js-deobfuscator

General purpose JavaScript deobfuscator

A simple but powerful deobfuscator to remove common JavaScript obfuscation techniques. Open an issue if there is a feature you think should be implemented.

Online version at deobfuscate.io

Install via npm install js-deobfuscator

Features

• Unpacks arrays containing literals (strings, numbers etc) and replaces all references to them
• Removes simple proxy functions (calls to another function), array proxy functions and arithmetic proxy functions (binary expressions)
• Simplifies arithmetic expressions
• Simplifies string concatenation
• Renames unreadable hexadecimal identifiers (e.g. _0xca830a)
• Converts computed to static member expressions and beautifies the code
• Experimental function evaluation

Examples

See bottom for more complicated example with features chained together.

Array Unpacking

Before

const a = ['\x20', '\x57\x6f\x72\x6c\x64', '\x48\x65\x6c\x6c\x6f'];

console.log(a[2] + a[0] + a[1]);

After

console.log("Hello" + " " + "World");

Proxy Functions

An example with simple proxy functions for other functions

Before

function a(b, c) {
    return someFunction(b, c);
}

const result = a(5, 6);

After

const result = someFunction(5, 6);

An example with proxy functions for arithmetic

Before

function a(b, c) {
    return c + 2 * b;
}

const result = a(5, 6);

After

const result = 6 + 2 * 5;

An example with chained proxy functions

Before

function a(b, c) {
    return c + 2 * b;
}
function b(c, d) {
    return a(c, d);
}
function c(d, e) {
    return b(d, e);
}

const result = c(5, 6);

After

const result = 6 + 2 * 5;

Expression Simplification

An example with numbers

Before

let total = 0x2 * 0x109e + -0xc * -0x16a + -0x3234;
for (let i = 0x1196 + 0x97b * 0x3 + -0x2e07; i < -0x95 * -0x38 + -0x1a75 + -0x619; i++) {
    total += i;
}

After

let total = 0;
for (let i = 0; i < 10; i++) {
    total += i;
}

An example with strings.

Before

console.log('He' + 'll' + 'o' + ' Wo' + 'r' + 'ld');

After

console.log("Hello World");

Overall Example

All these features can be chained together to simplify code.

Before

const ar = ['\x48\x65\x6c\x6c\x6f', 0x95, '\x20', 0x1a75, '\x57\x6f\x72\x6c\x64', -0x53, '\x6c\x6f\x67']
const a = function(b, c) {
    return c + 2 * b;
}, b = function(c, d) {
    return a(c, d);
}, c = function(d, e) {
    return b(d, e);
};
const message = ar[0] + ar[2] + ar[4];
const result = c(ar[1] * 0x38 + ar[3] + 0x619, 0x12 * ar[5] + 0x1a13 + 0x621);
console[ar[6]](message + ' ' + result);

After

const message = "Hello World";
const result = 40106;
console.log(message + " " + result);

Advanced Usage

Function Evaluation

Often obfuscated scripts don't just use an array of strings, instead they have string decoder functions that execute more complex logic, such as the example below.

function _0x29e92(_0x337a9) {
    const _0x38a2db = ['\x48\x65\x6c\x6c\x6f', '\x20', '\x57\x6f\x72\x6c\x64'];
    const _0x9ca21 = _0x337a9 - 0x1;
    const _0xa8291 = _0x38a2db[_0x9ca21];
    return _0xa8291;
}

const _0x78e2 = _0x29e92(1) + _0x29e92(2) + _0x29e92(3);
console.log(_0x78e2);

To tell the deobfuscator to execute this function, you can use the "#execute" directive like so:

function _0x29e92(_0x337a9) {
    "#execute";
    const _0x38a2db = ['\x48\x65\x6c\x6c\x6f', '\x20', '\x57\x6f\x72\x6c\x64'];
    const _0x9ca21 = _0x337a9 - 0x1;
    const _0xa8291 = _0x38a2db[_0x9ca21];
    return _0xa8291;
}

const _0x78e2 = _0x29e92(1) + _0x29e92(2) + _0x29e92(3);
console.log(_0x78e2);

The deobfuscator will then evaluate this function and attempt to replace any calls to it with the correct values:

const a = "Hello World";
console.log(a);

A few important points about function evaluation:

• BE CAREFUL when using function evaluation, this executes whatever functions you specify on your local machine so make sure those functions are not doing anything malicious.
• This feature is still somewhat experimental, it's probably easier to use via the CLI as it's easier to find errors than the online version.
• If the function is not a function declaration (i.e. a function expression or an arrow function expression) then the deobfuscator will not be able to detect the name of it automatically. To provide it use "#execute[name=FUNC_NAME]" directive.
• You may need to modify the function to ensure it relies on no external variables (i.e. move a string array declaration inside the function) and handle any extra logic like string array rotation first.
• You must first remove any anti tampering mechanisms before using function evaluation, otherwise it may cause an infinite loop.

Config

interface Config {
    arrays: {
        unpackArrays: boolean;
        removeArrays: boolean;
    };
    proxyFunctions: {
        replaceProxyFunctions: boolean;
        removeProxyFunctions: boolean;
    };
    expressions: {
        simplifyExpressions: boolean;
        removeDeadBranches: boolean;
    };
    miscellaneous: {
        beautify: boolean;
        simplifyProperties: boolean;
        renameHexIdentifiers: boolean;
    };
}

To Run

Put the obfuscated script in input/source.js and run:
npm start

Or use the online version at deobfuscate.io