CVE-2026-81097 affects rails-mcp-server versions 1.4.0 through 1.6.0, where the execute_ruby tool's sandbox can be escaped via the pseudo-terminal (PTY) library's spawn entry points. The tool was documented as a read-only Ruby sandbox enforced by a pattern denylist and replacements for Kernel process-spawning methods. However, PTY spawn entry points were never included in the denylist, allowing an attacker to start a shell and execute arbitrary OS commands as the server's running account. The vulnerability was introduced when the denylist was added in version 1.4.0 and persisted through 1.6.0. Version 1.6.1 mitigates the issue by restricting allowed requires to a data-only list and blocking dynamic dispatch to execution entry points. Version 2.0.0 removes the vulnerable tool entirely.