[Security] Session.cancelBackgroundTask has race conditions and missing authorization

[randomm]

Summary

Adversarial review of cancel_task tool revealed issues in the underlying Session.cancelBackgroundTask() function.

Issues Found

CRITICAL: Missing Authorization Control

• tryCancel() does NOT validate caller has permission to cancel a specific task
• Any session can cancel ANY task in the system

CRITICAL: TOCTOU Race Condition

• Time-of-check-time-of-use between status check (line 562) and actual cancellation (line 569)
• Task can complete between check and cancel, leading to incorrect state

CRITICAL: Double-Modify State Corruption

• cancelBackgroundTask() mutates backgroundTaskResults after task completion
• Completed tasks can be incorrectly marked as "failed"
• Result data lost, audit trail corrupted

MEDIUM: Slot Release Race

• release_slot() called before deleting from maps
• Brief window for double-reservation

Location

/packages/opencode/src/session/index.ts

• Lines 499-548: cancelBackgroundTask()
• Lines 550-581: tryCancel()

Impact

For current use case (PM cancelling own tasks), risk is low. But these should be fixed before:

• Multi-user scenarios
• Cross-session task management
• Production deployment

Recommended Fixes

1. Add session ownership check before cancel
2. Make status check + cancel atomic
3. Never mutate already-completed task results
4. Reorder slot release to after map deletion

[randomm]

Fixed in commit 126240a — fix(session): add authorization and race condition guards to cancelBackgroundTask