CVE-2026-6621

High

Published: 20 April 2026

Published

20 April 2026

Modified

22 April 2026

KEV Added

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Patch

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CVSS Score 7.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L

EPSS Score 0.0006 18.2th percentile

Risk Priority 15 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-6621 is a high-severity Code Injection (CWE-94) vulnerability. Its CVSS base score is 7.3 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 18.2th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified are NIST 800-53 SA-22 (Unsupported System Components) and SI-10 (Information Input Validation).

Threat & Defense at a Glance

What attackers do: exploitation maps to Exploit Public-Facing Application (T1190) and 1 other technique. What defenders deploy: see the NIST 800-53 controls recommended below.

Threat & Defense Details

Mitigating Controls (NIST 800-53 r5)AI

SI-10 Information Input Validation good match

prevent

Information input validation directly prevents manipulation of the __proto__ argument by rejecting or sanitizing inputs that could cause prototype pollution.

SI-2 Flaw Remediation good match

preventrecover

Flaw remediation mandates timely identification, patching, or removal of the vulnerable extend-deep package affected by this prototype pollution issue.

SA-22 Unsupported System Components good match

prevent

Prohibiting unsupported system components addresses the inactive repository with no patches available for this CVE, preventing use of the vulnerable package.

MITRE ATT&CK Enterprise TechniquesAI

T1190 Exploit Public-Facing Application Initial Access

Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.

attack.mitre.org →

T1059.007 JavaScript Execution

Adversaries may abuse various implementations of JavaScript for execution.

attack.mitre.org →

Why these techniques?

Remote unauthenticated code injection via prototype pollution in a public-facing JS package directly enables T1190 exploitation and T1059.007 JavaScript command/script execution.

Confidence: HIGH · MITRE ATT&CK Enterprise v18.1

NVD Description

A vulnerability was determined in 1024bit extend-deep up to 0.1.6. The impacted element is an unknown function of the file index.js. This manipulation of the argument __proto__ causes improperly controlled modification of object prototype attributes. Remote exploitation of the attack…

is possible. The exploit has been publicly disclosed and may be utilized. The code repository of the project has not been active for many years.

Deeper analysisAI

CVE-2026-6621 affects the 1024bit extend-deep package up to version 0.1.6, specifically an unknown function in the index.js file. The vulnerability involves manipulation of the __proto__ argument, resulting in improperly controlled modification of object prototype attributes. It is classified under CWE-94 (code injection) and CWE-1321 (prototype pollution), with a CVSS v3.1 base score of 7.3 (AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L). The issue was published on 2026-04-20.

Remote exploitation is possible without authentication or user interaction, allowing unauthenticated attackers accessible over the network to trigger the prototype pollution. Successful exploitation can lead to low impacts on confidentiality, integrity, and availability through modification of object prototypes.

Advisories referenced in VulDB entries (vuldb.com/vuln/358256 and related pages) document the issue, while a GitHub security research repository provides a proof-of-concept (PoC.md) demonstrating the prototype pollution. The project's code repository has not been active for many years, indicating no official patches or updates are available.

The exploit has been publicly disclosed and may be utilized by attackers.