Description

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.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "8.4",
  "pubDate": "2026-08-27T17:20:52.110Z",
  "pubdate": "2026-08-27T17:20:52.110Z",
  "executiveSummary": "The execute_ruby tool, utilized within the server environment, contained a critical sandbox escape vulnerability due to insufficient filtering of process-spawning entry points. The security architecture relied on a denylist-based approach and selective method replacement, which failed to account for the PTY (pseudo-terminal) library. This flaw allowed unauthorized arbitrary command execution outside the intended sandboxed constraints, effectively granting an attacker the privileges of the server process. Impacted versions include 1.4.0 through 1.6.0. The risk is significant, as it allowed for potential system compromise, data exfiltration, or lateral movement within the infrastructure. Exploitation required access to the tool interface, enabling an attacker to bypass the restricted environment by leveraging secondary execution paths not covered by existing security controls.",
  "technicalDetails": "The vulnerability resides in the implementation of the execute_ruby sandbox, which attempted to restrict Ruby code execution through a combination of a string-based denylist and method overriding for standard process-spawning primitives in the Kernel module. The root cause is an incomplete security perimeter that failed to consider the broader Ruby standard library, specifically the PTY (pseudo-terminal) module. Because the PTY library provides its own set of spawn-related entry points for interacting with pseudo-terminals, it functioned as an overlooked vector for process invocation.\nThe attack flow proceeds as follows: an attacker with access to the tool executes Ruby code designed to require the PTY library. Once the library is successfully loaded, the attacker invokes PTY.spawn or associated entry points. Since these methods were not included in the existing denylist or subject to monkey-patching/method replacement, the underlying OS process-creation syscalls are triggered directly by the Ruby interpreter. This effectively bypasses the sandboxing logic, executing arbitrary commands with the identity and permissions of the host server process.\nThis vulnerability is present in versions 1.4.0 through 1.6.0. While the system attempted to enforce isolation by filtering core Kernel methods, the reliance on an exhaustive denylist—rather than a strict allowlist—proved insufficient. The ability to load additional modules facilitated the discovery of secondary execution paths that were not effectively guarded by the initial implementation. Version 1.6.1 addressed the root cause by transitioning to an allowlist for required libraries and blocking dynamic dispatch to sensitive execution entry points, while version 2.0.0 completely removed the tool, eliminating the attack surface entirely.\nThe impact of a successful exploitation is complete sandbox breakout, enabling the attacker to execute shell commands, read/write files accessible to the application user, and potentially perform further system enumeration. There are no authentication requirements specific to the flaw itself, assuming the attacker has already gained access to the execute_ruby interface, and the network exposure is contingent upon the accessibility of the host application."
}