The Claude Code RCE: When Eager Parsing Leads to Remote Execution

A critical Remote Code Execution (RCE) vulnerability was discovered in Anthropic's Claude Code CLI by security researcher Joernchen from 0day.click, exposing a flaw in how the command line tool handled external inputs. This vulnerability, which has since been patched in version 2.1.118, allowed attackers to execute arbitrary commands on a user's machine by exploiting a classic input validation problem in the tool's deeplink handler. The issue arose from a function named eagerParseCliFlag, which failed to properly distinguish between command line flags and their values, allowing an attacker to inject additional flags through a maliciously crafted link. This vulnerability underscores the importance of robust input validation and context-aware parsing, especially for tools with deeplink support. The attack vector leveraged the claude-cli:// deeplink protocol to trick users into executing malicious settings, bypassing security prompts and gaining unauthorized access. Anthropic addressed the flaw by enhancing their argument parsing method, and the incident serves as a critical lesson for developers to avoid manual string matching on raw argument arrays, advocating for the use of well-established libraries to handle command line syntax.

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