GHSA-qjv7-627w-8qjv: Fiber vulnerable to XSS in AutoFormat Content Negotiation
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."DetailsThe 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.ImpactThis 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 PatchThe 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.`html.EscapeString` escapes `<`, `>`, `&`, `'`, `"`, which is sufficient for an element-text context. Apply the same change to v2's `(*Ctx).Format()`.Proof of ConceptBenign JSONHTML sink enables XSS
CVSS v4.0: 5.3