What happened
NVD published CVE-2026-67531 (CVSS 9.3, Critical) on August 6, 2026, describing a sandbox-escape vulnerability in FrontMCP's code-execution tool caused by a Proxy-trap bypass against Zod v4 schema internals.
Why it matters
Code-execution sandboxes are the primary security boundary between an LLM agent's generated code and the host system; a sandbox escape in an MCP code-exec tool is a novel agent-execution attack class that directly enables arbitrary host code execution from what should be an isolated agent tool call.
Attack vector
The sandboxed codecall:execute tool exposes live host Zod schema instances to the sandboxed script via getTool(); because Zod v4 defines the internal _zod property as non-configurable/non-writable, the ECMAScript Proxy sandbox boundary can be bypassed, allowing sandboxed code to reach into and manipulate host-realm objects — effectively a sandbox escape from an MCP code-execution tool.
Affected systems
agentfront FrontMCP < 1.5.7
Mitigation
Upgrade to FrontMCP 1.5.7 or later per the vendor fix commit.