Cookie-state corruption via prototype-bearing parser objects
Published Mar 18, 2026 · Updated Mar 18, 2026
Prototype pollution in Elysia before 1.4.27 allows remote attackers to override application-visible cookie values with a crafted cookie name. The parseCookie function inserted attacker-controlled names such as proto into ordinary JavaScript objects, allowing inherited properties to appear during cookie access and iteration. No authentication or user interaction is required, but the consequence depends on application code trusting or iterating the cookie jar and is limited to request-derived cookie state.
Summary
What happened
Prototype pollution in Elysia before 1.4.27 allows remote attackers to override application-visible cookie values with a crafted cookie name. The parseCookie function inserted attacker-controlled names such as proto into ordinary JavaScript objects, allowing inherited properties to appear during cookie access and iteration. No authentication or user interaction is required, but the consequence depends on application code trusting or iterating the cookie jar and is limited to request-derived cookie state.
The record
- CVE
- CVE-2026-31865
- Published
- Mar 18, 2026
- Updated
- Mar 18, 2026
- Vendor
- ElysiaJS
- Product
- Elysia
- Classifications
- CWE-1321, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Mar 18, 2026CVE publishedPublication date reported by the CVE source.
- Mar 18, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 1.4.27
What conditions does exploitation require?
What is affected?
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Crafted __proto__ cookie payloadproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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