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Elysia Cookie Value Prototype Pollution

Moderate severity GitHub Reviewed Published Mar 16, 2026 in elysiajs/elysia • Updated Mar 17, 2026

Package

npm elysia (npm)

Affected versions

< 1.4.27

Patched versions

1.4.27

Description

Impact

Elysia cookie can be overridden by prototype pollution , eg. __proto__

Sending cookie with the follows name can override cookie value:

__proto__=%7B%22injected%22%3A%22polluted%22%7D

Patches

Patched by 1.4.27

Workarounds

  1. Use t.Cookie validation to enforce validation value
  2. Prevent iterable over cookie if possible

References

@SaltyAom SaltyAom published to elysiajs/elysia Mar 16, 2026
Published to the GitHub Advisory Database Mar 17, 2026
Reviewed Mar 17, 2026
Last updated Mar 17, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

EPSS score

Weaknesses

Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype. Learn more on MITRE.

CVE ID

CVE-2026-31865

GHSA ID

GHSA-8hq9-phh3-p2wp

Source code

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