The execute_ruby tool is documented as a read-only Ruby sandbox and is enforced by a pattern denylist together with replacements for the process-spawning methods on Kernel. The pseudo-terminal library's spawn entry points are neither in the denylist nor replaced, so a normal tool call could reach them and start a shell, executing commands as the account running the server and outside the guarded methods. The denylist was introduced with the tool in 1.4.0 and never covered those entry points through 1.6.0. Version 1.6.1 restricts the requires the sandbox permits to a data-only list and blocks dynamic dispatch to execution entry points; 2.0.0 removes the tool.

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.