CVE-2026-31865: Elysia Elysia Cookie Value Prototype Pollution Vulnerability

MODERATE
Advisory Identifier: GHSA-8hq9-phh3-p2wp
Share
CVSS 6.5
CWE-1321
Published
Advisory Summary

CVE-2026-31865 is a MODERATE (CVSS 6.5) elysia cookie value prototype pollution vulnerability (CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')) affecting Elysia libraries in the npm ecosystem. The issue involves elysia cookie value prototype pollution. Remediation is available by upgrading affected packages to version 1.4.27.

CVE-2026-31865 at a Glance

CVE Identifier CVE-2026-31865
GHSA Identifier GHSA-8hq9-phh3-p2wp
Severity MODERATE
CVSS v3.1 Base Score 6.5
CWE Classification CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
Affected Product Elysia
Package Ecosystem npm
Published Date
Last Updated

Security Impact

CVSS: 6.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality Low (C:L)
Integrity Low (I:L)
Availability None (A:N)

How to Fix CVE-2026-31865

To remediate this vulnerability, upgrade all affected Elysia dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

elysia fixed: 1.4.27 
npm install elysia@1.4.27
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

npm ls elysia

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-31865, inspect your project dependencies using npm:

npm ls elysia

Analyze the terminal output to locate any version of Elysia in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.4.27), your application is affected.

Note: The npm ls elysia command inspects dependencies declared in your package.json and lockfile; it does not perform automated runtime dynamic scanning.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution'), where 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. In Elysia, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### Impact Elysia cookie can be overridden by prototype pollution , eg. `__proto__` Sending cookie with the follows name can override cookie value: ```bash __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

CVE-2026-31865 Affected Elysia Versions

1 component(s)
Fixed Versions:
1.4.27
Product / Package Ecosystem Affected Versions Fixed Version
elysia
npm/elysia
npm
< 1.4.27
1.4.27

References

GitHub Advisory / Wiki ADVISORY
https://github.com/elysiajs/elysia/security/advisories/GHSA-8hq9-phh3-p2wp
NVD — CVE-2026-31865 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-31865
Security Fix Commit FIX COMMIT
https://github.com/elysiajs/elysia/commit/e9d6b1743fa7368ef942dce181f6a089757f6aab
Elysia Source Repository REPOSITORY
https://github.com/elysiajs/elysia

Frequently Asked Questions

What is CVE-2026-31865?
CVE-2026-31865 is a security vulnerability in Elysia involving Elysia Cookie Value Prototype Pollution under CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution').
Which Elysia versions are affected?
Affected versions include releases prior to 1.4.27 across affected packages: elysia.
What version fixes CVE-2026-31865?
Upgrading to 1.4.27 (or the latest patched release for your respective branch) resolves the vulnerability.
Is elysia affected?
Yes, elysia is affected in releases prior to 1.4.27. Upgrade your dependency to 1.4.27 or later.
What is the CVSS score of CVE-2026-31865?
CVE-2026-31865 has a CVSS v3.1 base score of 6.5 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N.
What CWE is associated with CVE-2026-31865?
CVE-2026-31865 is classified under CWE-1321: 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.).
How can I check npm dependencies for CVE-2026-31865?
Run the dependency tree command: 'npm ls elysia'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-8hq9-phh3-p2wp",
  "modified" : "2026-03-19T19:01:12Z",
  "published" : "2026-03-17T16:17:41Z",
  "aliases" : [ "CVE-2026-31865" ],
  "summary" : "Elysia Cookie Value Prototype Pollution",
  "details" : "### Impact\nElysia cookie can be overridden by prototype pollution , eg. `__proto__`\n\nSending cookie with the follows name can override cookie value:\n```bash\n__proto__=%7B%22injected%22%3A%22polluted%22%7D\n```\n\n### Patches\nPatched by 1.4.27\n\n### Workarounds\n1. Use t.Cookie validation to enforce validation value\n2. Prevent iterable over cookie if possible",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "elysia"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.4.27"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/elysiajs/elysia/security/advisories/GHSA-8hq9-phh3-p2wp"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-31865"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/elysiajs/elysia/commit/e9d6b1743fa7368ef942dce181f6a089757f6aab"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/elysiajs/elysia"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1321" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-17T16:17:41Z",
    "nvd_published_at" : "2026-03-18T04:17:19Z"
  }
}