AntiDebug Implementation

Recently while using Wenxin Yiyan (Baidu’s AI), I opened the developer tools and found that it entered a breakpoint in an anonymous function. When clicking to continue execution, the page directly turned into a blank tab. This is clearly an anti-debugging measure. Here I’ll analyze this implementation technique.

Analysis

1. Examining the call stack at the breakpoint revealed setInterval.
2. The tag for the anonymous function is displayed as VM, so the anonymous function containing the breakpoint is code generated by eval or new Function.
3. After the breakpoint executes, it runs window.location.replace(“about:blank”);. I couldn’t determine exactly how it detects entering a breakpoint because the code is obfuscated, but one approach is to use time difference detection. When entering a debugger, the logic execution will be significantly delayed, so calculating a time difference can determine if developer tools are open.

Example

  (function () {
    setInterval(() => {
      const startTime = performance.now();
      eval('(function() { debugger;})()');
      const stopTime = performance.now();
      if ((stopTime - startTime) > 1000) {
        window.location.replace("about:blank");
      }
    }, 400)
  })()

Xinghua’s Approach

I looked at Xinghua (iFlytek’s AI) and found they also implemented anti-crawling measures, but with a different approach. Xinghua uses the fact that opening developer tools compresses the webpage’s height or width, and detects this difference to make a determination. However, this approach has a drawback - if the developer tools are opened in a separate window, this detection becomes ineffective.

const widthThreshold = window.outerWidth - window.innerWidth > threshold;
const heightThreshold = window.outerHeight - window.innerHeight > threshold;
const orientation = widthThreshold ? 'vertical' : 'horizontal';

Counter-AntiDebug

Having roughly analyzed the implementation methods, the corresponding solutions are quite simple. For Xinghua’s approach, as mentioned earlier, simply using developer tools in a separate window breaks their detection.

For Baidu’s method which uses timers, we can simply override the timer implementation. Using Tampermonkey is recommended:

  window.setInterval = function(callback, delay) {
        // Here, we simply don't call the original setInterval function, thus blocking all timers
      console.log('Blocked a setInterval call with delay ' + delay);
   };

Never Pause Here

Conclusion

1. From a purely technical perspective, AntiDebug is interesting - for example, understanding the trigger mechanism of debuggers, learning HACK techniques to detect when developer tools are open, and how to break AntiDebug implementations.
2. Setting aside the technical aspects, why are “self-developed” AIs like Baidu and Xinghua afraid of frontend debugging? What are they 100% nervous about?

Related Documentation

• https://x-c3ll.github.io/posts/javascript-antidebugging/