Evading JavaScript anti-debugging techniques

[8chanAnon]

Interesting though it involves recompiling the web browser. I have encountered this issue on many websites and my response is to stream the website through a proxy server which can then save the content (both outgoing and incoming) to the local disk for analysis. Using the browser's debugging tool is a lost cause when you're dealing with obfuscated code. The approach that I use is to isolate the target JS, modify it by including calls to a websocket, save the code to disk and instruct the proxy server to load the code from disk instead of from the website. This way the website appears to work normally except with my modification. In some cases, it may be necessary to isolate an additional file or two due to dependencies.

The reason for the websocket is that the browser console is also rendered inoperable due to the debugger statements and console clear commands emanating from the website JS. A websocket is then the only way to transfer actionable information (such as a password or a secret link). It's not an easy or quick process but, by inserting websocket calls in interesting places, it is possible to figure out what the JS is doing. It also helps a lot to prettify the JS in order to study it. There are websites that can do that for you. Unfortunately, the prettification of the JS may break it so you're still stuck with doing the modifications in the original JS.

I built my own proxy server for this task but I imagine that the same may be possible with a tool like HTTP Toolkit but that means getting the Pro version.

[connor4312]

In the VS Code JS debugger, there's an option to "exclude caller" on a call frame that which prevents stacks with the given caller from pausing at a location. As mentioned elsewhere, browser devtools have something similar with "Never pause here." Do you think there's more than tools can do to make your process easier?

I maintain the vscode debugger and found both the article and your comment interesting--there's a large overlap between "programs with anti-debugger techniques" and "programs that are hard to debug."

[js2]

> Interesting though it involves recompiling the web browser.

Years ago I really wanted to disable the blink tag, so I just ran `perl -pie "s/blank/abcde/g"` on the binary and that worked well enough.

I'll bet you could so something similar with "debugger". On macOS, you'd break code signing, but you could re-sign it or strip the signing and let it run unsigned.

[leptons]

Been doing stuff similar to this for decades(?) using the Fiddler proxy. It does so much stuff browser extensions or browser inspectors don't. One of my most important tools for website debugging/hacking/workarounds.

[hoten]

Would the "Local Overrides" feature of chrome devtools simplify this workflow for you?

[jkingsman]

I'm surprised to not see Chrome's handy "Never pause here" menu that appears when you right click any line of JS, including debug breakpoints. This is typically what I do when there's a debug in an intervaled function (simple anti-debug commonly found on some video sites).

Example: https://i.imgur.com/BsphnEu.png

[nullpt_rs]

I knew I forgot to mention something :) I do love this option but I wasn't able to get it to work with this obfuscation technique either.

[bunga-bunga]

I don’t think that works with eval code because it doesn’t have a file:0:0 address

[jalino23]

thats interesting. I've always seen that but always ignored it, not knowing what I could use it for

[gmerc]

Unfortunately that won’t be an option with Web Integrity….

[Affric]

Glad you have said it.

And before a developer for these commerce websites jumps up and says “ah but supreme are trying to prevent bots from buying up all of their merch and scalping it”:

Supreme are restricting supply so they can maximise profits.

They are selling on the web rather than through traditional retail outlets using this method not to reach a wider audience for the audience’s sake but to have a larger number of people who are willing to pay an even higher price.

The web, the system that brings free information to the masses requiring no knowledge of the underlying technologies, is too important to compromise for these e-commerce platforms attempting to have their cake and eat it to.

[TYT]

For people who don't want to compile the anti-debugging firefox themselves, I have set up a github repo to do it automatically: https://github.com/Sec-ant/anti-anti-debugging-debugger-fire...

[rasz]

>By renaming it to something like "banana," the debugger would no longer trigger on occurrences of the debugger keyword. To achieve this, we built customized version of Firefox.

heavy handed approach. I have some moderate success intercepting setInterval/setTimeout and manually sifting to find that one call that starts the ball rolling. Things get old fast when the code you are looking at looks like

    0[_0x199d1e(0x815*-0x2+0x1735+0x13f*-0x5)](_0x199d1e(0x3b3*0xa+0x1c1+-0x260d),_0x199d1e(0x2149*0x1+0x9f7+0x1*-0x29f5)))[_0x

[29ebJCyy]

Just record a Replay (https://replay.io). Done!

[crazygringo]

> Once upon a time, whenever you tried to open your devtools on Supreme's website, you found yourself trapped in a pesky debugger loop.

Could somebody here explain what that means, since the article doesn't? What's a debugger loop? What is the actual JavaScript code that somehow prevents debugging, and how does it accomplish that?

[SyrupThinker]

Using a `debugger;` statement allows you to trigger a breakpoint with code.

This only gets activated when the devtools window is opened, so placing this statement in a frequently executed piece code will continuously interrupt whatever you are doing in the devtools when you use them.

I assume in the past the tooling might not have had the necessary configuration options to suppress that, but nowadays you can just disable debugger statement breakpoints to avoid it.

[8chanAnon]

The Javascript statement is simply "debugger". Very easy to abuse. Of course, there are other techniques for breaking devtools. There are JS libraries designed for the purpose of detecting that the dev console is open. The response may be to run the debugger command, freeze the code, reload the web page or, worse, do some serious hanky-panky (it's not hard to crash the web browser; an endless loop can do that).

[badrabbit]

The SANS course for this still teaches to use IE for debugging JS because it is the only browser that lets you break at arbitrary points in the code instead of newline boundaries.

[rezonant]

Chrome has breakpoints within the line too. They aren't particularly great though.

[38]

note if all you care about is capturing the web requests, you can use something like MITM Proxy:

https://github.com/mitmproxy/mitmproxy

[lini]

You can also use a MITM proxy tool to intercept the JS files and modify their response body to remove or replace the `debugger;` statements with something else. Might require inspecting the JS files first to see what needs to be replaced exactly, but should not take more than a few minutes.

[chmod775]

That will not pass integrity checks (the script inspecting its own code).

It will also not work if the script is some initially obfuscated string that is passed to eval() or something more complex assembling the actual code on the fly.

[grishka]

This assumes that the script contains the word "debugger" in clear text, however it may not. It may decrypt or descramble a string and then eval() it. Your approach wouldn't catch that.

[j0hnyl]

This won't work with obfuscated JS.

[ezekiel68]

Ah yes, the eternal arms race.