vulnerability / NVI-00013253 / Revision 1
Prototype Pollution via parse() in NodeJS flatted
GitHub lists this reviewed advisory for flatted. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
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01 / Meaning
Why it matters
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02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
flatted
source listed<= 3.4.1
--- **Summary** The parse() function in flatted can use attacker-controlled string values from the parsed JSON as direct array index keys, without validating that they are numeric. Since the internal input buffer is a JavaScript Array, accessing it with the key "\_\_proto\_\_" returns Array.prototype via the inherited getter. This object is then treated as a legitimate parsed value and assigned as a property of the output object, effectively leaking a live reference to Array.prototype to the consumer. Any code that subsequently writes to that property will pollute the global prototype. --- **Root Cause** File: esm/index.js:29 (identical in cjs/index.js) ``` const resolver = (input, lazy, parsed, $) => output => { for (let ke = keys(output), {length} = ke, y = 0; y < length; y++) { const k = ke[y]; const value = output[k]; if (value instanceof Primitive) { const tmp = input[value]; // Bug is here ``` No validation that value is a safe numeric index input is built as a plain Array. JavaScript's property lookup on arrays traverses the prototype chain for non-numeric keys. The key "\_\_proto\_\_" resolves to Array.protot
03 / Response
What to do next
- investigate
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04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
05 / Provenance
Sources you can inspect
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- GHSA-rf6f-7fwh-wjgh ↗
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- Revision 1 ·
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