| Gói / sản phẩm | Phiên bản bị ảnh hưởng | Phiên bản đã vá |
|---|---|---|
| github.com/gofiber/fiber/v2 | < 2.52.13 | 2.52.13 |
| github.com/gofiber/fiber/v3 | < 3.2.0 | 3.2.0 |
| fiber | ≥ 3.0.0, < 3.1.0< 2.52.12 | 3.1.02.52.12 |
Description
A Cross-Site Scripting (CWE-79) vulnerability in Go Fiber allows a remote attacker to inject arbitrary HTML/JavaScript by supplying Accept: text/html on any request whose handler passes attacker-influenced data to the AutoFormat() feature. This affects github.com/gofiber/fiber/v3 (DefaultRes.AutoFormat) through version 3.1.0 and github.com/gofiber/fiber/v2 (Ctx.Format) through version 2.52.12.
The developer opts into content negotiation by calling AutoFormat(), but does not opt into raw HTML emission for a particular request; Fiber chooses that branch from attacker-controlled Accept. Five of the six branches of the same method already escape. JSON, XML, MsgPack, and CBOR all route through encoders that neutralize markup; the txt branch emits text/plain and cannot execute. The html branch is the sole outlier in a method whose name (AutoFormat) and symmetrical structure actively telegraph "safe, format-agnostic reply."
The issue resides in res.go within (*DefaultRes).AutoFormat(). The method negotiates against the request Accept header, selects one of html | json | txt | xml | msgpack | cbor, and serializes the caller-supplied body accordingly.
The "html" branch concatenates the stringified body directly into HTML markup with no output encoding:
accept comes from r.c.Accepts(...), i.e. is fully attacker-controlled. An attacker can force the "html" branch on any AutoFormat() call regardless of which format the developer tested against.b is produced from body via direct assignment (string / []byte) or fmt.Sprintf("%v", body). No html.EscapeString is applied.text/html; charset=utf-8, so browsers render it as active HTML.// res.go
func (r *DefaultRes) AutoFormat(body any) error {
accept := r.c.DefaultReq.Accepts("html", "json", "txt", "xml", "msgpack", "cbor")
r.Type(accept)
var b string
switch val := body.(type) {
case string:
b = val
case []byte:
b = r.c.app.toString(val)
default:
b = fmt.Sprintf("%v", val)
}
switch accept {
case "txt":
return r.SendString(b)
case "json":
return r.JSON(body)
case "xml":
return r.XML(body)
case "html":
return r.SendString("<p>" + b + "</p>")
case "msgpack":
return r.MsgPack(body)
case "cbor":
return r.CBOR(body)
}
return r.SendString(b)
}
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Quét site miễn phíThis impacts all current v3 releases ≤ 3.1.0 containing DefaultRes.AutoFormat, and all current v2 releases ≤ 2.52.12 where the identical "<p>" + b + "</p>" construction exists in (*Ctx).Format(). Exploitation requires that an application call c.AutoFormat(v) where v (or a field stringified by %v) contains request-influenced data.
A handler that uses AutoFormat() to serve multiple representations of the same data can be turned into an HTML XSS sink when the client sends Accept: text/html, even if the developer only tested the JSON path.
This may result in:
AutoFormat.AutoFormat.The injection surface is r.Type("html") followed by r.SendString(b) with unescaped caller data, where it constructs markup on the caller's behalf around a value whose HTML-ness the caller did not declare. A few options:
AutoFormat() should treat body as data, not markup, in the "html" branch and escape it before concatenating it into the framework-generated <p> wrapper. Callers that need raw negotiated HTML should use Format() with an explicit HTML handler.AutoFormat alone for backward compatibility.HTML-escape the value in the "html" branch before concatenating it into the <p> wrapper.
import "html"
// ...
case "html":
return r.SendString("<p>" + html.EscapeString(b) + "</p>")
html.EscapeString escapes <, >, &, ', ", which is sufficient for an element-text context. Apply the same change to v2's (*Ctx).Format().
# Create project directory
mkdir fiber-xss-poc && cd fiber-xss-poc
# Initialize Go module
go mod init fiber-xss-poc
# Install Fiber v3
go get github.com/gofiber/fiber/v3
# Create the PoC file
cat > main.go << 'EOF'
package main
import (
"github.com/gofiber/fiber/v3"
)
type User struct {
ID int `json:"id"`
Name string `json:"name"`
}
func main() {
app := fiber.New()
app.Get("/api/user", func(c fiber.Ctx) error {
user := User{
ID: 1,
Name: c.Query("name", "anonymous"),
}
return c.AutoFormat(user)
})
app.Listen(":3000")
}
EOF
# Run it
go run main.go
}
Benign JSON
curl -s 'http://127.0.0.1:3000/api/user?name=Alice' -H 'Accept: application/json'
{"id":1,"name":"Alice"}
HTML sink enables XSS
curl -s 'http://127.0.0.1:3000/api/user?name=<script>alert(document.domain)</script>' -H 'Accept: text/html'
<p>{1 <script>alert(document.domain)</script>}</p>