Summary
Echo vulnerable to Reflected Cross-Site Scripting (XSS) via unsanitized JSONP callback in Context.JSONP / Context.JSONPBlob
The Context.JSONP, Context.JSONPBlob, and internal jsonPBlob helper in context.go directly concatenate the caller-supplied callback parameter into a JavaScript response without any validation, escaping, or allow-listing.
This creates a high-impact reflected XSS that can be triggered by any route passing attacker-controlled data (query string, form, header, path param, etc.) into the callback.
Present in current master and v5.1.0 (latest release).
Vulnerable Code (verbatim)
func (c *Context) jsonPBlob(code int, callback string, i any) (err error) {
c.writeContentType(MIMEApplicationJavaScriptCharsetUTF8)
c.response.WriteHeader(code)
if _, err = c.response.Write([]byte(callback + "(")); err != nil {
return
}
if err = c.echo.JSONSerializer.Serialize(c, i, ""); err != nil {
return
}
if _, err = c.response.Write([]byte(");")); err != nil {
return
}
return
}
func (c *Context) JSONP(code int, callback string, i any) (err error) {
return c.jsonPBlob(code, callback, i)
}
func (c *Context) JSONPBlob(code int, callback string, b []byte) (err error) {
c.writeContentType(MIMEApplicationJavaScriptCharsetUTF8)
c.response.WriteHeader(code)
if _, err = c.response.Write([]byte(callback + "(")); err != nil {
return
}
if _, err = c.response.Write(b); err != nil {
return
}
_, err = c.response.Write([]byte(");"))
return
}
No sanitization is performed on the callback string.
Exploit (one-request repro)
Any route using the official pattern is vulnerable:
Goe.GET("/api", func(c echo.Context) error {
return c.JSONP(200, c.QueryParam("callback"), map[string]string{"status": "ok"})
})
Attacker request:
GET /api?callback=alert(document.domain)%2f%2f HTTP/1.1
Host: target.com
Server response:
Content-Type: application/javascript; charset=utf-8
alert(document.domain)//({"status":"ok"});
→ Immediate arbitrary JavaScript execution (full reflected XSS).
Affected Versions
- All versions ≤ v5.1.0 (including current master)
Workarounds
Manually sanitize the callback before calling JSONP/JSONPBlob:
Gofunc safeCallback(cb string) string {
if matched, _ := regexp.MatchString(`^[a-zA-Z0-9_.\[\]]+$`, cb); !matched {
return "callback"
}
return cb
}
Or migrate away from JSONP entirely (use CORS + application/json).
References
- https://github.com/labstack/echo/blob/master/context.go
- https://echo.labstack.com/docs/cookbook/jsonp
Workarounds
Validate the callback in your handler before calling Context.JSONP, for example against ^[A-Za-z_$][A-Za-z0-9_$]*(\.[A-Za-z_$][A-Za-z0-9_$]*)*$, and set X-Content-Type-Options: nosniff. JSONP responses can be read by any website with the user's cookies, so do not serve data that requires authentication through JSONP.
Impact
- Session hijacking, cookie theft, account takeover, keylogging, defacement.
- Works even with strict CSP (response is served as application/javascript).
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
CVE-2026-118652 has a CVSS score of 6.1 (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 (5.4.0, 4.16.0); 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.
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Fixed in github.com/labstack/echo/v5 v5.4.0 and github.com/labstack/echo/v4 v4.16.0. Context.JSONP and Context.JSONPBlob accept only a callback that is empty, a JavaScript identifier or a dot-separated path of identifiers (ASCII letters, digits, _ and $); any other callback returns a 400 Bad Request error wrapping ErrInvalidJSONPCallback and nothing is written. JSONP responses also carry X-Content-Type-Options: nosniff.
Frequently Asked Questions
- What is CVE-2026-118652? CVE-2026-118652 is a medium-severity cross-site scripting (XSS) vulnerability in github.com/labstack/echo/v5 (go), affecting versions < 5.4.0. It is fixed in 5.4.0, 4.16.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- How severe is CVE-2026-118652? CVE-2026-118652 has a CVSS score of 6.1 (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-2026-118652?
github.com/labstack/echo/v5(go) (versions < 5.4.0)github.com/labstack/echo/v4(go) (versions < 4.16.0)
- Is there a fix for CVE-2026-118652? Yes. CVE-2026-118652 is fixed in 5.4.0, 4.16.0. Upgrade to this version or later.
- Is CVE-2026-118652 exploitable, and should I be worried? Whether CVE-2026-118652 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-2026-118652 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-2026-118652?
- Upgrade
github.com/labstack/echo/v5to 5.4.0 or later - Upgrade
github.com/labstack/echo/v4to 4.16.0 or later
- Upgrade