Velocity.js has a Prototype Pollution vulnerability through #set path assignment

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-44966: 0.55% estimated exploitation activity in the next 30 days; percentile 45.78 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary A prototype pollution vulnerability was discovered in Velocity.js <= 2.1.5. This issue occurs during the processing of #set directives in Velocity templates. If an application renders a template controlled by an attacker, it is possible to modify Object.prototype, potentially leading to Denial of Service (DoS) or Remote Code Execution (RCE) depending on the server environment. Details The root cause is located in the #set path assignment logic within the source code: - File: /src/compile/set.ts - Issue: The engine accepts arbitrary path keys and performs assignments using the logic (baseRef as Record<string, unknown)[key] = val. Because there is no validation or filtering to block sensitive keys such as \\proto\\, constructor, or prototype, an attacker can traverse the prototype chain and pollute the global Object.prototype. PoC Impact - Vulnerability Type: Prototype Pollution - Who is impacted: Any application that renders Velocity templates where the template content can be…

Published by the source

9 May 2026, 00:40 UTC

Last modified by the source

8 Jun 2026, 23:45 UTC

Source last checked

26 Oct 2026, 12:53 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-j658-c2gf-x6pq

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-j658-c2gf-x6pq

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-44966

Advisory: Velocity.js has a Prototype Pollution vulnerability through #set path assignment
IDs: CVE-2026-44966, GHSA-j658-c2gf-x6pq
Record: https://patchnext.com/advisories/cve-2026-44966 (revision 2, source checked 26 Oct 2026, 12:53 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor response not established. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm velocityjs: affected <= 2.1.5

## Suggested inspection (read-only)
- `npm ls velocityjs --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on velocityjs.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-j658-c2gf-x6pq (OSV record GHSA-j658-c2gf-x6pq lists affected ranges: npm velocityjs >=0 <=2.1.5)
- Verified: https://github.com/advisories/GHSA-j658-c2gf-x6pq (GitHub-reviewed advisory GHSA-j658-c2gf-x6pq lists affected ranges: npm velocityjs >=0 <=2.1.5)

Commands are suggestions: review each one before running it.