CVE-2026-42554: Fiber vulnerable to XSS in AutoFormat Content Negotiation

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.”