Caddy: stripHTML template function bypass in github.com/caddyserver/caddy
Caddy: stripHTML template function bypass in github.com/caddyserver/caddy
02 / AFFECTED SOFTWARE
Affected packages
97 explicit affected versions
03 / CONNECTIONS
Connected vulnerabilities
04 / EVIDENCE
Source records
### 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 } ``` Template file (index.html) ```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.
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.
Caddy: stripHTML template function bypass in github.com/caddyserver/caddy
05 / REFERENCES