Prototype Pollution

Prototype Pollution Playbook

Prototype pollution is the JavaScript equivalent of a universal gadget: plant a property on Object.prototype and it appears on every object in the runtime. Worthless in isolation, deadly in chain (__proto__.isAdmin = true → auth bypass; __proto__.shell = "/bin/bash" → RCE via spawn).

1. Sinks — the libraries that still introduce sinks

Keep a running list per engagement. These continue to ship sinks in 2026:

• Deep-merge: lodash.merge, deepmerge (pre-fix), merge-deep, deepExtend, hoek.merge, mixme
• Deep-clone: lodash.defaultsDeep, lodash.zipObjectDeep, set-value (pre-3.0.3)
• URL-to-obj: qs, express-fileupload, jquery.extend(true, ...)
• Config loaders: node-config recursive merge, dotenv-extended, rc
• Template engines: Handlebars helpers fed from untrusted ctx

# Every JS/TS project: sweep known-bad versions
jq '.dependencies,.devDependencies | to_entries[] | select(.key | test("merge|lodash|set-value|dot-object|dot-prop|node-pg"))' /workspace/src/package.json
npm ls lodash set-value dot-prop 2>/dev/null | grep -E '[0-9]'

2. Sources

Any user input deserialized into a nested object:

• JSON body parsers (body-parser, express.json)
• Query string parsers (qs with default config parses a[b][__proto__][c]=1)
• YAML uploads
• Form-data / multipart

3. Audit workflow

1. Find every deep merge call site.
2. Trace each one backwards — is the right-hand object user-controlled?
3. If yes: check the merge function's prototype-pollution fix version.
4. Even if the merge is fixed, check whether a copy (lodash.set, dot-path-value, jsonpath.set) creates a pollution path.

4. Exploitation gadgets

Poisoning Object.prototype doesn't do anything by itself — you need a gadget that reads a property that didn't exist before.

Classic gadgets:

• child_process.spawn(cmd, args, opts) — opts has a shell option. Poison __proto__.shell = "/bin/bash" then any subsequent spawn call executes through bash and interprets args as shell strings.
• Express middleware — most middlewares check options.someFlag with if (opts.someFlag). Poisoning that flag flips security defaults.
• Templating — Handlebars and EJS read helpers and partials from the context object; pollution adds helpers that execute code.
• lodash.template — if the template source is built from _.template(tpl, ctx) you can inject via polluted escape/evaluate keys.
• mongoose — polluting Schema.Types causes subsequent schema definitions to use attacker-controlled types.

5. PoC template

# Classic lodash.merge RCE via child_process.spawn
curl -X POST https://target.com/api/settings \
  -H 'Content-Type: application/json' \
  -d '{"__proto__": {"shell": "/bin/bash", "env": {"PATH": "/tmp:/usr/bin"}}}'

# Second request triggers the gadget
curl https://target.com/api/render-pdf
# → any subsequent child_process.spawn call now runs through /bin/bash

6. Success signals for validate_finding

• Server error with stack trace referencing Object.prototype
• Output of injected command reflected in next response
• Admin-only endpoint now returns 200 after poisoning __proto__.isAdmin

Negative control: same payload with proto (no leading underscores) — should have no effect. If it does, the endpoint is treating that key specially and the finding is unrelated.

7. Default CVSS

8. Chain promotion

Prototype pollution is almost always the first hop of a chain. After validation, add an enables edge from the pollution vuln to:

• the spawn / template gadget vuln (RCE chain)
• the auth check vuln (privilege escalation chain)
• the SSRF vuln if the downstream request lib has an options pollution surface

Chain weight 0.4 — pollution is cheap once the merge sink is known.