NSA Codebreaker 2025 — Task 6: Crossing the Channel
A lesson in destination-specific authorization
Task 6 shifted the investigation into a Mattermost environment. The challenge supplied persistent application data and an account with access to a limited part of the system. The objective concerned reaching the channel used by the fictional adversary.
Reviewing the custom integration
I examined the provided bot plugins and found a flaw in the logic used to manage private negotiation channels. The relevant checks established that users belonged to the current channel, but did not adequately establish their authority to access the destination channel.
That distinction was the central finding. Validating that a user exists, or belongs somewhere in the application, does not establish permission to perform an operation on a different resource.
Understanding the impact
The provided PostgreSQL data helped me understand relationships among users and channels. My notes describe using those relationships to assess the reach of the authorization flaw in the challenge environment.
The issue was in the supplied custom bot behavior. This writeup should not be read as a claim that ordinary Mattermost installations share the same flaw.
Defensive implications
A channel-management integration needs to authorize the requested operation against the destination resource. Creating a channel and restoring an existing channel also deserve distinct checks: an archived private channel can retain a security boundary that should survive its archived state.
Useful regression cases would include requests by users who are valid members of the source channel but have no permission over the destination, including previously archived private channels. Changes to membership should be auditable so that unexpected access can be investigated.
What I took away
This task was a different kind of puzzle from unpacking malware. The code could look reasonable line by line while still enforcing the wrong permission boundary.
The broader lesson was to ask what each check actually proves. Source membership answered one question; the operation required an answer about destination access.
Based on my Task 6 repository notes.