Extension JavaScript Behavior Analyzer

Extension JavaScript behavior analyzer

Short answer

An extension JavaScript behavior analyzer inspects source files for patterns that indicate what the extension could do: execute dynamic code, load remote scripts, inject HTML, access cookies, store data, send messages or contact external hosts. Static indicators describe capability and code structure, not observed runtime behavior.

Drop an extension package hereor choose a file from your deviceChoose a fileSupported: CRX, XPI, ZIP, manifest.json. Maximum size: 25 MB.

Processed locallyCode is never executed

Your file stays in this browser. Parsing happens in a Web Worker; the extension code is read, never executed.

What the scan checks

Permissions, host access, content scripts, background scripts, CSP, externally_connectable, web-accessible resources, JavaScript patterns, referenced domains and risk categories.

MV3MV2CRXXPIZIPJSON

Static analysis shows references and patterns. It cannot prove that network activity happened or that an extension is malicious.

Compare two versions

See permission, domain and finding changes between two packages.

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How behavior is inferred statically

The analyzer parses JavaScript with an AST parser, then walks calls, assignments and member access. It records file, line, evidence and confidence. This is more precise than searching for words in minified text.

It also scans HTML for remote script tags and inline scripts, and extracts URLs for domain classification.

Prioritizing findings

Start with high-severity findings that combine access and transmission: eval or remote code references, native messaging, cookie access near network calls, password field references, and obfuscation with encoded payloads.

Read the surrounding code and check whether the path is reachable. A finding in a bundled library or dead code is different from one in the main content script.

Finding categories
Category	Example	What it suggests
Dynamic code	eval, new Function	Code built at runtime
Remote code	script src, importScripts	Executable code outside the package
DOM injection	innerHTML, document.write	Page content modification
Sensitive data	document.cookie, password fields	Potential data access
Network	fetch, WebSocket	Communication capability
Obfuscation	hex identifiers, base64 blobs	Readability barrier

The limits of static behavior analysis

It cannot observe executed branches, server responses or user-specific data flows. It cannot prove that a network request happened. It is a review accelerator, not a sandbox.

Frequently asked questions

Does the analyzer run the JavaScript?

No. It parses the source as text and inspects the syntax tree. The code is never evaluated.

Why does a clean extension still get findings?

Bundled libraries, optional features and generic APIs can produce findings. Use severity, confidence, file and line to judge relevance.

Can it detect data exfiltration?

It can flag combinations such as cookie access near network calls, but it cannot prove that data was sent. Runtime monitoring is required for confirmation.

Last reviewed: October 31, 2026