Fiber vulnerable to XSS in AutoFormat Content Negotiation
Date: 2026-05
Affected: go/github.com/gofiber/fiber/v2, go/github.com/gofiber/fiber/v3
Attack vector: xss
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-42554
CWE: CWE-79

## 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


Mitigations:

Architectural layers (MAESTRO):