Vikunja: Any user can enumerate every team and its members by attaching arbitrary teams to a throwaway project

Summary

When you share a project with a team, the API lets you attach any team on the instance, including teams you have nothing to do with, as long as you're an admin of the project. Listing a project's teams then returns each team's full member roster. So any logged-in user can spin up a throwaway project, attach team IDs one by one, and read back the name, description, and complete member list of every team on the instance. Normally you can only see teams you belong to; this path ignores that.

Details

Sharing a project with a team is a two-step flow: you PUT a team to the project, then you GET the project's teams to see them. The share endpoint only enforces that you're an admin of the target project, it doesn't care whether the team you're referencing is one you're allowed to see. Any existing team ID is accepted.

The listing endpoint is even more permissive: read access to the project is enough, which you obviously have on a project you created. And its response isn't just a list of team names, it includes each team's description, creator, and a full members array with every member's username, display name, and admin flag.

Put together, an attacker who owns a single project can walk the team ID space and pull the roster of every team in the instance, including teams they aren't a member of. This contradicts how team visibility works everywhere else in the product, where you only ever see teams you belong to. Emails are stripped from the roster, so what leaks is identities and org structure rather than contact details.

The fix is to require that the caller actually has access to the team they're attaching, not just to the project.

PoC

The attacker needs nothing but an ordinary account.

1. Create a throwaway project you own:
2. Attach team IDs one by one (sweep the range you want):

PUT /api/v1/projects/<your_project_id>/teams
{<SNIP>, "team_id": <N>, <SNIP>}

3. Read them back with their rosters:

GET /api/v1/projects/<your_project_id>/teams

Each entry contains the team name, description, creator, and a members array with every member's username, display name, and admin flag, for teams you are not a member of.

Impact

Information disclosure of the instance-wide team directory: team names, descriptions, and full membership, for teams the attacker has no relationship with. This maps out the organisation's group structure and who belongs where, useful for targeting and social engineering. Any authenticated account is enough; no share link, no admin rights, no victim interaction.