Vulnerability in handlebars (CVE-2026-106445)
Summary
Vulnerability in handlebars (CVE-2026-106445). Risk of unauthorized operations or information disclosure. Exploitable via Function. Mitigation: upgrade to 4.7.10 or later.
Original description
The description as published by the source.
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:
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:
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 requires only an accessible function in the template context:
- Access a function's prototype: {{lookup myFunction "proto"}} resolves to Function.prototype. The proto property is not in the deny list and is permitted when allowProtoMethodsByDefault is true (since the result is a function).
- Access co…
Source: NIST National Vulnerability Database NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-106445 has been found in handlebars.
Attackers can target a specific entry point like Function over the network to misuse the product.
Risk of unauthorized operations or information disclosure.
What to do: upgrade handlebars to 4.7.10 or later.
If unsure, ask your IT team or search "handlebars CVE-2026-106445" on the vendor's site.
CVE-2026-106445 (handlebars) — CWE-184
Attack surface: Function / Function.prototype / constructor / lookupProperty
Patched: 4.7.10 — apply immediately
Workaround: set allowProtoMethodsByDefault: true
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-184) exists in handlebars. Attackers reach the vulnerable code path via Function.
Check the vendor advisory for the affected versions of handlebars. Attack surface: Function / Function.prototype / constructor / lookupProperty.
Severity: ?. Risk of unauthorized operations or information disclosure.
Update to 4.7.10.
Workaround: set allowProtoMethodsByDefault: true
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'handlebars' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
grep -r 'handlebars' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
handlebarsを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply temporary workaround mitigate
set allowProtoMethodsByDefault: trueパッチが適用されるまでの応急処置として、set allowProtoMethodsByDefault: true を実施。回避策の副作用 (機能低下) を確認した上で。
-
3Apply patch patch
Upgrade handlebars to 4.7.10ステージング環境で 4.7.10 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
-
4Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。