Caddy: stripHTML template function bypass
Date: 2026-06
Affected: go/github.com/caddyserver/caddy, go/github.com/caddyserver/caddy/v2
Attack vector: xss
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-52846
CWE: CWE-116
CVSS: 4.2 (CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N)

### 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 } ``` Templat


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