Fixing Httpx HTML Stack Overflow: A Practical Guide

by Alex Johnson 52 views

Have you encountered the dreaded html: open stack of elements exceeds 512 nodes error when using httpx? This error, often a result of deeply nested or excessively complex HTML-like content, can halt your scans and leave you scratching your head. In this guide, we'll break down the cause of this panic, explore a real-world example, and provide practical strategies to resolve it, ensuring your httpx scans run smoothly. Let's dive in!

Understanding the "html: open stack of elements exceeds 512 nodes" Error

At its core, this error arises from the Go's HTML parser's limitations. The parser maintains a stack of open HTML elements as it processes a document. When this stack exceeds a predefined limit, typically 512 nodes, the parser panics, leading to the html: open stack of elements exceeds 512 nodes error.

This issue often surfaces when httpx, a versatile HTTP probing tool, encounters web pages or resources with extremely deep nesting, malformed HTML, or, as seen in the provided example, minified JavaScript files that resemble HTML structures. The PageTypeClassifier within httpx, responsible for determining the content type, utilizes the HTML parser, thus triggering the panic when the stack limit is breached.

The error message itself, panic: html: open stack of elements exceeds 512 nodes, clearly indicates the problem. The subsequent stack trace pinpoints the exact location within the httpx codebase where the panic occurred, specifically in the htmlToText function of the pagetypeclassifier package. This function attempts to convert HTML content to plain text, a process that involves parsing the HTML structure. When the HTML is too complex or deeply nested, the parser's stack overflows, causing the program to crash.

To effectively address this error, it's essential to understand the context in which it occurs. Identifying the specific URL or type of content that triggers the panic is the first step. Once identified, you can then explore strategies to either avoid processing such content or modify the httpx configuration to mitigate the issue. Remember, this error is not necessarily a bug in your code but rather a limitation of the HTML parser when dealing with exceptionally complex or malformed HTML-like structures.

Case Study: httpx Crashing with a Large JavaScript File

Let's examine a specific scenario where httpx crashes due to this error. The provided example involves scanning the following URL:

https://unpkg.com/three@0.150.0/build/three.min.js

This URL points to a minified JavaScript file, three.min.js, which is part of the Three.js library. Minified JavaScript often contains long lines of code without proper HTML structure, but when the PageTypeClassifier attempts to analyze it as HTML, it leads to the stack overflow.

The command used to trigger the crash is:

echo https://unpkg.com/three@0.150.0/build/three.min.js | httpx -title -tech-detect -status-code -location -content-length -cname -web-server -follow-redirects -websocket

This command instructs httpx to perform several checks on the specified URL, including retrieving the title, detecting technologies, checking the status code, and more. The -tech-detect flag is particularly relevant, as it likely triggers the PageTypeClassifier and, consequently, the HTML parsing process.

The panic output confirms the issue:

panic: html: open stack of elements exceeds 512 nodes

goroutine 3033 [running]:
github.com/projectdiscovery/httpx/common/pagetypeclassifier.htmlToText(...)
        /home/runner/work/httpx/httpx/common/pagetypeclassifier/pagetypeclassifier.go:36
github.com/projectdiscovery/httpx/common/pagetypeclassifier.(*PageTypeClassifier).Classify(0xc0096fb488, {0xc00f0d6000?, 0xc?})
        /home/runner/work/httpx/httpx/common/pagetypeclassifier/pagetypeclassifier.go:26 +0x6f
github.com/projectdiscovery/httpx/runner.(*Runner).analyze(_, _, {_, _}, {{0xc00b60e200, 0x32}, {0x0, 0x0}, {0x0, 0x0}}, ...)
        /home/runner/work/httpx/httpx/runner/runner.go:2349 +0x7555
github.com/projectdiscovery/httpx/runner.(*Runner).process.func1({{0xc00b60e200, 0x32}, {0x0, 0x0}, {0x0, 0x0}}, {0x1686161?, 0x0?}, {0xc00b60e200, 0x5})
        /home/runner/work/httpx/httpx/runner/runner.go:1444 +0x125
created by github.com/projectdiscovery/httpx/runner.(*Runner).process in goroutine 1
        /home/runner/work/httpx/httpx/runner/runner.go:1442 +0x8a6

The stack trace clearly shows that the panic originates from the htmlToText function within the pagetypeclassifier package, confirming that the HTML parser is the culprit.

