Prototype Pollution

This gives Claude the full offensive playbook for finding and exploiting prototype pollution in JavaScript applications. It covers both `__proto__` and `constructor.prototype` injection paths, quick probes for client and server contexts, and concrete black-box detection signals like polluting Express `parameterLimit` or `json spaces` settings. The gadget table walks through real RCE chains in EJS and child_process scenarios. Load this when you're merging untrusted input into objects, auditing query parsers or deep assign logic, or hunting for post-pollution sinks that turn property injection into command execution. The decision tree and tool list make it practical for both initial recon and chaining to full exploits.

skills.sh installs: 3.8K
GitHub stars: 2.6K
last updated: Aug 21, 2026

View on GitHub

Install

$ npx -y skills add yaklang/hack-skills --skill prototype-pollution --agent claude-code

Installs into .claude/skills of the current project.

Files

SKILL: Prototype Pollution — Expert Attack Playbook

AI LOAD INSTRUCTION: Expert prototype pollution for client and server JS. Covers __proto__ vs constructor.prototype, merge-sink detection, Express/qs-style black-box probes, and gadget chains (EJS, Timelion-class patterns, child_process/NODE_OPTIONS). Assumes you know object spread and prototype inheritance — focus is on parser behavior and post-pollution sinks.

Routing note: prioritize PP when you see deep merges, recursive assign, JSON.parse followed by Object.assign, or URL queries converted to nested objects.

0. QUICK START

Client-side first probes

#__proto__[polluted]=1
#__proto__[polluted]=polluted
#constructor[prototype][polluted]=1

When input can reflect into DOM or framework routing, pair with alert(1) / console checks to observe whether global object properties were polluted.

#__proto__[xxx]=alert(1)

Server-side first probes（JSON / form）

{"__proto__":{"polluted":true}}

{"constructor":{"prototype":{"polluted":true}}}

SKILL.md on GitHub

Details

Repository: github.com/yaklang/hack-skills
Categories: Testing & QA, Automation & Workflows
First seen: Jun 3, 2026