GHSA-vcc4-2c75-vc9v — v2
MEDIUMGHSA-vcc4-2c75-vc9v is a medium-severity (CVSS 4.2) CWE-116 vulnerability in github.com/caddyserver/caddy/v2. A fix is available for github.com/caddyserver/caddy/v2 — see the affected versions and patch details below.
Caddy: stripHTML template function bypass
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for GHSA-vcc4-2c75-vc9v.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
GHSA-vcc4-2c75-vc9v plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,166 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
github.com/caddyserver/caddy/v2🐹github.com/caddyserver/caddyReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
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
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/caddyserver/caddy/v2 | all versions | 2.11.4go get github.com/caddyserver/caddy/v2@v2.11.4 |
| 🐹Go | github.com/caddyserver/caddy | all versions | No fix |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/caddyserver/caddy/v2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/caddyserver/caddy/v2 to 2.11.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-vcc4-2c75-vc9v is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like GHSA-vcc4-2c75-vc9v can be triaged on real exposure rather than presence alone.
Tailored to GHSA-vcc4-2c75-vc9v. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-vcc4-2c75-vc9v in your dependencies?
O3 Security finds GHSA-vcc4-2c75-vc9v across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.