CVE-2026-76216

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:

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)).

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 present on latest release tag v2.4.0 (git show v2.4.0:<file>). No fix commits touch these files between v2.4.0 and HEAD (the only post-tag commit to link_sharing.go, c580d51, merely shadows an embedded Update method).

Affected Versions

<= 2.4.0 (latest release; also present on HEAD of main). Requires default-enabled link sharing.

Exploitability Constraint (reflected in AC:H)

The attacker cannot freely choose the colliding id, link_shares.id is autoincrement. Exploitation is (a) opportunistic (a guest holding a share with id N attacks the user whose users.id == N) or (b) targeted (self-register and walk the autoincrement toward a chosen id; low-numbered shares collide with low-numbered/early/admin accounts). This is the identical constraint the accepted CVE-2026-68581 had; it affects target selection (AC), not reachability of the boundary crossing.