This case study highlights a common scenario where httpx can encounter this error. When scanning URLs that serve non-HTML content, such as JavaScript files, the PageTypeClassifier may incorrectly attempt to parse the content as HTML, leading to the stack overflow. In the next section, we'll explore practical solutions to address this issue.

Practical Solutions to Resolve the Stack Overflow Error

Now that we understand the root cause and have examined a real-world example, let's explore practical solutions to resolve the html: open stack of elements exceeds 512 nodes error in httpx:

1. Exclude problematic URLs

The simplest solution is to exclude the URLs that trigger the panic from your httpx scans. This can be achieved by maintaining a list of known problematic URLs or URL patterns and filtering them out before running httpx. For instance, if you know that certain file extensions, such as .js or .css, consistently cause issues, you can exclude them from your scans.

This approach is effective when you have a good understanding of the URLs you're scanning and can identify potential problem areas in advance. However, it may not be feasible if you're dealing with a large number of URLs or if the problematic URLs are not easily identifiable.

2. Adjust the PageTypeClassifier

Another approach is to modify the behavior of the PageTypeClassifier within httpx. Since the classifier is responsible for triggering the HTML parsing process, you can configure it to be more selective in its analysis. This might involve:

  • Disabling the PageTypeClassifier: If you don't need the content type classification functionality, you can disable it altogether. This will prevent httpx from attempting to parse potentially problematic content as HTML.
  • Modifying the classification rules: You can adjust the rules used by the PageTypeClassifier to determine whether a given resource should be treated as HTML. This might involve adding exceptions for certain file extensions or content types.

However, modifying the PageTypeClassifier requires a deeper understanding of the httpx codebase and may not be suitable for all users.

3. Pre-process the HTML content

In some cases, you may be able to pre-process the HTML content before feeding it to httpx. This could involve:

  • Removing deeply nested elements: If the HTML structure is excessively nested, you can use a tool to flatten it or remove unnecessary elements.
  • Correcting malformed HTML: If the HTML contains syntax errors, you can use an HTML validator to fix them.

However, pre-processing the HTML content can be time-consuming and may not always be feasible, especially when dealing with a large number of URLs.

4. Increase the Stack Size (Use with Caution)

While not recommended as a primary solution, you could technically try increasing the stack size for the Go runtime. This is highly discouraged unless you fully understand the implications. It's more of a last resort and could lead to other stability issues. If you choose to explore this, research how to set GOMAXPROCS and the stack size appropriately.

5. File a Bug Report/Contribute to httpx

Finally, consider reporting the issue to the httpx developers. Providing them with the URL that triggers the panic and the steps to reproduce the error can help them identify and fix the underlying problem. Contributing to the project by submitting a pull request with a proposed solution is also a great way to give back to the open-source community.

By implementing one or more of these solutions, you can effectively address the html: open stack of elements exceeds 512 nodes error and ensure the smooth operation of your httpx scans.

Conclusion

The html: open stack of elements exceeds 512 nodes error in httpx can be a frustrating obstacle, but by understanding its cause and implementing the appropriate solutions, you can overcome this challenge. Whether it's excluding problematic URLs, adjusting the PageTypeClassifier, or pre-processing the HTML content, there are several strategies available to mitigate this issue. Remember to carefully consider the context in which the error occurs and choose the solution that best fits your needs. By taking these steps, you can ensure that your httpx scans run smoothly and efficiently. For more information on how HTML works, check out this Mozilla Developer Network Documentation.