CVE-2026-52846: Caddy StripHTML Template Function Bypass Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-vcc4-2c75-vc9v",
  "modified" : "2026-06-16T21:28:55Z",
  "published" : "2026-06-16T21:28:55Z",
  "aliases" : [ "CVE-2026-52846" ],
  "summary" : "Caddy: stripHTML template function bypass",
  "details" : "### Summary\nCaddy’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.\n\n---\n\n### Details\nThe vulnerability originates from `funcStripHTML` in:\n\n[caddy/caddy/caddyhttp/templates/tplcontext.go](https://github.com/caddyserver/caddy/blob/77e9ce7404c4a76853e101a9f5687a929ee56654/modules/caddyhttp/templates/tplcontext.go)\n\n```go\nfunc (TemplateContext) funcStripHTML(s string) string {\n    var buf bytes.Buffer\n    var inTag, inQuotes bool\n    var tagStart int\n    for i, ch := range s {\n        if inTag {\n            if ch == '>' && !inQuotes {\n                inTag = false\n            } else if ch == '<' && !inQuotes {\n                // false start\n                buf.WriteString(s[tagStart:i])\n                tagStart = i\n            } else if ch == '\"' {\n                inQuotes = !inQuotes\n            }\n            continue\n        }\n        if ch == '<' {\n            inTag = true\n            tagStart = i\n            continue\n        }\n        buf.WriteRune(ch)\n    }\n    if inTag {\n        // false start\n        buf.WriteString(s[tagStart:])\n    }\n    return buf.String()\n}\n```\n\n### POC\n\nCaddyfile setup\n\n```\n:8080 {\n    root * ./site\n    file_server\n    templates\n}\n```\n\nTemplate file (index.html)\n\n```html\n<!DOCTYPE html>\n<html lang=\"en\">\n<head>\n    <meta charset=\"UTF-8\">\n    <title>StripHTML Bypass Test</title>\n</head>\n<body>\n    <p>{{ stripHTML \"<<>img src=x onerror=alert('XSS')>\" }}</p>\n</body>\n</html>\n```\n\nThe 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.\n\nTested in v2.11.3\n\n### Impact\n\nMalformed HTML can bypass stripHTML, potentially allowing arbitrary HTML or JavaScript to be rendered if the output is used unsafely, leading to client-side XSS.\n\n### AI Disclosure\n\nAI assisted in writing the report description; however, the discovery of the issue has been done manually.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/caddyserver/caddy/v2"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.11.4"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.11.3"
    }
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/caddyserver/caddy"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "last_affected" : "1.0.5"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/caddyserver/caddy/security/advisories/GHSA-vcc4-2c75-vc9v"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/caddyserver/caddy"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-116" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-16T21:28:55Z",
    "nvd_published_at" : null
  }
}