VulnerabilityPublished 18 Mar 2026

CVE-2026-31865: elysiajs elysia · Prototype Pollution

Affects elysiajs elysiaSource CVE Database V5

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Not known to be exploited · CVSS 6.5 Medium : upgrade to 1.4.27 when convenient.

Data as of 31 Oct 2026, 08:20 UTC

CVSS 3.1 base score

6.5Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.2% chance of exploitation in 30 days – higher than 15% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 1.4.27

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: Low
Integrity: Low
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 18 Mar 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 30 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade elysia and elysiajs/elysia to 1.4.27.

Affected software

Product	Affected	Fixed in
elysiajselysia	Affected <1.4.27	Fixed in 1.4.27
npmelysiajs/elysia`pkg:npm/elysiajs/elysia`Other threats	Affected <1.4.27	Fixed in 1.4.27

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Upgrade all Elysia framework instances to version 1.4.27 or later immediately to apply the official patch addressing prototype pollution.
Implement strict validation on cookies using t.Cookie validation or equivalent mechanisms to enforce allowed values and prevent iterable or prototype property manipulation.
Review application code for any direct or indirect use of __proto__ or other prototype properties in cookie handling or request processing and refactor to avoid unsafe prototype modifications.
Employ runtime protections such as object freezing or sealing where feasible to prevent prototype pollution attacks.
Conduct thorough security testing, including fuzzing and static analysis, focused on prototype pollution vectors in the application.
Monitor application logs for unusual cookie or prototype-related activity that may indicate attempted exploitation.
Educate developers on the risks of prototype pollution and secure coding practices in JavaScript/Typescript frameworks.
Consider implementing Content Security Policy (CSP) and other web security headers to reduce attack surface. These steps combined will reduce the risk of exploitation and limit the impact if an attack occurs.

Description

Elysia is a Typescript framework for request validation, type inference, OpenAPI documentation, and client-server communication. Prior to version 1.4.27, an Elysia cookie can be overridden by prototype pollution , eg. `__proto__`. This issue is patched in 1.4.27. As a workaround, use t.Cookie validation to enforce validation value and/or prevent iterable over cookie if possible.

Impact

The primary impact of this vulnerability is on the confidentiality and integrity of application data, specifically cookie values managed by the Elysia framework. An attacker exploiting this flaw can manipulate prototype attributes to override cookie objects, potentially leading to unauthorized access, session manipulation, or bypass of security controls relying on cookie data. While availability is not directly affected, the integrity compromise can facilitate further attacks or data leakage. Since exploitation requires no authentication or user interaction and can be performed remotely, the vulnerability poses a moderate risk to any organization using vulnerable Elysia versions. Applications relying heavily on cookies for authentication or state management are particularly at risk. The scope includes all web applications using Elysia versions before 1.4.27, which may be significant given the framework's adoption in Typescript web development. Although no active exploits are reported, the ease of exploitation and potential for chaining with other vulnerabilities make timely remediation important.

CWE-1321Improperly Controlled Modification of Object Prototype Attributes (Prototype Pollution) (opens in a new tab)
Variant weakness
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.
Implementation
By freezing the object prototype first (for example, Object.freeze(Object.prototype)), modification of the prototype becomes impossible.
Architecture and Design
By blocking modifications of attributes that resolve to object prototype, such as proto or prototype, this weakness can be mitigated.
Implementation · Input Validation
When handling untrusted objects, validating using a schema can be used.
OWASP Cheat Sheets
Mass Assignment (opens in a new tab)
OWASP ASVS 4.0.3
V5.1.2 (opens in a new tab)
Attack patterns
Accessing Functionality Not Properly Constrained by ACLs (opens in a new tab)
Exploiting Incorrectly Configured Access Control Security Levels (opens in a new tab)
Manipulating User-Controlled Variables (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 18 Mar 2026, 03:44 UTC

WeaknessesCWE-1321

Technical details

References

Community reviews

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