CVE-2026-68746

This vulnerability exposes a dangerous flaw in Livebook's authorization caching: when handle_call/3 cannot resolve a user's group, it treats this as "enforcement disabled" rather than as an indeterminate state. This is a fundamental violation of the security principle that cached identity state should never expand privileges during connectivity disruptions or when referents are deleted. The root cause is a single fallback clause in handle_call/3 that conflates two semantically distinct conditions: (1) enforcement was explicitly turned off, and (2) the group cannot be resolved. The latter includes scenarios where the group was deleted, the cache is stale, or network issues prevent resolution. The function treats all of these as equivalent to "allow access," which is the wrong default in every case. What makes this worse is that two adjacent authentication functions in the same module correctly interpret "unresolved" as a denial condition. This internal inconsistency is the strongest signal that the handle_call/3 clause was never threat-modeled—it represents a code review failure, not an architectural mystery. For defenders: verify your deployment is on Livebook 0.19.9 or later. More importantly, examine your authentication logs for any cases where users accessed resources after their group was deleted or unavailable—the vulnerability may have allowed unauthorized access during exactly those periods. If you cannot upgrade immediately, any user whose group resolution fails should be treated as denied by default until the fix is applied. The deeper architectural question is whether Livebook should implement a "deny-until-confirmed" model: any identity resolution failure—whether from network issues, stale cache, or deleted groups—should default to denial rather than assuming enforcement is off. This would require cache invalidation on group deletion events and integration between identity resolution and team membership lifecycle, not merely time-based expiration.

Reviewed through automated stages and approved by a human before publication.