CVE-2023-24539: Improper sanitization of CSS values in html/template

Background

The stdlib package in Go is a collection of standard libraries that provide fundamental functionality for Go programs. The html/template package within the stdlib package is used for generating HTML output. It allows developers to define templates with placeholders that can be filled with dynamic data. However, the package is vulnerable to improper sanitization of CSS values, which can lead to the injection of unexpected HTML if executed with untrusted input.

Vulnerability Detail

The vulnerability arises from the inadequate handling of angle brackets (<>) in CSS contexts. Normally, angle brackets are not considered dangerous characters in CSS. However, when templates containing multiple actions separated by a '/' character are used, the CSS context can unexpectedly close, enabling the injection of unexpected HTML. This can have serious security implications, as it allows for the execution of arbitrary code or the manipulation of the rendered HTML.

go get stdlib@vgo1.25.5

Workaround

If updating the Go stdlib package is not immediately feasible, developers can implement a workaround by manually sanitizing the CSS values before using them in the html/template package. This can be done by escaping any angle brackets (<>) in the CSS values to prevent them from being interpreted as HTML tags. Here is an example of how this can be achieved in Go:

import (
	"html/template"
	"strings"
)

func sanitizeCSSValue(value string) string {
	return strings.ReplaceAll(value, "<", "\u003c").ReplaceAll(value, ">", "\u003e")
}

// Usage:
func main() {
	cssValue := "<script>alert('XSS')</script>"
	sanitizedValue := sanitizeCSSValue(cssValue)
	// Use the sanitized value in the html/template package
}

Conclusion

The improper sanitization of CSS values in the html/template package of the Go stdlib package can lead to the injection of unexpected HTML, potentially enabling various attacks. It is crucial to update to the recommended versions of the package or implement the provided workaround to mitigate the risk. For further assistance or support in vulnerability remediation, please contact DevNack.com.

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