Prototype pollution via unsafe _fillValue assignment
Published May 7, 2026 · Updated May 7, 2026
Prototype pollution in query-string-parser 1.0.0 allows remote attackers to modify Object.prototype via crafted query parameters. The _fillValue function in index.js accepts proto path segments from fromQuery and assigns their values into a newly created object's prototype. Exploitation requires an application to pass attacker-controlled query strings to fromQuery; polluted properties then appear on subsequently created objects.
Summary
What happened
Prototype pollution in query-string-parser 1.0.0 allows remote attackers to modify Object.prototype via crafted query parameters. The _fillValue function in index.js accepts proto path segments from fromQuery and assigns their values into a newly created object's prototype. Exploitation requires an application to pass attacker-controlled query strings to fromQuery; polluted properties then appear on subsequently created objects.
The record
- CVE
- CVE-2025-63704
- Published
- May 7, 2026
- Updated
- May 7, 2026
- Vendor
- Unknown vendor
- Product
- Unknown product
- Classifications
- CWE-1321
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- May 7, 2026CVE publishedPublication date reported by the CVE source.
- May 7, 2026Record updatedLatest update available in the CVE record.
Exploitability
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What is affected?
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Published CVSS scores
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Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
- query-string-parser prototype-pollution payloadproof of concept · unverified
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Technologies
Your stack
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Your stack
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