Vulnerability-Lookup

CVE-2026-52846 (GCVE-0-2026-52846)

Vulnerability from cvelistv5 – Published: 2026-06-23 17:47 – Updated: 2026-06-25 12:42

Title

Caddy: stripHTML template function bypass

Summary

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.

Severity

4.2 (Medium)


                        
                          CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator (v2.0.3)

CWE

CWE-116 - Improper Encoding or Escaping of Output

Assigner

GitHub_M

References

1 reference

URL	Tags
https://github.com/caddyserver/caddy/security/adv…	x_refsource_CONFIRM

Impacted products

1 product

Vendor	Product	Version
caddyserver	caddy	Affected: < 2.11.4

Show details on NVD website

JSON

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GHSA-VCC4-2C75-VC9V

Vulnerability from github – Published: 2026-06-16 21:28 – Updated: 2026-06-16 21:28

Summary

Caddy: stripHTML template function bypass

Details

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

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)

<!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 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.

Severity

4.2 (Medium)


                  
                    CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 2.11.3"
      },
      "package": {
        "ecosystem": "Go",
        "name": "github.com/caddyserver/caddy/v2"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.11.4"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "Go",
        "name": "github.com/caddyserver/caddy"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "last_affected": "1.0.5"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-52846"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-116"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-16T21:28:55Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "### Summary\nCaddy\u2019s `stripHTML` template function cannot reliably remove all HTML tags from input strings. Certain malformed HTML, such as `\u003c\u003c\u003eimg src=x onerror=alert()\u003e`, 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 == \u0027\u003e\u0027 \u0026\u0026 !inQuotes {\n                inTag = false\n            } else if ch == \u0027\u003c\u0027 \u0026\u0026 !inQuotes {\n                // false start\n                buf.WriteString(s[tagStart:i])\n                tagStart = i\n            } else if ch == \u0027\"\u0027 {\n                inQuotes = !inQuotes\n            }\n            continue\n        }\n        if ch == \u0027\u003c\u0027 {\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\u003c!DOCTYPE html\u003e\n\u003chtml lang=\"en\"\u003e\n\u003chead\u003e\n    \u003cmeta charset=\"UTF-8\"\u003e\n    \u003ctitle\u003eStripHTML Bypass Test\u003c/title\u003e\n\u003c/head\u003e\n\u003cbody\u003e\n    \u003cp\u003e{{ stripHTML \"\u003c\u003c\u003eimg src=x onerror=alert(\u0027XSS\u0027)\u003e\" }}\u003c/p\u003e\n\u003c/body\u003e\n\u003c/html\u003e\n```\n\nThe payload exploits the false start branch to smuggle a literal \u003c back into the output, then uses the following \u003e to terminate the parser\u2019s tag state, leaving a valid \u003cimg ...\u003e 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.",
  "id": "GHSA-vcc4-2c75-vc9v",
  "modified": "2026-06-16T21:28:55Z",
  "published": "2026-06-16T21:28:55Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/caddyserver/caddy/security/advisories/GHSA-vcc4-2c75-vc9v"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/caddyserver/caddy"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Caddy: stripHTML template function bypass"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

CVE-2026-52846 (GCVE-0-2026-52846)

GHSA-VCC4-2C75-VC9V

Summary

Details

POC

Impact

AI Disclosure

Tags

Sightings

Nomenclature