How to verify whether the input is a valid Html tag in golang

Go language is a fast, efficient and strongly typed programming language, which is widely used in network service development, cloud computing, data science, Internet finance and other fields. Input validation is a very important issue in web application development, and it is a common requirement to verify whether the HTML tags in the input are valid. Below we will introduce how to implement this requirement in Go language.

HTML tags play an important role in Web pages. They define the structure, style and interactive behavior of the page. But when processing user input, we need to pay attention to the risk that HTML tags may be abused, such as potential XSS attacks (cross-site scripting attacks) and other security issues. Therefore, some applications verify whether the input contains malicious or illegal tags to ensure the security and reliability of the page. Below we will introduce how to verify whether the input is a valid HTML tag in the Go language.

The first method is to use Go's native library. We can use the html.Parse function to parse the HTML code into a node tree, and then check the node type and attributes. The following is a sample code:

package main

import (
    "fmt"
    "golang.org/x/net/html"
    "strings"
)

func isValidHTMLTags(html string) bool {
    doc, err := html.Parse(strings.NewReader(html))
    if err != nil {
        fmt.Println(err)
        return false
    }
    for c := doc.FirstChild; c != nil; c = c.NextSibling {
        if c.Type == html.ElementNode {
            switch c.Data {
            case "a", "em", "strong":
                // 检查<a>、<em>、<strong>标签是否包含 href 和 title 属性
                if !containsAttributes(c, "href", "title") {
                    return false
                }
            case "img":
                // 检查<img>标签是否包含 src、alt、和 title 属性
                if !containsAttributes(c, "src", "alt", "title") {
                    return false
                }
            default:
                // 其他不允许的标签
                return false
            }
        }
    }
    return true
}

func containsAttributes(n *html.Node, attrs ...string) bool {
    for _, attr := range attrs {
        found := false
        for _, a := range n.Attr {
            if a.Key == attr {
                found = true
                break
            }
        }
        if !found {
            return false
        }
    }
    return true
}

func main() {
    html1 := "<p>Hello, <em>world!</em></p>"
    fmt.Println(isValidHTMLTags(html1))   // output: true

    html2 := "<script>alert('XSS');</script>"
    fmt.Println(isValidHTMLTags(html2))   // output: false

    html3 := "<a href='https://www.google.com' title='Google'>Google</a>"
    fmt.Println(isValidHTMLTags(html3))   // output: true

    html4 := "<img src='image.png' alt='Image' title='My image'/>"
    fmt.Println(isValidHTMLTags(html4))   // output: true

    html5 := "<audio src='music.mp3'></audio>"
    fmt.Println(isValidHTMLTags(html5))   // output: false
}

In the above code, we first use the html.Parse function to parse the input HTML code into a node tree. Then iterate through each node, if the node's type is ElementNode, you need to check the node's label name and properties. In this example, we only allow <a>, <em>, <strong>, and <img> tag, returns false if other tags are found. For allowed tags, we also need to check whether they contain the necessary attributes. For example, the <a> tag needs to contain the href and title attributes, while # The ##<img> tag needs to contain the src, alt and title attributes. When checking attributes, we can use the containsAttributes function, which accepts a node and a list of attributes and checks whether the node contains all the specified attributes.

The second method is to use a third-party library. Some third-party libraries in the Go language can help us verify the HTML tags in the input more easily, such as github.com/microcosm-cc/bluemonday and github. com/theplant/htmlsanitizer. These libraries provide some simple APIs that allow us to easily define whitelists or blacklists and filter out tags that do not meet the requirements. For example, the following is a sample code using the bluemonday library:

package main

import (
    "fmt"
    "github.com/microcosm-cc/bluemonday"
)

func main() {
    html := "<p>Hello, <em>world!</em></p>"
    policy := bluemonday.StrictPolicy()
    sanitizedHTML := policy.Sanitize(html)
    fmt.Println(sanitizedHTML)   // output: <p>Hello, <em>world!</em></p>
}

In the above code, we first define a default security policy (bluemonday.StrictPolicy()), and then use the policy.Sanitize function to filter the input HTML code. According to the default security policy, we allow the

<em> tag but not other tags. Since bluemonday supports a higher degree of customization, we can define our own security policy based on it. Please refer to its documentation for specific usage.

Verify that user input is a valid HTML tag. This is a common and important requirement. The above briefly introduces how to use Go native libraries and third-party libraries to achieve this requirement. I hope it will be helpful to you.

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