id-guessing barrier). 3.

**Sink:** `DELETE /api/v1/user/bots/{botId}` → `CanDelete` → `isOwner` → `u.BotOwnerID == a.GetID()` (`bot_users_permissions.go:55`) → true; `Delete` calls `DeleteUser`. **Bypass proof:** no `a.(*LinkSharing)` guard here (Create-only, :28). **Impact:** disable/rename/permanently delete victim's bot automation identities.

### Sink 3 — Team.CanRead (info disclosure) 1. **Entry:** link-share JWT id N. **Guard:** signature-only; no reject on `GET /teams/:team`. 2. **Sink:** `GET /api/v1/teams/{T}` → `CanRead` runs `Where("team_id=?", t.ID).And("user_id=?", a.GetID()).Get(tm)` (`teams_permissions.go:74-77`). **Bypass proof:** `a.GetID()` = N matches user N's `team_members` row → `can = true`; no `a.

(*LinkSharing)` check (contrast `IsAdmin` at :47). **Impact:** read roster + metadata of a team the link share is not part of.

## Bypass Evidence - `link_sharing.go:83-85` `GetID()` returns raw positive `share.ID` (NOT the negated `getUserID()` at :126-128). - `team_members_permissions.go:36` raw `u.ID == a.GetID()` before `IsAdmin`; the LinkSharing guard sits at :50 on a branch never reached when the fast path returns true. - `bot_users_permissions.go:55` raw `u.BotOwnerID == a.GetID()`; the `a.(*user.

User)` guard at :28 is Create-only and NOT replicated on `isOwner`. - `teams_permissions.go:76` raw `a.GetID()` in `CanRead`; sibling `IsAdmin` has the guard at :47, `CanRead` omits it. - All three sinks verified p

Affected package ranges reported by GitHub: code.vikunja.io/api: < 2.5.0

Scores and how they were measured

CVSS base severity

7.5

Version 3.1As of

CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

Not available

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-32r8-5843-4qw2 (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-32r8-5843-4qw2, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 27). Vikunja: Link-share principal-type confusion enables cross-account team removal, bot takeover, and roster disclosure (CVE-2026-76216). https://kevwire.com/record/CVE-2026-76216

MLA

"Vikunja: Link-share principal-type confusion enables cross-account team removal, bot takeover, and roster disclosure (CVE-2026-76216)." KEVWire, 27 Oct. 2026, kevwire.com/record/CVE-2026-76216.

Chicago

KEVWire. "Vikunja: Link-share principal-type confusion enables cross-account team removal, bot takeover, and roster disclosure (CVE-2026-76216)." Last modified October 27, 2026. https://kevwire.com/record/CVE-2026-76216.

Evidence summary (plain text)

Vikunja: Link-share principal-type confusion enables cross-account team removal, bot takeover, and roster disclosure (Oct 9, 2026).
1. **`TeamMember.CanDelete`** (`pkg/models/team_members_permissions.go:31-40`): the self-removal fast path `if u.ID == a.GetID() { return true }` (:36) executes **before** `IsAdmin`. `IsAdmin` (:48-51) is the ONLY place that rejects link shares (`if _, is := a.(*LinkSharing); is { return false }`, :50) — and it is never reached when the fast path returns true.
2. **`BotUser.isOwner`** (`pkg/models/bot_users_permissions.go:47-56`): `return u.BotOwnerID == a.GetID()` (:55), used by `CanRead`/`CanUpdate`/`CanDelete` (:36-45). Unlike `CanCreate` (:27-30) which type-asserts `a.(*user.User)`, these three paths have no principal guard.
3. **`Team.CanRead`** (`pkg/models/teams_permissions.go:68-78`): matches membership on `And("user_id = ?", a.GetID())` (:76) with no link-share guard, unlike sibling `IsAdmin` (:45-49, guard at :47).

Link-share JWTs reach these routes: `SetupTokenMiddleware` validates the signature only, `GetAuthFromClaims` returns `*models.LinkSharing`, and the `/user`/`/teams` route groups add no link-share rejection. Link sharing is enabled by default (`config.go` `ServiceEnableLinkSharing.setDefault(true)`).
CVSS 7.5 (version 3.1; evidence as of Oct 27, 2026; https://github.com/advisories/GHSA-32r8-5843-4qw2).
Sources:
GitHub-reviewed advisory GHSA-32r8-5843-4qw2: https://github.com/advisories/GHSA-32r8-5843-4qw2
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
Record ID: CVE-2026-76216.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 16:00 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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