CVE-2026-106445
Description
A flaw was found in Handlebars. The lookupProperty own-property check returns Function.prototype.constructor before the prototype-method deny list is applied. When an attacker can render a controlled template with allowProtoMethodsByDefault enabled and an accessible function in the template context, arbitrary JavaScript can execute with the application process privileges.
Statement
This flaw has an Important impact. Exploitation requires rendering a controlled template with allowProtoMethodsByDefault set to true (off by default) and an accessible function in the template context, allowing bypass of the constructor deny list via Function.prototype. The issue is not Critical for Red Hat scoring because those non-default runtime conditions raise attack complexity (AC:H); default Handlebars configurations that keep allowProtoMethodsByDefault disabled are not exploitable for this issue.
Mitigation
Do not set allowProtoMethodsByDefault to true when compiling or rendering untrusted templates or untrusted data.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 8.1 | N/A | N/A |
| Attack Vector | Network | N/A | N/A |
| Attack Complexity | High | N/A | N/A |
| Privileges Required | None | N/A | N/A |
| User Interaction | None | N/A | N/A |
| Scope | Unchanged | N/A | N/A |
| Confidentiality | High | N/A | N/A |
| Integrity Impact | High | N/A | N/A |
| Availability Impact | High | N/A | N/A |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
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