Decidim: JWT-backed authentication can be replayed across organizations

Description

A JWT issued to an Org 1 account is accepted on the Org 2 API and can read the admin-only GraphQL participantDetails field for an Org 2 participant. The same trust-boundary problem also affects API-user authentication: an Org 1 API user can use a JWT on the Org 1 host and replay that JWT to the Org 2 API to read Org 2 participant personal data and reach Org 2's proposal.answer mutation path.

Technical description

The current host selects the Decidim organization context, but JWT-backed API authentication is not sufficiently bound to that host organization. As a result, the API can process a request in Org 2's context while still trusting an authenticated principal from Org 1.

Reproduction steps:

1. Use an API key provided by the system administrator that is assigned to organization 1 to create the JWT token or get the JWT token shown in the response when logged in as the organization admin.

<img width="1080" height="1119" alt="decidim-jwt-01" src="https://github.com/user-attachments/assets/6195a250-faef-41d5-8f64-4d77d4077e96" />

2. When using this JWT token it is possible to retrieve details from other organisations. Notice the change of the host header in the request below to that of another tenant org2.localhost:3001

<img width="1085" height="1047" alt="decidim-jwt-02" src="https://github.com/user-attachments/assets/d40825e3-0d36-44f3-bede-86d247bbe6d0" />

Note that using a participant-generated JWT did not allow showing these results.

Impact

A JWT issued for one organization can be replayed successfully against another organization's API and used to retrieve sensitive details from that organization.

Patches

See https://github.com/decidim/decidim/pull/16673 and https://github.com/decidim/decidim/pull/16756

Workarounds

Disable JWT credentials on system panel (/system)

References

OWASP A01:2021 Broken Access Control

Credits

This issue was discovered in a security audit organized by the Decidim Association and made by Radically Open Security against Decidim financed by NGI.