Affected: CleanStart: caddy

Security vulnerability affects the caddy package. Caddy is an extensible server platform that uses TLS by default.

Caddy is an extensible server platform that uses TLS by default. Prior to 2.11.4, 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. This vulnerability is fixed in 2.11.4.

Affected: Go: github.com/caddyserver/caddy/v2, Go: github.com/caddyserver/caddy

### 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](https://github.com/caddyserver/caddy/blob/77e9ce7404c4a76853e101a9f5687a929ee56654/modules/caddyhttp/templates/tplcontext.go)

```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 } ```

Templa…

Affected: Ubuntu:Pro:24.04:LTS: caddy, Ubuntu:25.10: caddy, Ubuntu:Pro:26.04:LTS: caddy

Caddy is an extensible server platform that uses TLS by default. Prior to 2.11.4, 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. This vulnerability is fixed in 2.11.4.