CVE-2026-106445: Handlebars: JavaScript Injection via Own Property Check Bypass

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.4%, percentile 34: FIRST’s model estimates a 0.4% chance of exploitation activity in the next 30 days, higher than 34% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes handlebars (npm) >= 3.0.0, <= 4.7.9, fixed in 4.7.10. Check your lockfile, including transitive dependencies.

What it is

Handlebars provides the power necessary to let users build semantic templates. From 3.0.0 until 4.7.10, Handlebars lookupProperty returns Function.prototype.constructor before applying the prototype-access deny list because constructor is an own property of Function.prototype. When an attacker can render a controlled template with allowProtoMethodsByDefault enabled and an accessible function in the template context, the template can traverse from that function through its prototype to Function.prototype and then obtain the Function constructor through the own-property bypass. This permits attacker-controlled JavaScript to execute with the server application's privileges. This issue is fixed in version 4.7.10.

From the GitHub advisory

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:

Normally, prototype-derived values reach resultIsAllowed, which blocks dangerous method names such as constructor. However, constructor is itself an own property of prototype objects:

Weakness type: CWE-184, CWE-1289.

Product: handlebars (npm) · Assigned by: GitHub_M · Published: 2026-10-06

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: present (depends on deployment conditions)
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.41% probability of exploitation activity in the next 30 days; percentile 34.

Timeline

• 2026-10-06 — CVE record published by GitHub_M
• 2026-10-08 — GitHub-reviewed advisory GHSA-p8wg-vrv2-v86f published (critical)
• 2026-10-12 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-p8wg-vrv2-v86f github.com
2. CVE.org record CVE-2026-106445 cve.org
3. NVD entry CVE-2026-106445 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com

Critical npm advisories · Today’s Security Digest