CVE-2026-106445 handlebars-lang CVE debrief

Who should care

Defenders responsible for Handlebars deployments, especially those using allowProtoMethodsByDefault, should assess exposure and prioritize verification and remediation efforts. This includes reviewing current Handlebars versions, restricting usage of allowProtoMethodsByDefault, and monitoring for exploitation attempts. Security teams and operators managing Handlebars-based applications should also review compensating controls and asset inventories to limit

Why it matters

Defenders should care about CVE-2026-106445 because it allows for Remote Code Execution on the server through JavaScript injection in Handlebars templates when allowProtoMethodsByDefault is true. Roles responsible for Handlebars deployments, especially those using this setting, should assess exposure and prioritize verification and remediation.

• Remote Code Execution on the server
• Arbitrary JavaScript injection and execution
• Potential for exploitation with crafted templates

Technical 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. This vulnerability affects Handlebars deployments using allowProtoMethodsByDefault, particularly those with unpatched versions below 4.7.10.

Defensive priority

Defenders should prioritize verifying Handlebars versions and updating to 4.7.10 or later if using a vulnerable version.

Recommended defensive actions

• Verify Handlebars version and update to 4.7.10 or later if vulnerable
• Review and restrict usage of allowProtoMethodsByDefault
• Monitor for and restrict exploitation attempts
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The source corpus provides details on a JavaScript injection vulnerability in Handlebars, which can lead to Remote Code Execution on the server when allowProtoMethodsByDefault is true. The vulnerability arises from Handlebars' prototype property handling, allowing attackers to inject and execute arbitrary JavaScript. Evidence is based on public CVE Program and NVD records, as well as developer documentation and source code analysis. Defenders should verify Handlebars versions and update to 4.7.10 or later if vulnerable.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.