Prototype Pollution

Prototype Pollution Detection

When to Use

Audit merge/clone/deep-assign utilities, query string parsers, JSON parsers, config mergers, and any package that recursively sets object properties from untrusted input.

Key insight: Only ~50% acceptance rate. Must demonstrate REAL impact beyond just polluting prototype.

Process

Step 1: Find Object Manipulation Sinks

grep -rn "Object\.assign\|Object\.defineProperty\|Object\.create" .
grep -rn "merge\|extend\|deepMerge\|deepExtend\|deepAssign\|mixin" .
grep -rn "clone\|deepClone\|cloneDeep\|deepCopy" .
grep -rn "set\|setPath\|setValue\|lodash\.set\|_.set" .
grep -rn "\[.*\]\s*=" . --include="*.js"  # Bracket notation assignment

Step 2: Check for Recursive Property Setting

Look for patterns where object keys from user input are used as property paths:

// VULNERABLE: recursive merge without key filtering
function merge(target, source) {
  for (const key in source) {
    if (typeof source[key] === 'object') {
      target[key] = merge(target[key] || {}, source[key]);
    } else {
      target[key] = source[key];
    }
  }
}

Step 3: Check Key Filtering

grep -rn "__proto__\|constructor\|prototype" . | grep -i "filter\|block\|skip\|ignore\|reject"
grep -rn "Object\.create(null)" .  # Null prototype objects are safe
grep -rn "hasOwnProperty\|Object\.keys\|Object\.entries" .

Step 4: Assess Impact

Prototype pollution alone is often not enough. Look for impact:

• DoS: Polluted property causes TypeError crash (toString, valueOf)
• Property injection: Polluted property affects security logic (isAdmin, role, auth)
• Gadget chains: Polluted property reaches dangerous sink (eval, template)
• Method clobbering: toString/valueOf overwritten causing crash

Dangerous Keys

| Key | Effect | Impact | |-----|--------|--------| | __proto__ | Sets properties on Object.prototype | All objects affected | | constructor.prototype | Same effect via constructor chain | All objects affected | | constructor | Overwrites constructor reference | Type confusion | | toString | Overwrites string conversion | TypeError on string operations | | valueOf | Overwrites value conversion | TypeError on comparisons | | hasOwnProperty | Overwrites property check | Logic bypass |

CVSS Guidance

• Proto pollution + RCE gadget chain: CRITICAL 9.8
• Proto pollution + auth bypass: HIGH 8.1
• Proto pollution + DoS (crash): HIGH 7.5
• Proto pollution with no demonstrated impact: MEDIUM 5.3 (often rejected)

References

• [Sinks](references/sinks.md) -- Object manipulation sinks
• [False Positive Indicators](references/false-positive-indicators.md)
• [PoC Skeleton](references/poc-skeleton.md)

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: ByamB4
• Source: ByamB4/find-cve-agent
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.