vulnerability / NVI-00012050 / Revision 2
Handlebars: JavaScript Injection via Own Property Check Bypass
GitHub lists this reviewed advisory for handlebars. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
Published affected context helps an owner decide what evidence to gather. This report has not examined your application.
0.589%
FIRST EPSS model v2026.06.15; exploitation probability over 30 days, percentile 0.48165. Missing scores are unknown.
As of · FIRST EPSS
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
handlebars
source listed>= 3.0.0, <= 4.7.9
## Summary Handlebars can expose the `Function` constructor despite its prototype-access deny list. When a template reaches `Function.prototype`, its own `constructor` property is returned before the deny list is checked. An attacker who can render a controlled template with `allowProtoMethodsByDefault: true` can inject and execute arbitrary JavaScript, leading to Remote Code Execution on the server. ## Description `lookupProperty` trusts own properties: ```js if (Object.prototype.hasOwnProperty.call(parent, propertyName)) { return result; } ``` Normally, prototype-derived values reach `resultIsAllowed`, which blocks dangerous method names such as `constructor`. However, `constructor` is itself an own property of prototype objects: ```js Function.prototype.constructor === Function; String.prototype.constructor === String; Object.prototype.constructor === Object; ``` As a result, once a template reaches `Function.prototype`, looking up `constructor` returns `Function` without consulting the deny list. ## Proof of Concept An attacker needs to reach any prototype object where constructor is an own property, then access constructor to obtain Function. The shortest path requi
03 / Response
What to do next
- investigate
Identify the exact component and deployed version, examine the linked publisher’s conditions, and retain evidence of any mitigation.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. FIRST EPSS
A fixed release is not deployment evidence.
Check current vendor support and compatibility. After making a change, verify the deployed build and relevant runtime conditions. Keep the evidence, owner and time. An updated lockfile alone does not prove what is running.
04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
05 / Provenance
Sources you can inspect
Each assertion retains its source. Mirrored records may repeat one underlying publisher; additional links are not independent corroboration.
- GHSA-p8wg-vrv2-v86f ↗
GitHub Advisory Database · vendor advisory
Observed . GitHub Advisory Database; CC-BY-4.0. Each advisory links its original attribution. Null normalizes fields.
- CVE-2026-106445 ↗
FIRST EPSS · official catalog
Observed . FIRST EPSS; probability of exploitation in the next 30 days, not local compromise. Dataset date/model retained; FIRST services terms apply.
Public revision history
Only published revisions appear here. Routine fetches do not create a material-change event.
- Revision 2 ·
Dated EPSS estimate updated; this is not a new disclosure or proof of exploitation.
- Revision 1 ·
First indexed by Null; source disclosure date is shown separately.