Deobfuscate Javascript and reconstruct strings. Simplify cumbersome logic where possible while adhering to scope limitations.
Try it online @ restringer.tech.
For comments and suggestions feel free to open an issue or find me on Twitter - @ctrl__esc
npm install -g restringer
Requires Node 16 or newer.
git clone [email protected]:PerimeterX/restringer.git
cd restringer
npm install
The restringer.js uses generic deobfuscation methods that reconstruct and restore obfuscated strings and simplifies redundant logic meant only to encumber. REstringer employs the Obfuscation Detector to identify specific types of obfuscation for which there's a need to apply specific deobfuscation methods in order to circumvent anti-debugging mechanisms or other code traps preventing the script from being deobfuscated.
Usage: restringer input_filename [-h] [-c] [-q | -v] [-m M] [-o [output_filename]]
positional arguments:
input_filename The obfuscated JS file
optional arguments:
-h, --help Show this help message and exit.
-c, --clean Remove dead nodes from script after deobfuscation is complete (unsafe).
-q, --quiet Suppress output to stdout. Output result only to stdout if the -o option is not set.
Does not go with the -v option.
-m, --max-iterations M Run at most M iterations
-v, --verbose Show more debug messages while deobfuscating. Does not go with the -q option.
-o, --output [output_filename] Write deobfuscated script to output_filename.
<input_filename>-deob.js is used if no filename is provided.
Examples:
- Print the deobfuscated script to stdout.
restringer [target-file.js] - Save the deobfuscated script to output.js.
restringer [target-file.js] -o output.js - Deobfuscate and print debug info.
restringer [target-file.js] -v - Deobfuscate without printing anything but the deobfuscated output.
restringer [target-file.js] -q
import {REstringer} from 'restringer';
const restringer = new REstringer('"RE" + "stringer"');
if (restringer.deobfuscate()) {
console.log(restringer.script);
} else {
console.log('Nothing was deobfuscated :/');
}
// Output: 'REstringer';
REstringer is highly modularized. It exposes modules that allow creating custom deobfuscators that can solve specific problems.
The basic structure of such a deobfuscator would be an array of deobfuscation modules (either safe or unsafe), run via flAST's applyIteratively utility function.
Unsafe modules run code through eval (using isolated-vm to be on the safe side) while safe modules do not.
import {applyIteratively} from 'flast';
import {safe, unsafe} from 'restringer';
const {normalizeComputed} = safe;
const {resolveDefiniteBinaryExpressions, resolveLocalCalls} = unsafe;
let script = 'obfuscated JS here';
const deobModules = [
resolveDefiniteBinaryExpressions,
resolveLocalCalls,
normalizeComputed,
];
script = applyIteratively(script, deobModules);
console.log(script); // Deobfuscated script
With the additional candidateFilter function argument, it's possible to narrow down the targeted nodes:
import {unsafe} from 'restringer';
const {resolveLocalCalls} = unsafe;
import {applyIteratively} from 'flast';
let script = 'obfuscated JS here';
// It's better to define a function with a meaningful name that can show up in the log
function resolveLocalCallsInGlobalScope(arb) {
return resolveLocalCalls(arb, n => n.parentNode?.type === 'Program');
}
script = applyIteratively(script, [resolveLocalCallsInGlobalScope]);
console.log(script); // Deobfuscated script
You can also customize any deobfuscation method while still using REstringer without running the loop yourself:
import fs from 'node:fs';
import {REstringer} from 'restringer';
const inputFilename = process.argv[2];
const code = fs.readFileSync(inputFilename, 'utf-8');
const res = new REstringer(code);
// res.logger.setLogLevelDebug();
res.detectObfuscationType = false; // Skip obfuscation type detection, including any pre and post processors
const targetFunc = res.unsafeMethods.find(m => m.name === 'resolveLocalCalls');
let changes = 0; // Resolve only the first 5 calls
res.safeMethods[res.unsafeMethods.indexOf(targetFunc)] = function customResolveLocalCalls(n) {return targetFunc(n, () => changes++ < 5)}
res.deobfuscate();
if (res.script !== code) {
console.log('[+] Deob successful');
fs.writeFileSync(`${inputFilename}-deob.js`, res.script, 'utf-8');
} else console.log('[-] Nothing deobfuscated :/');
import {applyIteratively, treeModifier, logger} from 'flast';
// Optional loading from file
// import fs from 'node:fs';
// const inputFilename = process.argv[2] || 'target.js';
// const code = fs.readFileSync(inputFilename, 'utf-8');
const code = `(function() {
function createMessage() {return 'Hello' + ' ' + 'there!';}
function print(msg) {console.log(msg);}
print(createMessage());
})();`;
logger.setLogLevelDebug();
let script = code;
// Use this function to target the relevant nodes
const f = n => n.type === 'Literal' && replacements[n.value];
// Use this function to modify the nodes according to your needs.
// markNode(n) would delete the node, while markNode(n, {...}) would replace the node with the supplied node.
const m = (n, arb) => arb.markNode(n, {
type: 'Literal',
value: replacements[n.value],
});
const swc = treeModifier(f, m, 'StarWarsChanger');
script = applyIteratively(script, [swc]);
if (code !== script) {
console.log(script);
// fs.writeFileSync(inputFilename + '-deob.js', script, 'utf-8');
} else console.log(`No changes`);
